XR76121ELMTR-F [EXAR]
PowerBloxTM 20A Synchronous Step-Down COT Regulators;
| 型号： | XR76121ELMTR-F |
| 厂家： | 
EXAR CORPORATION |
| 描述： | PowerBloxTM 20A Synchronous Step-Down COT Regulators |
| 文件： | 总18页 (文件大小：1225K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
XR76121
TM
PowerBlox 20A Synchronous
Step-Down COT Regulators
Description
FEATURES
The XR76121 is a synchronous step-down regulator combining the
controller, drivers, bootstrap diode and MOSFETs in a single package
for point-of-load supplies. The XR76121 has a load current rating
of 20A. A wide 5V to 22V input voltage range allows for single supply
operation from industry standard 5V, 12V and 19.6V rails.
ꢀ■
20A step-down regulator
4.5V to 5.5V low VIN operation
5V to 22V wide single input voltage
3V to 22V operation with external
5V bias
≥0.6V adjustable output voltage
Proprietary constant on-time control
No loop compensation required
Ceramic output capacitor stable
operation
With a proprietary emulated current mode constant on-time (COT)
control scheme, the XR76121 provides extremely fast line and
load transient response using ceramic output capacitors. They
require no loop compensation, simplifying circuit implementation
and reducing overall component count. The control loop also
provides 0.1% load and 0.1% line regulation and maintains constant
operating frequency. A selectable power saving mode, allows the user
to operate in discontinuous mode (DCM) at light current loads thereby
significantly increasing the converter efficiency.
ꢀ■
Programmable 70ns-1µs on-time
Constant 200kHz-1MHz frequency
Selectable CCM or CCM/DCM
operation
Power-good flag with low impedance when
power removed
ꢀ■
ꢀ■
ꢀ■
ꢀ■
Precision enable
Programmable soft-start
5mm x 6mm QFN package
A host of protection features, including overcurrent, over temperature,
overvoltage, short-circuit, open feedback detect and UVLO, helps
achieve safe operation under abnormal operating conditions.
APPLICATIONS
The XR76121 is available in a RoHS compliant, green/halogen-free
space-saving 5mm x 6mm QFN package.
ꢀ■
Servers
Distributed power architecture
Point-of-load converters
FPGA, DSP and processor supplies
ꢀ■
ꢀ■
ꢀ■
ꢀ■
Base stations, switches/routers
Typical Application
100
C
L1
C
V
600kHz
ENABLE
BST
OUT
98
96
94
92
90
88
86
84
82
80
78
76
74
72
70
EN
BST
SW
V
IN
VIN
R
FF
LIM
PVIN
PGOOD
VCC
SS
ILIM
R1
R2
POWER GOOD
R
FF
800kHz
XR76121
FB
FCCM
VSNS
PGND
C
IN
C
OUT
R
5.0V
3.3V
2.5V
1.8V
1.5V
1.2V
1.0V
V
OUT
R1
TON
AGND
R
ON
R2
C
C
SS
VCC
0.1
5.1
10.1
15.1
20.1
I
( )
A
OUT
Figure 1. Typical Application
Figure 2. Efficiency
REV1A
1/18
XR76121
Absolute Maximum Ratings
Operating Conditions
These are stress ratings only and functional operation
of the device at these ratings or any other above those
indicated in the operation sections of the specifications
below is not implied. Stresses beyond those listed under
absolute maximum ratings may cause permanent damage
to the device. Exposure to any absolute maximum rating
condition for extended periods may affect device reliability
and lifetime.
PV ......................................................................3V to 22V
IN
V
V
.....................................................................4.5V to 22V
IN
...................................................................4.5V to 5.5V
CC
SW, ILIM ..........................................................-1V to 22V(2)
PGOOD, TON, SS, EN.................................-0.3V to 5.5V(2)
Switching frequency ...................................200kHz-1MHz(3)
Junction temperature range (T ).................. -40°C to 125°C
PV , V
-0.3V to 25V
-0.3V to 6.0V
J
IN
IN.........................................................................
Package power dissipation max at 25°C.....................4.1W
V
CC ..................................................................................
