MIC2085-LBQS [MICREL]
Single Channel Hot Swap Controllers; 单通道热插拔控制器
| 型号： | MIC2085-LBQS |
| 厂家： | 
MICREL SEMICONDUCTOR |
| 描述： | Single Channel Hot Swap Controllers |
| 文件： | 总28页 (文件大小：406K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
MIC2085/MIC2086
Single Channel Hot Swap Controllers
General Description
Features
The MIC2085 and MIC2086 are single channel positive
voltage hot swap controllers designed to allow the safe
insertionofboardsintolivesystembackplanes. TheMIC2085
and MIC2086 are available in 16-pin and 20-pin QSOP
packages, respectively. Using a few external components
and by controlling the gate drive of an external N-Channel
MOSFET device, the MIC2085/86 provide inrush current
limiting and output voltage slew rate control in harsh, critical
power supply environments. Additionally, a circuit breaker
function will latch the output MOSFET off if the current limit
threshold is exceeded for a programmed period of time. The
devices’ array of features provide a simplified yet robust
solution for many network applications in meeting the power
supply regulation requirements and affords protection of
critical downstream devices and components.
• MIC2085: Pin for pin functional equivalent to the
LTC1642
• 2.3V to 16.5V supply voltage operation
• Surge voltage protection to 33V
• Operating temperature range –40°C to 85°C
• Active current regulation limits inrush current
independent of load capacitance
• Programmable inrush current limiting
• Analog foldback current limiting
• Electronic circuit breaker
• Dual-level overcurrent fault sensing
• Fast response to short circuit conditions (< 1µs)
• Programmable output undervoltage detection
• Undervoltage lockout protection
• Power-on reset (MIC2085/86) and
power-good (MIC2086) status outputs
• /FAULT status output
All support documentation can be found on Micrel’s web
site at www.micrel.com.
• Driver for SCR crowbar on overvoltage
Applications
• RAID systems
• Cellular base stations
• LAN servers
• WAN servers
• InfiniBand™ Systems
• Industrial high side switching
Typical Application
RSENSE
0.007Ω
2%
Q1
Backplane PCB Edge
Connector Connector
Long
Pin
Si7884DP
(PowerPAKTM SO-8)
VIN
12V
1
2
VOUT
12V@5A
3
4
R1
3.3Ω
CLOAD
220
C1
µ
F
1
µ
F
R7
127kΩ
1%
*R6
10Ω
Short
Pin
VLOGIC
16
15
VCC
SENSE
14
GATE
COMP+
R5
47kΩ
R2
R10
47kΩ
R11
100kΩ
11
10
47kΩ
Output Signal
(Power Good)
1%
4
6
COMPOUT
PWRGD
LOGIC
ON
C2
/FAULT
0.022µF
/FAULT
CONTROLLER
5
7
R3
1.82kΩ
1%
MIC2085
/POR
FB
/RESET
Medium
(or Short)
Pin
Power-On Reset
Output
9
OV
R4
10kΩ
1%
R8
12
13
COMP—
REF
16.2kΩ
1%
1
Q2
2N4401
CRWBR
Q3
TCR22-4
C7
CPOR
3
GND CFILTER
8
0.033µF
2
**R9
180Ω
C3
0.1
C4
0.1
C5
8200pF
C6
0.01µF
µ
F
µF
GND
Long
Pin
Overvoltage (Input) = 13.3V
Undervoltage Lockout = 10.8V
Undervoltage (Output) &
POR/START-UP DELAY = 60ms
Circuit-Breaker Response Time = 500µs
*R6 is an optional component used for noise filtering
**R9 needed when using a sensitive gate SCR
Power-Good (Output) = 11.4V
InfiniBand is a trademark of InfiniBand Trade Association
PowerPAK is a trademark of Vishay Intertechnology Inc.
Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com
M0235-121903
January 2004
1
MIC2085/2086
Micrel
Ordering Information
Part Number
Fast Circuit Breaker Threshold
Discharge Output
Package
MIC2085-xBQS
x = J, 95mV
x = K, 150mV*
x = L, 200mV*
x = M, Off
NA
16-pin QSOP
MIC2086-xBQS
x = J, 95mV
x = K, 150mV*
x = L, 200mV*
x = M, Off
Yes
20-pin QSOP
*Contact factory for availability.
Pin Configuration
CRWBR
CFILTER
CPOR
ON
1
2
3
4
5
6
7
8
9
20 VCC
19 VCC
CRWBR
CFILTER
1
16 VCC
18 SENSE
17 GATE
16 REF
2
15 SENSE
14 GATE
CPOR
ON
3
4
5
6
7
8
/POR
13 REF
PWRGD
/FAULT
FB
15 DIS
/POR
/FAULT
FB
12 COMP–
11 COMP+
10 COMPOUT
14 COMP–
13 COMP+
12 COMPOUT
11 OV
GND
GND
9 OV
GND 10
MIC2085
16-Pin QSOP (QS)
MIC2086
20-Pin QSOP (QS)
Pin Description
Pin Number
MIC2086
Pin Number
MIC2085
Pin Name
Pin Function
1
1
CRWBR
Overvoltage Timer and Crowbar Circuit Trigger: A capacitor connected to
this pin sets the timer duration for which an overvoltage condition will trigger
an external crowbar circuit. This timer begins when the OV input rises above
its threshold as an internal 45µA current source charges the capacitor. Once
the voltage reaches 470mV, the current increases to 1.5mA.
2
3
2
3
CFILTER
CPOR
Current Limit Response Timer: A capacitor connected to this pin defines the
period of time (tOCSLOW) in which an overcurrent event must last to signal a
fault condition and trip the circuit breaker. If no capacitor is connected, then
tOCSLOW defaults to 5µs.
Power-On Reset Timer: A capacitor connected between this pin and ground
sets the start-up delay (tSTART) and the power-on reset interval (tPOR). When
VCC rises above the UVLO threshold, the capacitor connected to CPOR
begins to charge. When the voltage at CPOR crosses 1.24V, the start-up
threshold (VSTART), a start cycle is initiated if ON is asserted while capacitor
CPOR is immediately discharged to ground. When the voltage at FB rises
above VFB, capacitor CPOR begins to charge again. When the voltage at
CPOR rises above the power-on reset delay threshold (VTH), the timer
resets by pulling CPOR to ground, and /POR is deasserted.
If CPOR = 0, then tSTART defaults to 20µs.
M0235-121903
2
January 2004
MIC2085/2086
Micrel
Pin Description (Cont.)
Pin Number
MIC2086
Pin Number
MIC2085
Pin Name
Pin Function
4
4
ON
ON Input: Active high. The ON pin, an input to a Schmitt-triggered compara-
tor used to enable/disable the controller, is compared to a VTH reference
with 100mV of hysteresis. Once a logic high is applied to the ON pin
(VON > 1.24V), a start-up sequence is initiated as the GATE pin starts
ramping up towards its final operating voltage. When the ON pin receives a
low logic signal (VON < 1.14V), the GATE pin is grounded and /FAULT is
high if VCC is above the UVLO threshold. ON must be low for at least 20µs
in order to initiate a start-up sequence. Additionally, toggling the ON pin
LOW to HIGH resets the circuit breaker.
5
6
5
/POR
Power-On Reset Output: Open drain N-Channel device, active low. This pin
remains asserted during start-up until a time period tPOR after the FB pin
voltage rises above the power-good threshold (VFB). The timing capacitor
CPOR determines tPOR. When an output undervoltage condition is detected
at the FB pin, /POR is asserted for a minimum of one timing cycle, tPOR. The
/POR pin has a weak pull-up to VCC.
N/A
PWRGD
Power-Good Output: Open drain N-Channel device, active high. When the
voltage at the FB pin is lower than 1.24V, the PWRGD output is held low.
When the voltage at the FB pin is higher than 1.24V, then PWRGD is
asserted. A pull-up resistor connected to this pin and to VCC will pull the
output up to VCC. The PWRGD pin has a weak pull-up to VCC.
7
8
6
7
/FAULT
FB
Circuit Breaker Fault Status Output: Open drain N-Channel device, active
low. The /FAULT pin is asserted when the circuit breaker trips due to an
overcurrent condition. Also, this pin indicates undervoltage lockout and
overvoltage fault conditions. The /FAULT pin has a weak pull-up to VCC.
Power-Good Threshold Input: This input is internally compared to a 1.24V
reference with 3mV of hysteresis. An external resistive divider may be used
to set the voltage at this pin. If this input momentarily goes below 1.24V,
then /POR is activated for one timing cycle, tPOR, indicating an output
undervoltage condition. The /POR signal de-asserts one timing cycle after
the FB pin exceeds the power-good threshold by 3mV. A 5µs filter on this pin
prevents glitches from inadvertently activating this signal.