Package thermal resistance θJA .................................... 24°C/W(4)
BST................................................................-0.3V to 31V(1)
BST-SW............................................................. -0.3V to 6V
SW, ILIM........................................................ -1V to 25V(1)(2)
NOTES:
1. No external voltage applied.
2. SW pin’s DC range is -1V, transient is -5V for less than 50ns.
3. Recommended.
4. Measured on Exar evaluation board.
All other pins.........................................-0.3V to V + 0.3V
CC
Storage temperature.................................... -65°C to 150°C
Junction temperature................................................. 150°C
Power dissipation ...................................... Internally limited
Lead temperature (soldering, 10 second).................. 300°C
ESD rating (HBM – human body model) ....................... 2kV
ESD rating (CDM – charged device model) .................. 1kV
ESD rating (MM – machine model) .............................200V
Electrical Characteristics
Specifications are for operating junction temperature of T = 25°C only; limits applying over the full operating junction
J
temperature range are denoted by a •. Typical values represent the most likely parametric norm at T = 25°C, and are
J
provided for reference purposes only. Unless otherwise indicated, V = 12V, SW = AGND = PGND = 0V, C
= 4.7uF.
Units
IN
VCC
Symbol
Parameter
Conditions
•
•
Min
Typ
Max
Power Supply Characteristics
V
V
regulating
5
12
5.0
0.8
22
5.5
1.3
CC
V
IN
Input voltage range
V
tied to V
4.5
CC
IN
I
I
V
V
supply current
Not switching, V = 12V, V = 0.7V
•
•
mA
mA
VIN
IN
IN
FB
Not switching, V = V = 5V,
CC
IN
quiescent current
0.8
1.3
VCC
CC
V
FB
= 0.7V
f = 600kHz, R = 49.9k,
ON
I
I
V
supply current
17
1
mA
μA
VIN
IN
V
= 0.58V
FB
Shutdown current
Enable = 0V, PV = V = 12V
OFF
IN
IN
Enable and Undervoltage Lock-Out UVLO
V
V
EN pin rising threshold
EN pin hysteresis
•
1.8
1.9
60
2.0
V
mV
V
IH_EN
EN_HYS
V
CC
V
CC
UVLO start threshold, rising edge
UVLO hysteresis
•
•
4.00
100
4.25
170
4.40
mV
REV1A
2/18
XR76121
Electrical Characteristics (Continued)
Specifications are for operating junction temperature of T = 25°C only; limits applying over the full operating junction
J
temperature range are denoted by a •. Typical values represent the most likely parametric norm at T = 25°C, and are
J
provided for reference purposes only. Unless otherwise indicated, V = 12V, SW = AGND = PGND = 0V, C
= 4.7uF.
Units
IN
VCC
Symbol
Parameter
Conditions
•
Min
Typ
Max
Reference Voltage
V
V
= 5V - 22V, V regulating
0.597
0.596
0.600
0.600
0.603
0.604
V
V
IN
CC
= 4.5V - 5.5V, V tied to V
IN
CC
IN
V
Reference voltage
REF
V
IN
V
IN
= 5V - 22V, V regulating
CC
•
0.594
0.600
0.606
V
= 4.5V - 5.5V, V tied to V
CC
IN
DC load regulation
DC line regulation
0.1
0.1
%
%
CCM operation, closed loop,
applies to any C
OUT
Programmable Constant On-Time
On-time 1
R
= 5.90kΩ, V = 12V
•
•
170
360
425
478
90
200
415
500
550
110
250
230
490
575
647
135
350
ns
kHz
ns
ON
IN
f corresponding to on-time 1
On-time 2
V
= 1.0V
OUT
R
= 16.2kΩ, V = 12V
IN
ON
f corresponding to on-time 2
On-time 3
V
= 3.3V
kHz
ns
OUT
R
= 3.01kΩ, V = 12V
•
•
ON
IN
Minimum off-time
ns
Diode Emulation Mode
Zero crossing threshold
Soft-Start
DC value measured during test
-2
mV
I
Charge current
•
•
-14
1
-10
3
-6
µA
SS_CHARGE
I
Discharge current
Fault present
mA
SS_DISCHARGE
VCC Linear Regulator
V
V
= 6V to 22V, I
= 0 to 30mA
•
•
4.8
4.6
5.0
4.8
5.2
IN
LOAD
V
CC
Output voltage
V
= 5V, R = 16.2kΩ,
IN
ON
f
= 678kHz
SW
Power Good Output
Power good threshold
-10
-7.5
1
-5
4
%
%
V
Power good hysteresis
Power good
Minimum I
= 1mA
0.2
0.5
SINK
Power good, unpowered
I
= 1mA
V
SINK
Power good assertion delay,
FB rising
2
ms
µs
Power good de-assertion delay,
FB falling
65
REV1A
3/18
XR76121
Electrical Characteristics (Continued)
Specifications are for operating junction temperature of T = 25°C only; limits applying over the full operating junction
J
temperature range are denoted by a •. Typical values represent the most likely parametric norm at T = 25°C, and are
J
provided for reference purposes only. Unless otherwise indicated, V = 12V, SW = AGND = PGND = 0V, C
= 4.7uF.
Units
IN
VCC
Symbol
Parameter
Conditions
•
Min
2.4
Typ
Max
Mode Control (FCCM)
FCCM mode logic high threshold
FCCM rising
FCCM falling
•
•
V
V
FCCM mode logic low threshold
Input leakage current
0.4
100
nA
Open Feedback/OVP Detect (VSNS)
OVP trip high threshold
VSNS rising. Specified as % of V
•
•
•
115
0.5
120
115
1
125
%
%
REF
OVP trip low threshold
VSNS falling. Specified as % of V
VSNS rising
REF
OVP comparator delay
3.5
3.5
µs
Delay to turn off power stage from an
overvoltage event
VSNS rising
•
µs
Protection: OCP, OTP, Short-Circuit
Hiccup timeout
110
16.2
0.4
0
ms
µA/mΩ
%/°C
mV
I
I
I
I
/R
14.5
18.0
LIM DS
current temperature coefficient
LIM
LIM
LIM
comparator offset
comparator offset
-4.7
-8.0
4.7
8.0
•
•
•
0
mV
Current limit blanking
Thermal shutdown threshold
Thermal hysteresis
100
138
15
ns
Rising temperature
°C
°C
Percent of V , short circuit is active.
REF
Feedback pin short-circuit threshold
50
20
60
70
%
After PGOOD asserts high.
Output Power Stage
High-side MOSFET R
I
I
= 2A
= 2A
7.7
3.1
10
mΩ
mΩ
A
DS(ON)
DS
DS
Low-side MOSFET R
3.5
DS(ON)
Maximum output current
REV1A
4/18
XR76121
Pin Configuration
SW
12
PVIN
13
11
PGND
BST 14
EN 15
SS 16
10 VCC
17
AGND
9 VIN
8 VSNS
1
2
3
4
5
6
7
Pin Functions
Pin Number
Pin Name
Type
Description
1
2
3
4
5
6
7
8
9
FB
A
I
Feedback input to feedback comparator.
FCCM
Forcing this pin logic level high forces CCM operation.
AGND
A
Signal ground for control circuitry. Connect to AGND pad with a short trace.
TON
ILIM
A
A
Constant on-time programming pin. Connect with a resistor to AGND.
Overcurrent protection programming. Connect with a resistor to SW.
Power-good output. Open drain to AGND. Low Z when IC unpowered.
Sense pin for output OVP and open FB.
PGOOD
VSNS
VIN
OD
A
A
Supply input for the regulator’s LDO. Normally connected to PV .
IN
The output of regulators LDO. It requires a 4.7µF V bypass capacitor. For operation
using a 5V rail, VCC should be tied to VIN.
CC
10
11
12
VCC
PGND
SW
A
PWR
PWR
Ground of the power stage. Internally connected to source of the low-side MOSFET.
Switch node. Internally it connects source of the high-side MOSFET to drain of the
low-side MOSFET.