9,10
11
8
9
GND
OV
Ground Connection: Tie to analog ground.
OV Input: When the voltage on OV exceeds its trip threshold, the GATE pin
is pulled low and the CRWBR timer starts. If OV remains above its threshold
long enough for CRWBR to reach its trip threshold, the circuit breaker is
tripped. Otherwise, the GATE pin begins to ramp up one POR timing cycle
after OV drops below its trip threshold.
12
13
14
15
10
11
12
NA
COMPOUT
COMP+
COMP-
DIS
Uncommitted Comparator’s Open Drain Output.
Comparator’s Non-Inverting Input.
Comparator’s Inverting Input.
Discharge Output: When the MIC2086 is turned off, a 550Ω internal resistor
at this output allows the discharging of any load capacitance to ground.
16
17
13
14
REF
Reference Output: 1.24V nominal. Tie a 0.1µF capacitor to ground to ensure
stability.
GATE
Gate Drive Output: Connects to the gate of an external N-Channel
MOSFET. An internal clamp ensures that no more than 13V is applied
between the GATE pin and the source of the external MOSFET. The GATE
pin is immediately brought low when either the circuit breaker trips or an
undervoltage lockout condition occurs.
January 2004
3
M0235-121903
MIC2085/2086
Micrel
Pin Description (Cont.)
Pin Number
MIC2086
Pin Number
MIC2085
Pin Name
Pin Function
18
15
SENSE
Circuit Breaker Sense Input: A resistor between this pin and VCC sets the
current limit threshold. Whenever the voltage across the sense resistor
exceeds the slow trip current limit threshold (VTRIPSLOW), the GATE voltage
is adjusted to ensure a constant load current. If VTRIPSLOW (48mV) is
exceeded for longer than time period tOCSLOW, then the circuit breaker is
tripped and the GATE pin is immediately pulled low. If the voltage across the
sense resistor exceeds the fast trip circuit breaker threshold, VTRIPFAST, at
any point due to fast, high amplitude power supply faults, then the GATE pin
is immediately brought low without delay. To disable the circuit breaker, the
SENSE and VCC pins can be tied together.
The default VTRIPFAST for either device is 95mV. Other fast trip thresholds
are available: 150mV, 200mV, or OFF(VTRIPFAST disabled). Please contact
factory for availability of other options.
19,20
16
VCC
Positive Supply Input: 2.3V to 16.5V. The GATE pin is held low by an
internal undervoltage lockout circuit until VCC exceeds a threshold of 2.18V.
If VCC exceeds 16.5V, an internal shunt regulator protects the chip from
VCC and SENSE pin voltages up to 33V.
M0235-121903
4
January 2004
MIC2085/2086
Micrel
Absolute Maximum Ratings(1)
Operating Ratings(2)
(All voltages are referred to GND)
Supply Voltage (V ) .................................... 2.3V to 16.5V
CC
Supply Voltage (V ) ..................................... –0.3V to 33V
Operating Temperature Range .................. –40°C to +85°C
CC
SENSE Pin..........................................–0.3V to V + 0.3V
Junction Temperature (T ) ........................................ 125°C
CC
J
GATE Pin ....................................................... –0.3V to 22V
ON, DIS, /POR, PWRGD, /FAULT,
Package Thermal Resistance R
θ(J-A)
16-pin QSOP .....................................................112°C/W
20-pin QSOP .......................................................91°C/W
COMP+, COMP–, COMPOUT ....................... –0.3V to 20V
CRWBR, FB, OV, REF..................................... –0.3V to 6V
Maximum Currents
Digital Output Pins .....................................................10mA
(/POR, /FAULT, PWRGD, COMPOUT)
DIS Pin .......................................................................30mA
ESD Rating:
Human Body Model................................................... 2kV
Machine Model ........................................................200V
Electrical Characteristics(3)
VCC = 5.0V, TA = 25°C unless otherwise noted. Bold indicates specifications over the full operating temperature range of –40°C to +85°C.
Symbol
VCC
Parameter
Condition
Min
2.3
Typ
Max
16.5
2.5
Units
V
Supply Voltage
ICC
Supply Current
1.6
mA
VUV
Undervoltage Lockout Threshold
VCC rising
VCC falling
2.05
1.85
2.18
2.0
2.28
2.10
V
V
VUVHYST
VFB
VFBHYST
VOV
UV Lockout Hysteresis
180
1.24
3
mV
V
FB (Power-Good) Threshold Voltage FB rising
FB Hysteresis
1.19
1.19
1.29
mV
mV
mV
OV Pin Threshold Voltage
OV pin rising
2.3V < VCC < 16.5V
1.24
5
1.29
15
∆VOV
OV Pin Threshold Voltage
Line Regulation
VOVHYST
IOV
OV Pin Hysteresis
OV Pin Current
3
mV
µA
V
0.2
VTH
POR Delay and Overcurrent (CFILTER) VCPOR, VCFILTER rising
Timer Threshold
1.19
–2.5
–30
445
1.24
1.29
ICPOR
ITIMER
Power-On Reset Timer Current
Timer on
Timer off
–2.0
5
–1.5
–15
µA
mA
Current Limit /Overcurrent
Timer Current (CFILTER)
Timer on
Timer off
–20
2.5
µA
mA
VCR
CRWBR Pin Threshold Voltage
2.3V < VCC < 16.5V
2.3V < VCC < 16.5V
470
4
495
mV
mV
∆VCR
CRWBR Pin Threshold Voltage
Line Regulation
15
ICR
CRWBR Pin Current
CRWBR On, VCRWBR = 0V
CRWBR On, VCRWBR = 2.1V
CRWBR Off, VCRWBR = 1.5V
–60
–45
–1.5
3.3
–30
–1.0
µA
mA
mA
VTRIP
Circuit Breaker Trip Voltage
(Current Limit Threshold)
VTRIP = VCC –VSENSE
2.3V ≤ VCC ≤ 16.5V
VTRIPSLOW
40
80
48
55
mV
VTRIPFAST x = J
x = K
95
150
200
110
mV
mV
mV
x = L
VGS
External Gate Drive
VGATE – VCC
VCC < 3V
4
8
12
9
V
V
V
5V < VCC < 9V
9V < VCC < 15.0V
11
4.5
13
13
21–VCC
January 2004
5
M0235-121903
MIC2085/2086
Micrel
Electrical Characteristics (Cont.)
Symbol
Parameter
Condition
Min
Typ
Max
Units
IGATE
GATE Pin Pull-up Current
Start cycle, VGATE = 0V
VCC =16.5V
VCC = 2.3V
–22
–20
–16
–14
–8
–8
µA
µA
IGATEOFF
GATE Pin Sink Current
/FAULT = 0, VGATE>1V
VCC = 16.5V
25
12
50
20
mA
mA
VCC = 2.3V
VON
ON Pin Threshold Voltage
ON rising
ON falling
1.19
1.09
1.24
1.14
1.29
1.19
V
V
VONHYST
ION
ON Pin Hysteresis
100
mV
µA
V
ON Pin Input Current
VON = VCC
0.5
VSTART
Undervoltage Start-up
Timer Threshold
VCPOR rising
1.19
1.24
1.29
VOL
/FAULT, /POR, PWRGD Output
Voltage
IOUT = 1.6mA
(PWRGD for MIC2086 only)
0.4
V
IPULLUP
Output Signal Pull-up Current
/FAULT, /POR, PWRGD, COMPOUT (PWRGD for MIC2086 only)
/FAULT, /POR, PWRGD = GND
–20
µA
VREF
Reference Output Voltage
Reference Line Regulation
Reference Load Regulation
Reference Short-Circuit Current
Comparator Offset Voltage
Comparator Hysteresis
ILOAD = 0mA; CREF = 0.1µF
2.3V < VCC < 16.5V
IOUT = 1mA
1.21
1.24
5
1.27
10
V
∆VLNR
∆VLDR
IRSC
mV
mV
mA
mV
mV
Ω
2.5
3.5
7.5
VREF= 0V
VCOS
VCHYST
RDIS
VCM = VREF
–5
5
VCM = VREF
3
Discharge Pin Resistance
ON pin toggles from HI to LOW
100
550
1000
AC Electrical Characteristics(4)
Symbol
Parameter
Condition
Min
Typ
Max
Units
tOCFAST
Fast Overcurrent Sense to GATE
Low Trip Time
VCC = 5V
VCC –VSENSE = 100mV
1
µs
CGATE = 10nF, See Figure 1
tOCSLOW
Slow Overcurrent Sense to Gate
Low Trip Time
VCC = 5V
VCC –VSENSE = 50mV
5
µs
CFILTER = 0, See Figure 1
tONDLY
tFBDLY
ON Delay Filter
FB Delay Filter
20
20
µs
µs
Notes:
1. Exceeding the absolute maximum rating may damage the device.
2. The device is not guaranteed to function outside its operating rating.
3. Specification for packaged product only.
4. Specification for packaged product only.
M0235-121903
6
January 2004
MIC2085/2086
Micrel
Timing Diagrams
Figure 1. Current Limit Response
1.24V
FB
tPOR
0
0
0
1.24V
CPOR
/POR
Figure 2. Power-On Reset Response
tONDLY
Arm Fast Comparator
Arm Slow Comparator
1.24V
ON
0
tSTART
tPOR
1.24V
CPOR
GATE
0
0
1.24V
FB
0
0
/POR
Figure 3. Power-On Start-Up Delay Timing
Figure 4. Foldback Current Limit Response
January 2004
7
M0235-121903
MIC2085/2086
Micrel
Typical Characteristics
Power-On Reset Timer Current
Supply Current
vs. Temperature
Power-On Reset Timer (Off) Current
vs. Temperature
vs. Temperature
4.0
2.6
10
9
8
3.5
2.4
VCC = 16.5V
VCC = 5V
2.2
3.0
VCC = 16.5V
7
VCC = 16.5V
2.5
6
5
VCC = 5V
2.0
2.0
VCC = 5V
4
3
2
1
1.5
1.0
1.8
VCC = 2.3V
VCC = 2.3V
VCC = 2.3V
1.6
1.4
0.5
0.0
0
-40 -20
0
20 40 60 80 100
-40 -20
0
20 40 60 80 100
-40 -20
0
20 40 60 80 100
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
Overcurrent Timer Current
vs. Temperature
Overcurrent Timer (Off) Current
Gate Pull-Up Current
vs. Temperature
vs. Temperature
34
30
26
22
18
14
10
5
30
25
20
15
10
5
4
VCC = 16.5V
3
VCC = 16.5V
VCC = 16.5V
2
VCC = 2.3V
VCC = 5V
VCC = 2.3V
VCC = 5V
VCC = 2.3V
VCC = 5V
1
0
0
-40 -20
0
20 40 60 80 100
-40 -20
0
20 40 60 80 100
-40 -20
0
20 40 60 80 100
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
External Gate Drive
Gate Pull-Up Current
External Gate Drive
vs. Temperature
vs. V
vs. V
CC
CC
22
20
18
16
14
12
10
8
25
20
15
10
5
16
14
12
10
8
VCC = 5V
VCC = 16.5V
6
6
4
4
VCC = 2.3V
2
2
0
0
0
2
4
6
8
10 12 14 16 18
(V)
CC
2
4
6
8
10 12 14 16 18
(V)
CC
-40 -20
0
20 40 60 80 100
TEMPERATURE (°C)
V
V
POR Delay/Overcurrent
Timer Threshold
Gate Sink Current
vs. Gate Voltage
Gate Sink Current
vs. Temperature
vs. Temperature
600
500
400
300
200
100
0
100
90
80
70
60
50
40
30
20
10
1.25
1.24
1.23
1.22
1.21
1.20
VCC = 16.5V
VCC = 16.5V
12VCC
VCC = 2.3V
VCC = 5V
VCC = 5V
5VCC
VCC = 2.3V
0
2
4
6
8
10 12 14
-40 -20
0
20 40 60 80 100
TEMPERATURE (°C)
-40 -20
0
20 40 60 80 100
(V)
TEMPERATURE (°C)
V
GATE
M0235-121903
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January 2004
MIC2085/2086
Micrel
Typical Characteristics
Current Limit Threshold
(Fast Trip)
Current Limit Threshold
(Slow Trip)
UVLO Threshold
vs. Temperature
vs. Temperature
vs. Temperature
120
2.5
2.4
2.3
2.2
2.1
2.0
1.9
1.8
1.7
55
53
51
49
47
45
115
110
UVLO+
105
VCC = 2.3V
VCC = 2.3V
100
95
VCC = 5V
90
VCC = 5V
VCC = 16.5V
UVLO–
VCC = 16.5V
85
80
-40 -20
0
20 40 60 80 100
-40 -20
0
20 40 60 80 100
-40 -20
0
20 40 60 80 100
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
ON Pin Threshold (Rising)
vs. Temperature
ON Pin Input Current
vs. Temperature
ON Pin Threshold (Falling)
vs. Temperature
1.30
1.25
1.20
1.15
1.20
1.15
1.10
1.05
40
35
30
25
20
15
10
5
VCC = 2.3V
VCC = 16.5V
VCC = 16.5V
VCC = 2.3V
VCC = 2.3V
VCC = 5V
VCC = 5V
VCC = 16.5V
VCC = 5V
0
-40 -20
0
20 40 60 80 100
-40 -20
0
20 40 60 80 100
-40 -20
0
20 40 60 80 100
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
FB (Power-Good) Threshold
vs. Temperature
1.30
Output Signal Pull-Up Current
Overvoltage Pin Threshold
vs. Temperature
vs. Temperature
1.30
26
VCC = 5V
VCC = 16.5V
VCC = 16.5V
VCC = 16.5V
VCC = 2.3V
22
18
14
10
1.25
1.20
1.15
1.25
1.20
1.15
VCC = 2.3V
VCC = 2.3V
VCC = 5V
-40 -20
0
20 40 60 80 100
-40 -20
0
20 40 60 80 100
-40 -20
0
20 40 60 80 100
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
Discharge Pin Resistance
vs. Temperature
Comparator Offset Voltage
vs. Temperature
1000
900
800
700
600
500
400
300
200
0.5
0.4
0.3
0.2
0.1
0.0
2.3V
5V
VCC = 5V
16.5V
VCC = 16.5V
VCC = 2.3V
-40 -20
0
20 40 60 80 100
-40 -20
0
20 40 60 80 100
TEMPERATURE (°C)
TEMPERATURE (°C)
January 2004
9
M0235-121903
MIC2085/2086
Micrel
Test Circuit
M0235-121903
10
January 2004
MIC2085/2086
Micrel
Functional Characteristics
12V Hot Insert Response
12V Turn On Response
VIN = 12V
RLOAD = 4.8Ω
VIN = 12V
CLOAD = 1000µF
RLOAD = 4.8Ω
CLOAD = 1000µF
TIME (20ms/div.)
TIME (20ms/div.)
Inrush Current Response
Power-Good Response
VIN = 12V
RLOAD = 4.8Ω
CLOAD = 1000µF
VIN = 12V
RLOAD = 3.4Ω
CLOAD = 5700µF
TIME (10ms/div.)
TIME (10ms/div.)
Turn Off — Normal Discharge
Turn Off — Crowbar Discharge
VIN = 12V
RDIS(External) = 0
RLOAD = 4.8Ω
CLOAD = 1000µF
SW2 = HIGH
VIN = 12V
RLOAD = 4.8Ω
CLOAD = 1000µF
SW2 = LOW
TIME (2.5ms/div.)
TIME (2.5ms/div.)
January 2004
11
M0235-121903
MIC2085/2086
Micrel
Functional Characteristics (continued)
Turn On Into Short Circuit
VIN = 12V
RLOAD = 0
CLOAD = 1000µF
TIME (10ms/div.)
M0235-121903
12
January 2004
MIC2085/2086
Micrel
Functional Block Diagram
MIC2086 Block Diagram
January 2004
13
M0235-121903
MIC2085/2086
Micrel
where V
in the electrical table and R
is the current limit slow trip threshold found
Functional Description
Hot Swap Insertion
TRIPSLOW
is the selected value that
SENSE
will set the desired current limit. There are two basic start-up
modes for the MIC2085/86: 1)Start-up dominated by load
capacitance and 2)start-up dominated by total gate capaci-
tance. The magnitude of the inrush current delivered to the
load will determine the dominant mode. If the inrush current
When circuit boards are inserted into live system backplanes
and supply voltages, high inrush currents can result due to
the charging of bulk capacitance that resides across the
supply pins of the circuit board. This inrush current, although
transient in nature, may be high enough to cause permanent
damage to on-board components or may cause the system’s
supply voltages to go out of regulation during the transient
period which may result in system failures. The MIC2085/86
acts as a controller for external N-Channel MOSFET devices
in which the gate drive is controlled to provide inrush current
limiting and output voltage slew rate control during hot plug
insertions.
is greater than the programmed current limit (I ), then load
LIM
capacitance is dominant. Otherwise, gate capacitance is
dominant. The expected inrush current may be calculated
using the following equation:
C
C
C
C
LOAD
LOAD
INRUSH I
×
15µA ×
GATE
(3)
GATE
GATE
where I
is the GATE pin pull-up current, C
is the
GATE
LOAD
Power Supply
load capacitance, and C
is the total GATE capacitance
GATE
VCC is the supply input to the MIC2085/86 controller with a
voltage range of 2.3V to 16.5V. The VCC input can withstand
transient spikes up to 33V. In order to help suppress tran-
sients and ensure stability of the supply voltage, a capacitor
of 1.0µF to 10µF from VCC to ground is recommended.