13
14
15
PVIN
BST
EN
PWR
Input voltage for power stage. Internally connected to drain of the high-side MOSFET.
High-side driver supply pin. Connect a 0.1µF bootstrap capacitor between BST and SW.
Precision enable pin. Pulling this pin above 2V will enable the regulator.
A
I
Soft-start pin. Connect an external capacitor between SS and AGND to program the soft-
start rate based on the 10µA internal source current.
16
17
SS
A
A
AGND PAD
Signal ground for control circuitry.
NOTE:
A = Analog, I = Input, O = Output, OD = Open Drain, PWR = Power.
REV1A
5/18
XR76121
Typical Performance Characteristics
Efficiency and Package Thermal Derating
Unless otherwise specified: T
= 25°C, no airflow, f = 800kHz. Efficiency data includes inductor losses, schematic
AMBIENT
from the Application Information section of this datasheet.
100
98
100
98
96
94
92
90
88
86
84
82
80
78
76
74
72
70
600kHz
600kHz
L = 0.4µH (1.0V, 1.2V, 1.5V, 1.8V)
L = 1µH (2.5V, 3.3V, 5.0V)
96
94
92
90
88
86
84
82
80
78
76
74
72
70
5.0V CCM
3.3V CCM
2.5V CCM
1.8V CCM
1.5V CCM
1.2V CCM
1.0V CCM
3.3V DCM
2.5V DCM
1.8V DCM
1.5V DCM
1.2V DCM
1.0V DCM
3.3V CCM
2.5V CCM
1.8V CCM
1.5V CCM
1.2V CCM
1.0V CCM
5.0V DCM
3.3V DCM
2.5V DCM
1.8V DCM
1.5V DCM
1.2V DCM
1.0V DCM
1.0
I
10.0
10.0
1.0
I
0.1
0.1
(A)
(A)
OUT
OUT
Figure 3. Efficiency, V = 12V
Figure 4. Efficiency, V = 5V, L = 0.4µH
IN
IN
120
120
110
100
90
110
100
90
80
80
70
70
60
60
3.3V, CCM, 600kHz
1.8V, CCM, 800kHz
1.0V, CCM, 800kHz
5.0V, CCM, 600kHz
2.5V, CCM, 800kHz
50
50
40
40
1.0V, CCM, 800kHz
30
30
20
20
4
6
8
10
12
(A)
14
16
18
20
4
6
8
10
12
14
16
18
20
I
I
(A)
OUT
OUT
Figure 5. Maximum T
vs. I
,
Figure 6. Maximum T
vs. I
,
AMBIENT
OUT
AMBIENT
OUT
V
IN
= 12V, No Airflow
V
IN
= 5V, No Airflow
REV1A
6/18
XR76121
Typical Performance Characteristics (Continued)
All data taken at V = 12V, V
= 1.8V, f = 800kHz, T = 25°C, no airflow, forced CCM. (Unless otherwise specified).
IN
OUT
A
Schematic from the Applications Information section of this datasheet.
Figure 7. Steady State, I
= 20A
Figure 8. Steady State, DCM, I
= 0A
OUT
OUT
Figure 9. Power-Up, I
= 20A
Figure 10. Power-Up, I
= 0A
OUT
OUT
Figure 11. Load Transient, Forced CCM,
0A-10A-0A
Figure 12. Load Transient, DCM,
1.8A-11.8A-1.8A
REV1A
7/18
XR76121
Typical Performance Characteristics (Continued)
All data taken at V = 12V, V
= 1.8V, f = 800kHz, T = 25°C, no airflow, forced CCM. (Unless otherwise specified).
IN
OUT
A
Schematic from the Applications Information section of this datasheet.
Figure 13. Load Transient, DCM or Forced CCM,
10A-20A-10A
Figure 14. Enable Functionality,
= 12V
V
IN
Figure 15. Power-Up with Pre-Bias Voltage,
Figure 16. Short-Circuit Recovery,
= 20A
I
= 0A
I
OUT
OUT
REV1A
8/18
XR76121
Typical Performance Characteristics (Continued)
All data taken at V = 12V, V
= 1.8V, f = 800kHz, T = 25°C, no airflow, forced CCM. (Unless otherwise specified).