Alternatively,alowpassfilter,showninthetypicalapplication
circuit,canbeusedtoeliminatehighfrequencyoscillationsas
well as help suppress transient spikes.
(C
of the external MOSFET and any external capacitor
ISS
connected from the MIC2085/86 GATE pin to ground).
Load Capacitance Dominated Start-Up
In this case, the load capacitance, C
, is large enough to
LOAD
cause the inrush current to exceed the programmed current
limit but is less than the fast-trip threshold (or the fast-trip
threshold is disabled, ‘M’ option). During start-up under this
condition, the load current is regulated at the programmed
Start-Up Cycle
current limit value (I ) and held constant until the output
LIM
voltage rises to its final value. The output slew rate and
equivalent GATE voltage slew rate is computed by the
following equation:
When the voltage on the ON pin rises above its threshold of
1.24V, the MIC2085/86 first checks that its supply (V ) is
CC
above the UVLO threshold. If so, the device is enabled and
an internal 2µA current source begins charging capacitor
I
LIM
C
to 1.24V to initiate a start-up sequence (i.e., start-up
Output Voltage Slew Rate, dV
/dt =
POR
OUT
(4)
C
delay times out). Once the start-up delay (t
) elapses,
LOAD
START
CPOR is pulled immediately to ground and a 15µA current
sourcebeginschargingtheGATEoutputtodrivetheexternal
where I
quently, the value of C
the overcurrent response time, t
needed for the output to reach its final value. For example,
given a MOSFET with an input capacitance C = C
is the programmed current limit value. Conse-
LIM
must be selected to ensure that
FILTER
MOSFET that switches V to V
. The programmed start-
IN
OUT
, exceeds the time
OCSLOW
up delay is calculated using the following equation:
=
GATE
is set to 6A with a 12V
V
ISS
TH
t
= C
×
0.62 × C
(µF)
4700pF, C
is 2200µF, and I
START
POR
POR
(1)
LOAD
LIMIT
I
CPOR
input, then the load capacitance dominates as determined by
the calculated INRUSH > I . Therefore, the output voltage
slew rate determined from Equation 4 is:
where V , the POR delay threshold, is 1.24V, and I
the POR timer current, is 2µA. As the GATE voltage contin-
,
LIM
TH
CPOR
ues ramping toward its final value (V + V ) at a defined
CC
GS
6A
V
slewrate(See“LoadCapacitance”/“GateCapacitanceDomi-
nated Start-Up” sections), a second CPOR timing cycle
begins if: 1)/FAULT is high and 2)CFILTER is low (i.e., not
an overvoltage, undervoltage lockout, or overcurrent state).
Output Voltage Slew Rate, dVOUT/dt =
= 2.73
2200µF
ms
and the resulting t
needed to achieve a 12V output is
approximately 4.5ms. (See “Power-On Reset, Start-Up, and
OCSLOW
Thissecondtimingcycle, t
FB pin exceeds its threshold (V ) indicating that the output
, startswhenthevoltageatthe
Overcurrent Timer Delays” section to calculate t
.)
POR
OCSLOW
GATE Capacitance Dominated Start-Up
FB
voltage is valid. The time period t
is equivalent to t
POR
START
In this case, the value of the load capacitance relative to the
GATEcapacitanceissmallenoughsuchthat theloadcurrent
during start-up never exceeds the current limit threshold as
and sets the interval for the /POR to go Low-to-High after
“power is good” (See Figure 2 of “Timing Diagrams”). Active
current regulation is employed to limit the inrush current
transient response during start-up by regulating the load
current at the programmed current limit value (See “Current
Limiting and Dual-Level Circuit Breaker” section). The fol-
lowing equation is used to determine the nominal current
limit value:
determined by Equation 3. The minimum value of C
that
GATE
will ensure that the current limit is never exceeded is given by
the equation below:
IGATE
CGATE(min) =
× CLOAD
(5)
ILIM
V
48mV
TRIPSLOW
I
=
=
LIM
(2)
R
R
SENSE
SENSE
M0235-121903
14
January 2004
MIC2085/2086
Micrel
where C
is the summation of the MOSFET input
Output Undervoltage Detection
GATE
capacitance (C ) and the value of the external capacitor
The MIC2085/86 employ output undervoltage detection by
monitoring the output voltage through a resistive divider
connected at the FB pin. During turn on, while the voltage at
ISS
connected to the GATE pin of the MOSFET. Once C
is
GATE
determined, use the following equation to determine
the output slew rate for gate capacitance dominated start-up.
the FB pin is below the threshold (V ), the /POR pin is
FB
asserted low. Once the FB pin voltage crosses V , a 2µA
FB
IGATE
current source charges capacitor C
. Once the CPOR pin
dVOUT/dt (output) =
POR
(6)
CGATE
voltage reaches 1.24V, the time period t
elapses as the
POR
CPOR pin is pulled to ground and the /POR pin goes HIGH.
Table1depictstheoutputslewrateforvariousvaluesofC
.
GATE
If the voltage at FB drops below V for more than 10µs, the
FB
/POR pin resets for at least one timing cycle defined by t
(see Applications Information for an example).
POR
IGATE = 15µA
CGATE
0.001µF
0.01µF
0.1µF
1µF
dVOUT/dt
15V/ms
Input Overvoltage Protection
The MIC2085/86 monitors and detects overvoltage condi-
tions in the event of excessive supply transients at the input.
Whenever the overvoltage threshold (V ) is exceeded at
the OV pin, the GATE is pulled low and the output is shut off.
The GATE will begin ramping one POR timing cycle after the
OVpinvoltagedropsbelowitsthreshold.AnexternalCRWBR
circuit, as shown in the typical application diagram, provides
a time period that an overvoltage condition must exceed in
order to trip the circuit breaker. When the OV pin exceeds the
1.5V/ms
0.150V/ms
0.015V/ms
OV
Table 1. Output Slew Rate Selection for GATE
Capacitance Dominated Start-Up
Current Limiting and Dual-Level Circuit Breaker
Manyapplicationswillrequirethattheinrushandsteadystate
supply current be limited at a specific value in order to protect
critical components within the system. Connecting a sense
resistor between the VCC and SENSE pins sets the nominal
current limit value of the MIC2085/86 and the current limit is
calculated using Equation 2. However, the MIC2085/86 ex-
hibits foldback current limit response. The foldback feature
allows the nominal current limit threshold to vary from 24mV
up to 48mV as the FB pin voltage increases or decreases.
When FB is at 0V, the current limit threshold is 24mV and for
FB ≥ 0.6V, the current limit threshold is the nominal 48mV.
(See Figure 4 for Foldback Current Limit Response charac-
teristic).
overvoltagethreshold(V ),theCRWBRtimerbeginscharg-
ingtheCRWBRcapacitorinitiallywitha45µAcurrentsource.
OV
Once the voltage at CRWBR exceeds its threshold (V ) of
CR
0.47V, the CRWBR current immediately increases to 1.5mA
andthecircuitbreakeristripped, necessitatingadevicereset
by toggling the ON pin LOW to HIGH.
Power-On Reset, Start-Up, and Overcurrent Timer
Delays
The Power-On Reset delay, t
, is the time period for the
POR
/POR pin to go HIGH once the voltage at the FB pin exceeds
the power-good threshold (V ). A capacitor connected to
TH
, and t
CPOR sets the interval, t
is equivalent to the
POR
POR
start-up delay, t
(see Equation 1).
START
The MIC2085/86 also features a dual-level circuit breaker
triggeredvia48mVand95mVcurrentlimitthresholdssensed
across the VCC and SENSE pins. The first level of the circuit
breakerfunctionsasfollows.Oncethevoltagesensedacross
these two pins exceeds 48mV, the overcurrent timer, its
A capacitor connected to CFILTER is used to set the timer
which activates the circuit breaker during overcurrent condi-
tions. When the voltage across the sense resistor exceeds
the slow trip current limit threshold of 48mV, the overcurrent
timer begins to charge for a period, t
, determined by
duration set by capacitor C
, starts to ramp the voltage
OCSLOW
FILTER
C
. If no capacitor is used at CFILTER, then t
at CFILTER using a 2µA constant current source. If the
voltage at CFILTER reaches the overcurrent timer threshold
FILTER
OCSLOW
defaults to 5µs. If t
elapses, then the circuit breaker
OCSLOW
is activated and the GATE output is immediately pulled to
ground. The following equation is used to determine the
(V ) of 1.24V, then CFILTER immediately returns to ground
TH
as the circuit breaker trips and the GATE output is immedi-
ately shut down. For the second level, if the voltage sensed
across VCC and SENSE exceeds 95mV at any time, the
circuit breaker trips and the GATE shuts down immediately,
bypassing the overcurrent timer period. To disable current
limit and circuit breaker operation, tie the SENSE and VCC
pins together and the CFILTER pin to ground.
overcurrent timer period, t
.
OCSLOW
V
TH
t
= C
×
0.062 × C
(µF)
(7)
FILTER
OCSLOW
FILTER
I
TIMER
where V , the CFILTER timer threshold, is 1.24V and
TH
I
, the overcurrent timer current, is 20µA. Tables 2 and
TIMER
3 provide a quick reference for several timer calculations
using select standard value capacitors.
January 2004
15
M0235-121903
MIC2085/2086
Micrel
Using some basic algebra and simplifying Equation 8 to
isolate R5, yields:
C
t
= t
POR
POR START
0.01µF
0.02µF
0.033µF
0.05µF
0.1µF
6ms
12ms
18.5ms
30ms
V
OUT(Good)
R5 = R6
where V
–1
(8.1)
V
FB(MAX)
= 1.29V, V
= 11V, and R6 is
FB(MAX)
OUT(Good)
60ms
13.3kΩ. Substituting these values into Equation 8.1 now
yields R5 = 100.11kΩ. A standard 100kΩ ± 1% is selected.
Now, consider the 11.4V minimum output voltage, the lower
tolerance for R6 and higher tolerance for R5, 13.17kΩ and
101kΩ, respectively. With only 11.4V available, the voltage
0.33µF
200ms
Table 2. Selected Power-On Reset and
Start-Up Delays
sensed at the FB pin exceeds V
, thus the /POR and
FB(MAX)
C
t
PWRGD (MIC2086) signals will transition from LOW to
HIGH, indicating “power is good” given the worse case
tolerances of this example.
FILTER
OCSLOW
100µs
290µs
500µs
620µs
1.2ms
2.0ms
3.0ms
6.2ms
1800pF
4700pF
8200pF
0.010µF
0.020µF
0.033µF
0.050µF
0.1µF
Input Overvoltage Protection
The external CRWBR circuit shown in Figure 5 consists of
capacitor C4, resistor R7, NPN transistor Q2, and SCR Q3.
The capacitor establishes a time duration for an overvoltage
condition to last before the circuit breaker trips. The CRWBR
timer duration is approximated by the following equation:
C4 × V
(
)
CR
t
0.01× C4(µF)
(9)
0.33µF
20.75ms
OVCR
I
CR
Table 3. Selected Overcurrent Timer Delays
where V , the CRWBR pin threshold, is 0.47V and I , the
CR
CR
CRWBR pin current, is 45µA during the timer period (see the
CRWBR timer pin description for further description). A
similar design approach as the previous undervoltage detec-
tion example is recommended for the overvoltage protection
circuitry, resistors R2 and R3 in Figure 5. For input overvolt-
age protection, the first consideration is to establish the input
voltage level that indicates an overvoltage triggering a sys-
tem (output voltage) shut down. For this example, the input
value for which a 12V supply will signal an “output shut down”
is 13.2V (+10%). Similarly, from the previous example:
Applications Information
Output Undervoltage Detection
Foroutputundervoltagedetection,thefirstconsiderationisto
establish the output voltage level that indicates “power is
good.” For this example, the output value for which a 12V
supplywillsignal“good”is11V. Next,considerthetolerances
of the input supply and FB threshold (V ). For this example,
FB
the 12V supply varies ±5%, thus the resulting output voltage
may be as low as 11.4V and as high as 12.6V. Additionally,
the FB threshold has ±50mV tolerance and may be as low as
1.19V and as high as 1.29V. Thus, to determine the values of
theresistivedividernetwork(R5andR6)attheFBpin, shown
in Figure 5, use the following iterative design procedure.
1) Choose R3 to satisfy 100µA condition.
VOV(MIN)
1.19V
R3 ≥
≥
≥ 11.9kΩ
.
100µA
100µA
R3 is chosen as 13.7kΩ ±1%.
1) Choose R6 so as to limit the current through the
2) Thus, following the previous example and
substituting R2 and R3 for R5 and R6, respec-
tively, and 13.2V overvoltage for 11V output
“good”, the same formula yields R2 of 138.3kΩ.
The next highest standard 1% value is 140kΩ.
divider to approximately 100µA or less.
VFB(MAX)
1.29V
R6 ≥
≥
≥ 12.9kΩ
.
100µA
100µA
R6 is chosen as 13.3kΩ ± 1%.
Now, consider the 12.6V maximum input voltage (V +5%),
CC
thehighertoleranceforR3andlowertoleranceforR2,13.84k
and 138.60kΩ, respectively. With a 12.6V input, the voltage
2) Next, determine R5 using the output “good”
voltage of 11V and the following equation:
sensed at the OV pin is below V
, and the MIC2085/86
OV(MIN)
R5 +R6
(
)
will not indicate an overvoltage condition until V exceeds
CC
V
= V
FB
(8)
OUT(Good)
at least 13.2V.
R6
M0235-121903
16
January 2004
MIC2085/2086
Micrel
RSENSE
0.012Ω
2%
Q1
IRF7822
(SO-8)
VIN
12V
1
2
VOUT
12V@3A
3
4
CLOAD
220µF
C1
1µF
R1
100kΩ
R5
R2
140kΩ
1%
R4
16
15
10Ω
100kΩ
1%
VCC
SENSE
14
7
GATE
FB
C2
0.022µF
4
ON
MIC2085
R6
13.3kΩ
1%
5
/POR
/FAULT
CRWBR
9
Downstream
Signals
OV
6
1
Q2
2N4401
R3
13.7kΩ
1%
CPOR
GND
Q3
TCR22-4
C4
0.01µF
3
8
C3
0.05µF
C5
0.033µF
*R7
180Ω
Overvoltage (Input) = 13.3V
Undervoltage (Output) = 11.0V
POR/START-UP Delay = 30ms
*R7 needed when using a sensitive gate SCR.
Additional pins omitted for clarity.
Figure 5. Undervoltage/Overvoltage Circuit
January 2004
17
M0235-121903
MIC2085/2086
Micrel
PCB Connection Sense
of 1.24V and the MIC2085/86 initiates a start-up cycle. In
Figure 6, the connection sense consisting of a logic-level
discrete MOSFET and a few resistors allows for interrupt
controlfromtheprocessororothersignalcontrollertoshutoff
the output of the MIC2085/86. R4 keeps the GATE of Q2 at
ThereareseveralconfigurationoptionsfortheMIC2085/86’s
ON pin to detect if the PCB has been fully seated in the
backplane before initiating a start-up cycle. In the typical
applications circuit, the MIC2085/86 is mounted on the PCB
with a resistive divider network connected to the ON pin. R2
is connected to a short pin on the PCB edge connector. Until
the connectors mate, the ON pin is held low which keeps the
GATE output charge pump off. Once the connectors mate,
theresistornetworkispulleduptotheinputsupply,12Vinthis
V
until the connectors are fully mated. A logic LOW at the
IN
/ON_OFF signal turns Q2 off and allows the ON pin to pull up
above its threshold and initiate a start-up cycle. Applying a
logic HIGH at the /ON_OFF signal will turn Q2 on and short
the ON pin of the MIC2085/86 to ground which turns off the
GATE output charge pump.
example, and the ON pin voltage exceeds its threshold (V
)
ON
RSENSE
0.008Ω
Q1
Backplane PCB Edge
Long
Connector Connector
Pin
Si7860DP
2%
(PowerPAKTM SO-8)
VIN
12V
1
2
VOUT
12V@5A
3
4
C1
1µF
CLOAD
220µF
R6
R5
10Ω
Short
Pin
127kΩ
1%
16
15
R4
VCC
ON
SENSE
10kΩ
14
GATE
4
C2
0.01µF
R1
20kΩ
7
R2
20kΩ
FB
R3
100Ω
R7
16.2kΩ
1%
MIC2085
/ON_OFF
*Q2
5
1
/POR
Downstream
Signals
/FAULT
PCB Connection Sense
CPOR
GND
3
8
C2
0.05µF
GND
Long
Pin
Undervoltage (Output) = 11.4V
POR/START-UP DELAY = 30ms
*Q2 is TN0201T (SOT-23)
Additional pins omitted for clarity.