IN
OUT
A
Schematic from the Applications Information section of this datasheet.
1.850
1.840
1.830
1.820
1.810
1.800
1.790
1.780
1.770
1.760
1.750
1.850
1.840
1.830
1.820
1.810
1.800
1.790
1.780
1.770
1.760
1.750
0
2
4
6
8
10
12
14
16
18
20
4
6
8
10
12
14
(V)
16
18
20
22
V
I
(A)
IN
OUT
Figure 17. Load Regulation
Figure 18. Line Regulation
1,000
900
800
700
600
500
400
300
200
100
0
450
400
350
300
250
200
150
100
Calculated
Typical
Calculated
Typical
4
6
8
10
12
14
(V)
16
18
20
22
0
10
15
20
(kΩ)
25
30
35
5
V
R
IN
ON
Figure 19. t vs. R
Figure 20. t vs. V , R = 5.9kΩ
ON
ON
ON
IN
ON
900
900
800
700
600
500
400
300
200
100
0
800
700
600
500
400
300
200
100
0
0
2
4
6
8
10
(A)
12
14
16
18
20
4
6
8
10
12
(V)
14
16
18
20
22
I
V
OUT
IN
Figure 21. Frequency vs. I
Figure 22. Frequency vs. V
IN
OUT
REV1A
9/18
XR76121
Typical Performance Characteristics (Continued)
All data taken at V = 12V, V
= 1.8V, f = 800kHz, T = 25°C, no airflow, forced CCM. (Unless otherwise specified).
IN
OUT
A
Schematic from the Applications Information section of this datasheet.
35
30
610
605
600
595
590
25
20
15
10
5
Calculated worst case
Typical
0
1
1.2
1.4
1.6
1.8
2
2.2
-40
-20
0
20
40
60
80
100
120
R
(kΩ)
T (°C)
J
LIM
Figure 24. V
vs. Temperature
Figure 23. I
vs. R
REF
OCP
LIM
300
250
200
150
100
-40
-20
0
20
40
T (˚C)
60
80
100
120
J
Figure 25. t vs. Temperature, R = 5.9k
ON
ON
REV1A
10/18
XR76121
Functional Block Diagram
VIN
VCC
PGOOD
V
UVLO
CC
4.25V
V
XR76121
CC
LDO
V
CC
BST
10µA
THERMAL
PVIN
SHUTDOWN
SS
HS
0.6V
POWER GOOD
LEVEL
DRV
ENABLING SWITCHING
SHIFT
AND
SW
NON
FB
OVERLAP
COT CONTROL LOOP
V
0.555V
CC
LS
DRV
CONTROL
ZC
SW
OVP
VSNS
SCCOMP
DELAY
PGND
FB
0.36V
V
= 1.2 x V
HICCUP
LIM
H
L
REF
REF
PGND
V = 1.15 x V
I
EN
V
1.9V
CC
FCCM
PGND
TON EN
ILIM
AGND
Figure 26. Functional Block Diagram
REV1A
11/18
XR76121
Applications Information
Programming the On-Time
Detailed Operation
The on-time t is programmed via resistor R according
The XR76121 uses a synchronous step-down proprietary
ON
ON
to following equation:
emulated current-mode Constant On-Time (COT) control
scheme. The on-time, which is programmed via R
,
ON
IN
V
× [t
ꢀ
ON
(2.5 × 10-8)]
3.45 × 10-10
versus R , using the above equation,
is inversely proportional to V and maintains a nearly
IN
R
=
constant frequency. The emulated current-mode control
allows the use of ceramic output capacitors.
ON
A graph of t
ON
ON
Each switching cycle begins with the high-side (switching)
FET turning on for a preprogrammed time. At the end
of the on-time, the high-side FET is turned off and the
low-side (synchronous) FET is turned on for a preset
minimum time (250ns nominal). This parameter is termed
the minimum off-time. After the minimum off-time the voltage
at the feedback pin FB is compared to an internal voltage
is compared to typical test data in Figure 19. The graph
shows that calculated data matches typical test data
within 3%.
The t
corresponding to a particular set of operating
conditions can be calculated based on empirical data from:
ON
ramp at the feedback comparator. When V drops below
FB
V
OUT
the ramp voltage, the high-side FET is turned on and the
cycle repeats. This voltage ramp constitutes an emulated
current ramp and allows for the use of ceramic capacitors,
in addition to other capacitor types, for output filtering.
t
=
ON
V
x 1.06 x f x Eff.
IN
Where:
ꢀ■
f
is the desired switching frequency at
Enable
nominal I
.
OUT
The enable input provides precise control for startup.
Where bus voltage is well regulated, the enable input
can be derived from this voltage with a suitable resistor
divider. This ensures that XR76121 does not turn on
until bus voltage reaches the desired level. Therefore the
enable feature allows implementation of undervoltage
ꢀ■
Eff. is the converter efficiency corresponding to
nominal I
.
OUT
Substituting for t in the first equation we get:
ON
V
OUT
1.06 x f x Eff.
ꢀ
[(2.5 × 10-8) x V ]
IN
lockout for the bus voltage PV . Simple sequencing can
IN
R
=
ON
be implemented by using the PGOOD signal as the enable
input of a succeeding XR76121. Sequencing can also
be achieved by using an external signal to control the
enable pin.
(3.45 × 10-10)
Now R can be calculated in terms of operating
conditions V , V
above equation.
ON
, f and efficiency using the
OUT
IN
Selecting the Forced CCM Mode
A voltage higher than 2.4V at the FCCM pin forces the
XR76121 to operate in continuous conduction mode (CCM).
Note that discontinuous conduction mode (DCM) is always
on during soft-start. DCM will persist following soft-start
until a sufficient load is applied to transition the regulator
to CCM. Magnitude of the load required to transition
At V = 12V, f = 800kHz, I
efficiency numbers from Figure 3 we get the following R
= 20A and using the
IN
OUT
:
ON
VOUT (V)
Eff. (%)
f (kHz)
RON (kΩ)
5.0
3.3
2.5
1.8
1.5
1.2
1.0
0.95
0.93
0.91
0.89
0.87
0.84
0.81
600
600
800
800
800
800
800
23.12
15.30
8.52
6.04
5.02
4.01
3.35
to CCM is ΔI /2, where ΔI is peak-to-peak inductor
L
L
current ripple. Once the regulator transitions to CCM it will
continue operating in CCM regardless of the load magnitude.
Selecting the DCM/CCM Mode
The DCM will always be available if a voltage less
than 0.4V is applied to the FCCM pin. XR76121 will
operate in either DCM or CCM depending on the
load magnitude. At light loads DCM significantly increases
efficiency as seen in Figures 3 and 4. A preload of 10mA
is recommended for DCM operation. This helps improve
voltage regulation when external load is less then 10mA
and may reduce voltage ripple.
XR76121 R for common output voltages,
ON
V
= 12V, I
= 20A
IN
OUT
REV1A
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XR76121
Applications Information (Continued)
Overcurrent Protection (OCP)
If the load current exceeds the programmed overcurrent
Overvoltage Protection (OVP)
The output OVP function detects an overvoltage condition
on V of the regulator. OVP is achieved by comparing
threshold I
for four consecutive switching cycles,
OCP
OUT
the regulator enters the hiccup mode of operation.
In hiccup mode the MOSFET gates are turned off for 110ms
(hiccup timeout). Following the hiccup timeout a soft-start
is attempted. If OCP persists, hiccup timeout will repeat.
The regulator will remain in hiccup mode until load current
the voltage at VSNS pin to an OVP threshold voltage
set at 1.2 x V . When VSNS voltage exceeds the
OVP threshold, an internal overvoltage signal asserts after
1us (typical). This OVP signal latches off the high-side FET,
turns on the low-side FET and also asserts PGOOD low.
The low-side FET remains on to discharge the output
REF
is reduced below the programmed I
. In order to program
OCP
overcurrent protection use the following equation:
capacitor until VSNS voltage drops below 1.15 x V
.