Figure 6. PCB Connection Sense with ON/OFF Control
M0235-121903
18
January 2004
MIC2085/2086
Micrel
Higher UVLO Setting
R1
R2
1+
×1.24V
. The GATE
to remain off until V exceeds
Once a PCB is inserted into a backplane (power supply), the
internal UVLO circuit of the MIC2085/86 holds the GATE
output charge pump off until V exceeds 2.18V. If VCC falls
below 2V, the UVLO circuit pulls the GATE output to ground
and clears the overvoltage and/or current limit faults. For a
higher UVLO threshold, the circuit in Figure 7 can be used to
delay the output MOSFET from switching on until the desired
input voltage is achieved. The circuit allows the charge pump
IN
drive output will be shut down when V falls below
IN
CC
R1
1+
×1.14V
. In the example circuit (Figure 7), the rising
R2
UVLO threshold is set at approximately 11V and the falling
UVLO threshold is established as 10.1V. The circuit consists
of an external resistor divider at the ON pin that keeps the
GATE output charge pump off until the voltage at the ON pin
exceeds its threshold (V ) and after the start-up timer
ON
elapses.
RSENSE
0.010Ω
Q1
IRF7822
2%
(SO-8)
VIN
12V
1
2
VOUT
12V@4A
CLOAD
3
4
C1
1µF
220µF
R4
R3
R1
392kΩ
1%
16
15
10Ω
127kΩ
1%
VCC
SENSE
14
7
GATE
FB
C2
0.01µF
4
ON
MIC2085
R5
16.2kΩ
1%
R2
49.9kΩ
1%
5
Downstream
Signal
/POR
CPOR
GND
3
8
C3
0.1µF
Undervoltage Lockout (Rising) = 11.0V
Undervoltage Lockout (Falling) = 10.1V
Undervoltage (Output) = 11.4V
POR/START-UP Delay = 60ms
Additional pins omitted for clarity.
Figure 7. Higher UVLO Setting
January 2004
19
M0235-121903
MIC2085/2086
Micrel
Fast Output Discharge for Capacitive Loads
(DIS pin output is low) once the ON pin is deasserted. Figure
8(a) illustrates the use of the discharge feature with an
optional resistor (R5) that can be used to provide added
resistance in the output discharge path. For an even faster
discharge response of capacitive loads, the configuration of
Figure 8(b) can be utilized to apply a crowbar to ground
through an external SCR (Q3) that is triggered when the DIS
pin goes low which turns on the PNP transistor (Q2). See the
different “Functional Characteristic” curves for a comparison
of the discharge response configurations.
Inmanyapplicationswhereaswitchcontrolleristurnedoffby
either removing the PCB from the backplane or the ON pin is
reset, capacitive loading will cause the output to retain
voltage unless a ‘bleed’ (low impedance) path is in place in
ordertodischargethecapacitance.TheMIC2086isequipped
with an internal MOSFET that allows the discharging of any
load capacitance to ground through a 550Ω path. The dis-
charge feature is configured by wiring the DIS pin to the
output (source) of the external MOSFET and becomes active
Figure 8. MIC2086 Fast Discharge of Capacitive Load
20
M0235-121903
January 2004
MIC2085/2086
Micrel
Auto-Retry Upon Overcurrent Faults
The circuit in Figure 10 distributes 12V from the backplane to
the MIC2182 DC/DC converter that steps down +12V to
+3.3V for local bias. The pass transistor, Q1, isolates the
MIC2182’s input capacitance during module plug-in and
allows the backplane to accommodate additional plug-in
moduleswithoutaffectingtheothermodulesonthebackplane.
The two control input signals are VBxEn_L (active LOW) and
a Local Power Enable (active HIGH). The MIC2085 in the
circuit of Figure 10 performs a number of functions. The gate
output of Q1 is enabled by the two bit input signal VBxEn_L,
LocalPowerEnable=[0,1].Also,theMIC2085limitsthedrain
current of Q1 to 7A, monitors VB_In for an overvoltage
conditiongreaterthan16V,andenablestheMIC2182DC/DC
converter downstream to supply a local voltage rail. The
uncommitted comparator is used to monitor VB_In for an
undervoltage condition of less than 10V, indicated by a logic
LOW at the comparator output (COMPOUT). COMPOUT
maybeusedtocontroladownstreamdevicesuchasanother
DC/DCconverter. Additionally, theMIC2085isconfiguredfor
auto-retry upon an overcurrent fault condition by placing a
diode(D1)betweenthe/FAULTandONpinsofthecontroller.
TheMIC2085/86canbeconfiguredforautomaticrestartafter
a fault condition. Placing a diode between the ON and
/FAULT pins, as shown in Figure 9, will enable the auto-
restart capability of the controller. When an application is
configured for auto-retry, the overcurrent timer should be set
to minimize the duty cycle of the overcurrent response to
prevent thermal runaway of the power MOSFET. See
“MOSFET Transient Thermal Issues” section for further
detail. A limited duty cycle is achieved when the overcurrent
timer duration (t
timer duration (t
equation:
) is much less than the start-up delay
) and is calculated using the following
OCSLOW
START
t
OCSLOW
Auto −Retry Duty Cycle =
×100%
(10)
t
START
An InfiniBand™ Application Circuit
The circuit in Figure 10 depicts a single 50W InfiniBand™
module using the MIC2085 controller. An InfiniBand™
backplane distributes bulk power to multiple plug-in modules
thatemployDC/DCconvertersforlocalsupplyrequirements.
RSENSE
0.012Ω
Q1
IRF7822
5%
(SO-8)
VIN
5V
1
2
VOUT
5V@2.5A
CLOAD
3
4
C1
1µF
220µF
R4
R3
R1
16
15
10Ω
34kΩ
47kΩ
1%
VCC
SENSE
14
7
GATE
FB
C2
0.022µF
R2
33kΩ
4
ON
ON SIGNAL
MIC2085
D1
1N914
R5
14.7kΩ
1%
/FAULT
6
/FAULT
OUTPUT
5
Downstream
Signal
/POR
CPOR
GND CFILTER
3
8
2
C3
0.02µF
C4
4700pF
Undervoltage (Output) = 4.27V
POR/START-UP Delay = 12ms
Circuit Breaker Response Time = 290µs
Auto-Retry Duty Cycle = 2.5%
Additional pins omitted for clarity.
Figure 9. Auto-Retry Configuration
January 2004
21
M0235-121903
MIC2085/2086
Micrel
InfiniBand™ Application
Figure 10. A 50W InfiniBand™ Application
Sense Resistor Selection
The next lowest standard value is 6.0mW. At the other set
of tolerance extremes for the output in question:
TheMIC2085andMIC2086usealow-valuesenseresistorto
measure the current flowing through the MOSFET switch
(and therefore the load). This sense resistor is nominally
56.7mV
I
=
= 9.45A
,
valued at 48mV/I
. To accommodate worst-case
LOAD(CONT,MAX)
LOAD(CONT)
6.0mΩ
almost 10A. Knowing this final datum, we can determine
the necessary wattage of the sense resistor, using P = I R,
tolerances for both the sense resistor (allow ±3% over time
and temperature for a resistor with ±1% initial tolerance) and
still supply the maximum required steady-state load current,
a slightly more detailed calculation must be used.
2
I will be I
, and R will be
where
(0.97)(R
LOAD(CONT, MAX)
).
These numbers yield the following:
SENSE(NOM)
The current limit threshold voltage (the “trip point”) for the
MIC2085/86 may be as low as 40mV, which would equate to
2
P
= (10A) (5.82mΩ)
= 0.582W.
In this example, a 1W sense resistor is sufficient.
MAX
a sense resistor value of 40mV/I
. Carrying the
LOAD(CONT)
MOSFET Selection
numbers through for the case where the value of the sense
resistor is 3% high yields:
Selecting the proper external MOSFET for use with the
MIC2085/86 involves three straightforward tasks:
40mV
38.8mV
R
=
=
SENSE(MAX)
• Choice of a MOSFET which meets minimum
(11)
I
1.03 I
(
)
(
LOAD(CONT)
)
LOAD(CONT)
voltage requirements.
Once the value of R
has been chosen in this manner,
• Selection of a device to handle the maximum
continuous current (steady-state thermal
issues).
SENSE
it is good practice to check the maximum I
which
LOAD(CONT)
the circuit may let through in the case of tolerance build-up in
the opposite direction. Here, the worst-case maximum cur-
rent is found using a 55mV trip voltage and a sense resistor
that is 3% low in value. The resulting equation is:
• Verify the selected part’s ability to withstand any
peak currents (transient thermal issues).
MOSFET Voltage Requirements
55mV
56.7mV
The first voltage requirement for the MOSFET is that the drain-
source breakdown voltage of the MOSFET must be greater
ILOAD(CONT,MAX)
=
=
(12)
RSENSE(NOM)
0.97 R
(
)
(
)
SENSE(NOM)
than V
. For instance, a 16V input may reasonably be
IN(MAX)
As an example, if an output must carry a continuous 6A
without nuisance trips occurring, Equation 11 yields:
expected to see high-frequency transients as high as 24V.
Therefore,thedrain-sourcebreakdownvoltageoftheMOSFET
must be at least 25V. For ample safety margin and standard
availability, the closest minimum value should be 30V.
38.8mV
R
=
= 6.5mΩ
.
SENSE(MAX)
6A
M0235-121903
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January 2004
MIC2085/2086
Micrel
The second breakdown voltage criterion that must be met is
a bit subtler than simple drain-source breakdown voltage. In
MIC2085/86 applications, the gate of the external MOSFET
is driven up to a maximum of 21V by the internal output
MOSFET. At the same time, if the output of the external
MOSFET (its source) is suddenly subjected to a short, the
gate-source voltage will go to (21V – 0V) = 21V. Since most
power MOSFETs generally have a maximum gate-source
breakdown of 20V or less, the use of a Zener clamp is
• Any knowledge you can get about the heat
sinking available to the device (e.g., can heat be
dissipated into the ground plane or power plane,
if using a surface-mount part? Is any airflow
available?).
The data sheet will almost always give a value of on resis-
tancegivenfortheMOSFETatagate-sourcevoltageof4.5V,
and another value at a gate-source voltage of 10V. As a first
approximation, addthetwovaluestogetheranddividebytwo
to get the on-resistance of the part with 8V of enhancement.
recommended in applications with V ≥ 8V. A Zener diode
CC
with 10V to 12V rating is recommended as shown in Figure
11. At the present time, most power MOSFETs with a 20V
gate-source voltage rating have a 30V drain-source break-
downratingorhigher.Asageneraltip,choosesurface-mount
deviceswithadrain-sourceratingof30Vormoreasastarting
point.
Call this value R . Since a heavily enhanced MOSFET acts
ON
as an ohmic (resistive) device, almost all that’s required to
2
determine steady-state power dissipation is to calculate I R.
The one addendum to this is that MOSFETs have a slight
increase in R
with increasing die temperature. A good
ON
approximation for this value is 0.5% increase in R per °C
ON
Finally, the external gate drive of the MIC2085/86 requires a
low-voltage logic level MOSFET when operating at voltages
lower than 3V. There are 2.5V logic level MOSFETs avail-
able. Please see Table 4, “MOSFET and Sense Resistor
Vendors” for suggested manufacturers.
riseinjunctiontemperatureabovethepointatwhichR was
initially specified by the manufacturer. For instance, if the
ON
selected MOSFET has a calculated R
of 10mΩ at a
ON
T = 25°C, and the actual junction temperature ends up
J
at 110°C, a good first cut at the operating value for R
ON
would be:
MOSFET Steady-State Thermal Issues
R
10mΩ[1 + (110 - 25)(0.005)] 14.3mΩ
TheselectionofaMOSFETtomeetthemaximumcontinuous
current is a fairly straightforward exercise. First, arm yourself
with the following data:
ON
The final step is to make sure that the heat sinking available
to the MOSFET is capable of dissipating at least as much
power (rated in °C/W) as that with which the MOSFET’s
performance was specified by the manufacturer. Here are a
few practical tips:
• The value of I
for the output in
LOAD(CONT, MAX.)
question (see “Sense Resistor Selection” ).
• The manufacturer’s data sheet for the candidate
1. The heat from a surface-mount device such as
an SO-8 MOSFET flows almost entirely out of
the drain leads. If the drain leads can be sol-
dered down to one square inch or more, the
copper will act as the heat sink for the part. This
copper must be on the same layer of the board
as the MOSFET drain.
MOSFET.
• The maximum ambient temperature in which the
device will be required to operate.
RSENSE
0.007Ω
Q1
IRF7822
(SO-8)
*D1
1N5240B
10V
2%
VIN
12V
1
2
VOUT
12V@5A
3
4
CLOAD
220µF
C1
1µF
R1
47kΩ
R4
100kΩ
1%
R3
16
15
10Ω
VCC
SENSE
14
7
GATE
FB
C2
0.01µF
4
ON
MIC2085
R5
13.3kΩ
1%
R2
33kΩ
6
5
/FAULT
/POR
Downstream
Signals
CPOR
GND
3
8
C3
0.1µF
Undervoltage (Output) = 11.0V
POR/START-UP Delay = 60ms
*Recommended for MOSFETs with gate-source
breakdown of 20V or less (IRF7822 V (MAX) = 12V)
GS
for catastrophic output short circuit protection.
Additional pins omitted for clarity.
Figure 11. Zener Clamped MOSFET GATE
January 2004
23
M0235-121903
MIC2085/2086
2. Airflow works. Even a few LFM (linear feet per
Micrel
Taking the simplest case first, we’ll assume that once a fault
event such as the one in question occurs, it will be a long time
– 10 minutes or more – before the fault is isolated and the
channel is reset. In such a case, we can approximate this as
a“singlepulse”event,thatistosay,there’snosignificantduty
cycle. Then, reading up from the X-axis at the point where
“SquareWavePulseDuration”isequalto0.1sec(=100msec),
minute) of air will cool a MOSFET down sub-
stantially. If you can, position the MOSFET(s)
near the inlet of a power supply’s fan, or the
outlet of a processor’s cooling fan.
3. The best test of a surface-mount MOSFET for
an application (assuming the above tips show it
to be a likely fit) is an empirical one. Check the
MOSFET's temperature in the actual layout of
the expected final circuit, at full operating
current. The use of a thermocouple on the drain
leads, or infrared pyrometer on the package, will
then give a reasonable idea of the device’s
junction temperature.
we see that the Z
event of this duration is only 8% of its continuous R
of this MOSFET to a highly infrequent
θ(J-A)
.
θ(J-A)
θ(J-A)
This particular part is specified as having an R
50°C/W for intervals of 10 seconds or less. Thus:
of
Assume T = 55°C maximum, 1 square inch of copper at the
A
drain leads, no airflow.
Recalling from our previous approximation hint, the part has
MOSFET Transient Thermal Issues
an R of (0.0335/2) = 17mΩ at 25°C.
ON
Having chosen a MOSFET that will withstand the imposed
Assume it has been carrying just about 2.5A for some time.
When performing this calculation, be sure to use the highest
) in which the
MOSFET will be operating as the starting temperature, and
2
voltage stresses, and the worse case continuous I R power
dissipation which it will see, it remains only to verify the
MOSFET’s ability to handle short-term overload power dissi-
pation without overheating. A MOSFET can handle a much
higher pulsed power without damage than its continuous
dissipationratingswouldimply.Thereasonforthisisthat,like
everything else, thermal devices (silicon die, lead frames,
etc.) have thermal inertia.
anticipated ambient temperature (T
A(MAX)
find the operating junction temperature increase (∆T ) from
J
thatpoint.Then,asshownnext,thefinaljunctiontemperature
isfoundbyaddingT
and∆T . Sincethisisnotaclosed-
A(MAX)
J
formequation, gettingacloseapproximationmaytakeoneor
two iterations, but it’s not a hard calculation to perform, and
tends to converge quickly.
In terms related directly to the specification and use of power
MOSFETs, this is known as “transient thermal impedance,”
Then the starting (steady-state)T is:
or Z
. Almost all power MOSFET data sheets give a
J
θ(J-A)
Transient Thermal Impedance Curve. For example, take the
followingcase:V =12V, t hasbeensetto100msec,
T
T
T
+ ∆T
J
J
A(MAX)
IN
OCSLOW
+ [R + (T
– T )(0.005/°C)(R )]
A ON
A(MAX)
ON
A(MAX)
I
is 2.5A, the slow-trip threshold is 48mV
LOAD(CONT. MAX)
2
x I x R
θ(J-A)
nominal, and the fast-trip threshold is 95mV. If the output is
accidentally connected to a 3Ω load, the output current from
T
T
55°C + [17mΩ + (55°C-25°C)(0.005)(17mΩ)]
J
J
the MOSFET will be regulated to 2.5A for 100ms (t
)
2
OCSLOW
x (2.5A) x (50°C/W)
before the part trips. During that time, the dissipation in the
MOSFET is given by:
(55°C + (0.122W)(50°C/W)
61.1°C
P = E x I
E
= [12V-(2.5A)(3Ω)] = 4.5V
MOSFET
Iterate the calculation once to see if this value is within a few
percent of the expected final value. For this iteration we will
start with T equal to the already calculated value of 61.1°C:
P
= (4.5V x 2.5A) = 11.25W for 100msec.