REF
Then low-side FET turns off to prevent complete discharge
of V . The high-side and low-side FETs remain latched
OUT
(I + (0.5 × ∆IL))
OCP
off until V or EN is recycled. In order to use this feature,
IN
R
=
ꢁ
0.16kΩ
LIM
I
LIM
connect VSNS to V
with a resistor divider as shown in
OUT
R
DS
the application circuit. Use the same resistor divider value
that was used for programming V
.
OUT
Where:
Programming the Output Voltage
Use a voltage divider as shown in Figure 1 to program the
output voltage V
ꢀ■
R
LIM
is resistor value in kΩ for programming I
OCP
ꢀ■
ꢀ■
ꢀ■
I
is the overcurrent value to be programmed
OCP
.
OUT
ΔI is the peak-to-peak inductor current ripple
L
V
I
/R is the minimum value of the parameter
OUT
LIM DS
R1 = R2 x
ꢀ 1
0.6
specified in the tabulated data
ꢀ■
ꢀ■
I
/R = 14.5uA/mΩ
LIM DS
The recommended value for R2 is 2kΩ.
0.16kΩ accounts for OCP comparator offset
Programming the Soft-Start
The above equation is for worst-case analysis and
safeguards against premature OCP. Typical value of I
Place a capacitor C between the SS and AGND pins to
SS
,
OCP
program the soft-start. In order to program a soft-start time
for a given R , will be higher than that predicted by
LIM
of t , calculate the required capacitance C
from the
SS
SS
the above equation. Graph of calculated I
vs. R
is
OCP
LIM
following equation:
compared to typical I
in Figures 23.
OCP
Short-Circuit Protection (SCP)
10µA
0.6V
C
= t x
SS SS
If the output voltage drops below 60% of its programmed
value (i.e., FB drops below 0.36V), the regulator will enter
hiccup mode. Hiccup mode will persist until short-circuit
is removed. The SCP circuit becomes active at the end
of soft-start. Hiccup mode and short-circuit recovery
waveform is shown in Figure 16.
Pre-Bias Startup
XR76121 has the capability to startup into a pre-charged
output. Typical pre-bias startup waveforms are shown in
Figure 15.
Over Temperature Protection (OTP)
OTP triggers at a nominal controller temperature of 138°C.
The gates of the switching FET and the synchronous FET
are turned off. When controller temperature cools down to
123°C, soft-start is initiated and regular operation resumes.
Maximum Allowable Voltage Ripple at FB Pin
The steady-state voltage ripple at feedback pin FB
(V
,
) must not exceed 50mV in order for the regulator
FB RIPPLE
to function correctly. If V
,
is larger than 50mV then
FB RIPPLE
C
OUT
and/or L should be increased as necessary in order to
keep the V
,
below 50mV.
FB RIPPLE
REV1A
13/18
XR76121
Applications Information (Continued)
Thermal Design
Feed-Forward Capacitor (C
)
FF
Proper thermal design is critical in controlling device
temperatures and in achieving robust designs. There are
a number of factors that affect the thermal performance.
One key factor is the temperature rise of the devices in
the package, which is a function of the thermal resistances
of the devices inside the package and the power
being dissipated.
The feed-forward capacitor C is used to set the necessary
phase margin when using ceramic output capacitors.
FF
Calculate C from the following equation:
FF
1
C
=
FF
2 x
π
x R1 x 5 x f
LC
The thermal resistance of the XR76121 is specified in
the Operating Ratings section of this datasheet. The θJA
thermal resistance specification is based on the XR76121
evaluation board operating without forced airflow. Since the
actual board design in the final application will be different,
the thermal resistances in the final design may be different
from those specified.
Where f , the output filter double-pole frequency is
calculated from:
LC
1
f
=
LC
2 x
π
x √ L x C
OUT
You must use manufacturer’s DC derating curves to
determine the effective capacitance corresponding to V
A load step test (and/or a loop transient response test)
The package thermal derating curves for the XR76121 are
shown in Figures 5 and 6. These correspond to input voltage
of 12V and 5V, respectively. The package thermal derating
curves for the XR76121 are shown in Figures 9 and 10.