MOSFET
At first glance, it would appear that a really hefty MOSFET is
required to withstand this sort of fault condition. This is where
the transient thermal impedance curves become very useful.
Figure12shows thecurvefortheVishay(Siliconix)Si4410DY,
a commonly used SO-8 power MOSFET.
J
T
T
T + [17mΩ + (61.1°C-25°C)(0.005)(17mΩ)]
J
J
A
2
x (2.5A) x (50°C/W)
( 55°C + (0.125W)(50°C/W) 61.27°C
Normalized Thermal Transient Impedance, Junction-to-Ambient
2
1
Duty Cycle = 0.5
0.2
Notes:
0.1
P
DM
0.1
0.05
t
1
t
2
t
t
1
1. Duty Cycle, D =
2
0.02
2. Per Unit Base = R
= 50°C/W
thJA
(t)
3. T – T = P
JM
Z
A
DM thJA
Single Pulse
4. Surface Mounted
0.01
–4
10
–3
–2
10
–1
10
10
1
10
30
Square Wave Pulse Duration (sec)
Figure 12. Transient Thermal Impedance
M0235-121903
24
January 2004
MIC2085/2086
Micrel
So our original approximation of 61.1°C was very close to the
multi-layer layout for the R
, Power MOSFET, timer(s),
SENSE
correct value. We will use T = 61°C.
overvoltage and feedback network connections. The feed-
back and overvoltage resistive networks are selected for a
12V application (from Figure 5). Many hot swap applications
will require load currents of several amperes. Therefore, the
J
Finally, add (11.25W)(50°C/W)(0.08) = 45°C to the steady-
state T to get T
= 106°C. This is an accept-
J
J(TRANSIENT MAX.)
able maximum junction temperature for this part.
power (V and Return) trace widths (W) need to be wide
CC
PCB Layout Considerations
enough to allow the current to flow while the rise in tempera-
ture for a given copper plate (e.g., 1 oz. or 2 oz.) is kept to a
maximum of 10°C ~ 25°C. Also, these traces should be as
short as possible in order to minimize the IR drops between
the input and the load. For a starting point, there are many
trace width calculation tools available on the web such as the
following link:
Becauseofthelowvaluesofthesenseresistorsusedwiththe
MIC2085/86 controllers, special attention to the layout must
be used in order for the device’s circuit breaker function to
operate properly. Specifically, the use of a 4-wire Kelvin
connection to measure the voltage across R
is highly
SENSE
recommended. Kelvin sensing is simply a means of making
sure that any voltage drops in the power traces connecting to
theresistorsdoesnotgetpickedupbythetracesthemselves.
Additionally, these Kelvin connections should be isolated
from all other signal traces to avoid introducing noise onto
these sensitive nodes. Figure 13 illustrates a recommended,
http://www.aracnet.com/cgi-usr/gpatrick/trace.pl
Finally, plated-through vias are utilized to make circuit con-
nections to the power and ground planes. The trace connec-
tions with indicated vias should follow the example shown for
the GND pin connection in Figure 13.
Figure 13. Recommended PCB Layout for Sense Resistor, Power MOSFET,
and Feedback/Overvoltage Network
January 2004
25
M0235-121903
MIC2085/2086
Micrel
MOSFET and Sense Resistor Vendors
the MOSFET Gate of Figure 13 must be redirected when
using MOSFETs packaged in this style. Contact the device
manufacturer for package information.
Devicetypesandmanufacturercontactinformationforpower
MOSFETs and sense resistors is provided in Table 4. Some
of the recommended MOSFETs include a metal heat sink on
the bottom side of the package. The recommended trace for
MOSFET Vendors
Key MOSFET Type(s)
*Applications
Contact Information
Vishay (Siliconix)
Si4420DY (SO-8 package)
Si4442DY (SO-8 package)
Si3442DV (SO-8 package)
Si7860DP (PowerPAK™ SO-8)
Si7892DP (PowerPAK™ SO-8)
Si7884DP (PowerPAK™ SO-8)
SUB60N06-18 (TO-263)
I
I
I
I
I
I
I
I
≤ 10A
www.siliconix.com
OUT
OUT
OUT
OUT
OUT
OUT
OUT
OUT
= 10A-15A, V ≤ 5V (203) 452-5664
≤ 3A, V ≤ 5V
≤ 12A
≤ 15A
≤ 15A
CC
CC
≥ 20A, V ≥ 5V
≥ 20A, V ≥ 5V
CC
SUB70N04-10 (TO-263)
CC
International Rectifier
IRF7413 (SO-8 package)
IRF7457 (SO-8 package)
IRF7822 (SO-8 package)
IRLBA1304 (Super220™)
I
I
I
I
≤ 10A
≤ 10A
= 10A-15A, V ≤ 5V
www.irf.com
(310) 322-3331
OUT
OUT
OUT
OUT
CC
≥ 20A, V ≥ 5V
CC
Fairchild Semiconductor
FDS6680A (SO-8 package)
FDS6690A (SO-8 package)
I
I
≤ 10A
≤ 10A, V ≤ 5V
www.fairchildsemi.com
(207) 775-8100
OUT
OUT
CC
Philips
Hitachi
PH3230 (SOT669-LFPAK)
HAT2099H (LFPAK)
I
I
≥ 20A
≥ 20A
www.philips.com
OUT
OUT
www.halsp.hitachi.com
(408) 433-1990
* These devices are not limited to these conditions in many cases, but these conditions are provided as a helpful reference for customer applications.
Resistor Vendors
Sense Resistors
Contact Information
Vishay (Dale)
“WSL” Series
www.vishay.com/docswsl_30100.pdf
(203) 452-5664
IRC
“OARS” Series
“LR” Series
(second source to “WSL”)
www.irctt.com/pdf_files/OARS.pdf
www.irctt.com/pdf_files/LRC.pdf
(828) 264-8861
Table 4. MOSFET and Sense Resistor Vendors
M0235-121903
26
January 2004
MIC2085/2086
Micrel
Package Information
PIN 1
DIMENSIONS:
INCHES (MM)
0.157 (3.99)
0.150 (3.81)
0.009 (0.2286)
REF
0.012 (0.30)
0.008 (0.20)
0.025 (0.635)
BSC
45¡
0.0098 (0.249)
0.0075 (0.190)
0.0098 (0.249)
0.0040 (0.102)
8¡
0¡
0.196 (4.98)
0.189 (4.80)
0.050 (1.27)
0.016 (0.40)
SEATING 0.0688 (1.748)
PLANE
0.0532 (1.351)
0.244 (6.20)
0.229 (5.82)
Rev. 04
16-Pin QSOP (QS)
0.344 (8.74)
0.337 (8.56)
0.0575 REF
8¡
0¡
0.157 (3.99)
0.150 (3.81)
0.244 (6.20)
0.229 (5.82)
0.009 (0.229)
0.007 (0.178)
0.012 (0.305)
0.008 (0.203)
0.025 BSC
(0.635)
Rev. 04
Note:
1. All Dimensions are in Inches (mm) excluding mold flash.
2. Lead coplanarity should be 0.004" max.
3. Max misalignment between top and bottom.
0.068 (1.73)
0.053 (1.35)
0.010 (0.254)
0.004 (0.102)
4. The lead width, B to be determined at 0.0075" from lead tip.
7¡ BSC
0.050 (1.27)
0.016 (0.40)
20-Pin QSOP (QS)
January 2004
27
M0235-121903
MIC2085/2086
Micrel
MICREL, INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131 USA
TEL + 1 (408) 944-0800 FAX + 1 (408) 944-0970 WEB http://www.micrel.com
The information furnished by Micrel in this data sheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its use.
Micrel reserves the right to change circuitry and specifications at any time without notification to the customer.
Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product can
reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant into
the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A Purchaser’s
use or sale of Micrel Products for use in life support appliances, devices or systems is at Purchaser’s own risk and Purchaser agrees to fully indemnify
Micrel for any damages resulting from such use or sale.
© 2003 Micrel, Incorporated.
M0235-121903
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January 2004
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