.
OUT
should be performed and if necessary C can be adjusted
FF
in order to get a critically damped transient load response.
In applications where output voltage ripple is less than
about 3mV, such as when a large number of ceramic
C
are paralleled, it is necessary to use ripple injection
OUT
from across the inductor. The circuit and corresponding
calculations are explained in the Exar design note.
Feed-Forward Resistor (R
)
FF
R
FF
is required when C is used. R , in conjunction with
FF FF
C
, functions similar to a high frequency pole and adds
FF
gain margin to the frequency response. Calculate R from:
FF
1
R
=
FF
2 x
π
x f x C
FF
Where f is the switching frequency.
If R is greater than 0.1 x R1, then instead of C /R , use
FF
FF FF
ripple injection circuit as described in Exar design note.
REV1A
14/18
XR76121
Applications Information
V
= 12V
IN
REN2 3.83k
REN1 10k
V
IN
4 x 22µF/25V/X6T/1206
2 x 0.1µF
C
SS
47nF
CBST
0.1µF
FB
1
2
3
4
5
6
7
FB
V
CC
800kHz, 1.8V, 0-20A
OUT
FCCM
AGND
AGND
TON
L1, IHLP-5050FD-01
0.4µH at 44A, 0.9m Ohm
V
SW
12 SW
R
6.19k
ON
4 x 0.1µF
5 x 100µF/6.3V/X6T/1206
XR76121
R
1.82k
ILIM
SW
LIM
PGOOD
C
FF
470pF
RPGOOD 10k
V
CC
R1
4.02k
R
FF
V
OUT
0.4k
V
CC
RSENS1
4.02k
FB
V
IN
R2
2k
RSENS2
2k
C
C
IN
0.1µF
VCC
4.7µF
Figure 27. Application Circuit Schematic
REV1A
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XR76121
Package Description
All dimensions are in mm and angles in degrees.
Figure 28. Package Description (1 of 2)
REV1A
16/18
XR76121
Package Description (Continued)
All dimensions are in mm and angles in degrees.
Figure 28. Package Description (2 of 2)
REV1A
17/18
XR76121
Ordering Information
Operating
Temperature Range
Environmental
Rating
Packaging
Quantity
Part Number
Package
Marking
XR76121EL-F
Bulk
XR76121EL
YYWWF
RoHS compliant
and Green(1)
XR76121ELMTR-F
XR76121ELTR-F
XR76121EVB
-40°C ≤ T ≤ 125°C
5mm x 6mm QFN
250/tape and reel
3K/tape and reel
J
XXXXXXXX(2)
XR76121 evaluation board
NOTE:
1. Visit www.exar.com for more information.
2. YY = Year, WW = Work Week, F = Halogen Free, XXXXXXXX = Lot Number.
Revision History
Revision
1A
Date
Description
July 2016
Initial Release
www.exar.com
48760 Kato Road
Fremont, CA 94538
USA
Tel.: +1 (510) 668-7000
Fax: +1 (510) 668-7001
Email: powertechsupport@exar.com
Exar Corporation reserves the right to make changes to the products contained in this publication in order to improve design, performance or reliability. Exar Corporation conveys
no license under any patent or other right and makes no representation that the circuits are free of patent infringement. While the information in this publication has been
carefully checked, no responsibility, however, is assumed for inaccuracies.
Exar Corporation does not recommend the use of any of its products in life support applications where the failure or malfunction of the product can reasonably be expected
to cause failure of the life support system or to significantly affect its safety or effectiveness. Products are not authorized for use in such applications unless Exar Corporation
receives, in writing, assurances to its satisfaction that: (a) the risk of injury or damage has been minimized; (b) the user assumes all such risks; (c) potential liability of
Exar Corporation is adequately protected under the circumstances.
Reproduction, in part or whole, without the prior written consent of Exar Corporation is prohibited. Exar, XR and the XR logo are registered trademarks of Exar Corporation.
All other trademarks are the property of their respective owners.
©2016 Exar Corporation
XR76121_DS_070116
REV1A
18/18
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