SC4524AEVB [SEMTECH]
28V 2A Step-Down Switching Regulator; 28V 2A降压型开关稳压器
| 型号： | SC4524AEVB |
| 厂家： | 
SEMTECH CORPORATION |
| 描述： | 28V 2A Step-Down Switching Regulator |
| 文件： | 总18页 (文件大小：741K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
SC4524A
28V 2A Step-Down Switching Regulator
POWER MANAGEMENT
Features
Description
Wide input range: 3V to 28V
2A Output Current
200kHz to 2MHz Programmable Frequency
Precision ꢀV Feedback Voltage
Peak Current-Mode Control
Cycle-by-Cycle Current Limiting
Hiccup Overload Protection with Frequency Foldback
Soft-Start and Enable
Thermal Shutdown
The SC4524A is a constant frequency peak current-mode
step-down switching regulator capable of producing 2A
output current from an input ranging from 3V to 28V. The
switching frequency of the SC4524A is programmable
up to 2MHz, allowing the use of small inductors and
ceramic capacitors for miniaturization, and high input/
output conversion ratio. The SC4524A is suitable for
next generation XDSL modems, high-definition TVs and
various point of load applications.
Thermally Enhanced 8-pin SOIC Package
Fully RoHS and WEEE compliant
Peak current-mode PWM control employed in the
SC4524Aachievesfasttransientresponsewithsimpleloop
compensation. Cycle-by-cycle current limiting and hiccup
overload protection reduces power dissipation during
output overload. Soft-start function reduces input start-
up current and prevents the output from overshooting
during power-up.
Applications
XDSL and Cable Modems
Set Top Boxes
Point of Load Applications
CPE Equipment
DSP Power Supplies
LCD and Plasma TVs
The SC4524A is available in SOIC-8 EDP package.
Typical Application Circuit
Efficiency
D1
90
85
80
10V – 28V
V
IN
1N4148
C4
2.2mF
C1
0.1mF
BST
SW
IN
L1
OUT
5V/2A
75
70
65
60
55
50
45
40
VIN = 12V
VIN =24V
8.2mH
R4
SC4524A
SS/EN
42.2k
FB
GND
COMP
ROSC
D2
20BQ030
R6
10.5k
C2
22mF
C7
R7
28.0k
R5
18.2k
10nF
C8
10pF
C5
2.2nF
0
0.5
1
1.5
2
L1: Coiltronics DR73-8R2
C2: Murata GRM31CR60J226K
C4: Murata GRM31CR71H225K
Load Current (A)
Figure 1. 1MHz 10V-28V to 5V/2A Step-down Converter
November 2, 2007
ꢀ
SC4524A
Pin Configuration
Ordering Information
Device
Package
SC4524ASETRT(ꢀ)(2)
SC4524AEVB
SOIC-8 EDP
SW
IN
1
2
3
4
8
7
6
5
BST
Evaluation Board
FB
Notes:
9
ROSC
GND
COMP
SS/EN
(ꢀ) Available in tape and reel only. A reel contains 2,500 devices.
(2) Available in lead-free package only. Device is fully WEEE and RoHS
compliant.
(8 - Pin SOIC - EDP)
Marking Information
yyww=Date code (Example: 0752)
xxxxx=Semtech Lot No. (Example: E90ꢀ0)
2
SC4524A
Absolute Maximum Ratings
Thermal Information
Junction to Ambient (ꢀ) ……………………………… 36°C/W
Junction to Case (ꢀ) ………………………………… 5.5°C/W
Maximum Junction Temperature……………………… ꢀ50°C
Storage Temperature ………………………… -65 to +ꢀ50°C
VIN Supply Voltage ……………………………… -0.3 to 32V
BST Voltage ……………………………………………… 42V
BST Voltage above SW …………………………………… 36V
SS Voltage ……………………………………………-0.3 to 3V
Lead Temperature (Soldering) ꢀ0 sec ………………… 300°C
FB Voltage …………………………………………… -0.3 to VIN
SW Voltage ………………………………………… -0.6 to VIN
SW Transient Spikes (ꢀ0ns Duration)……… -2.5V to VIN +ꢀ.5V
Peak IR Reflow Temperature …………………………. 260°C
ESD Protection Level(2) ………………………………… 2000V
Recommended Operating Conditions
Input Voltage Range ……………………………… 3V to 28V
Maximum Output Current ……………………………… 2A
Exceeding the above specifications may result in permanent damage to the device or device malfunction. Operation outside of the parameters specified in the
Electrical Characteristics section is not recommended.
NOTES-
(ꢀ) Calculated from package in still air, mounted to 3”x 4.5”, 4 layer FR4 PCB with thermal vias under the exposed pad per JESD5ꢀ standards.
(2) Tested according to JEDEC standard JESD22-Aꢀꢀ4-B.
Electrical Characteristics
Unless otherwise noted, VIN = ꢀ2V, VBST = ꢀ5V, VSS = 2.2V, -40°C < TA = TJ < ꢀ25°C, ROSC = ꢀ2.ꢀkΩ.
Parameter
Conditions
Min
Typ
Max
Units
Input Supply
Input Voltage Range
3
28
V
VIN Start Voltage
VIN Rising
2.70
2.82
225
2
2.95
V
VIN Start Hysteresis
mV
mA
µA
VIN Quiescent Current
VIN Quiescent Current in Shutdown
Error Amplifier
VCOMP = 0 (Not Switching)
VSS/EN = 0, VIN = ꢀ2V
2.6
50
40
Feedback Voltage
0.980
ꢀ.000
0.005
-ꢀ70
280
60
ꢀ.020
-340
V
%/V
nA
µΩ-ꢀ
dB
Feedback Voltage Line Regulation
FB Pin Input Bias Current
Error Amplifier Transconductance
Error Amplifier Open-loop Gain
COMP Pin to Switch Current Gain
COMP Maximum Voltage
COMP Source Current
COMP Sink Current
VIN = 3V to 28V
VFB = ꢀV, VCOMP = 0.8V
8
A/V
V
2.4
VFB = 0.9V
ꢀ7
VFB = 0.8V, VCOMP = 0.8V
VFB = ꢀ.2V, VCOMP = 0.8V
µA
25
Internal Power Switch
Switch Current Limit
(Note ꢀ)
2.6
3.3
4.3
A
Switch Saturation Voltage
ISW = -2.6A
250
400
mV
3
SC4524A
Electrical Characteristics (Cont.)
Unless otherwise noted, VIN = ꢀ2V, VBST = ꢀ5V, VSS = 2.2V, -40°C < TA = TJ < ꢀ25°C, ROSC = ꢀ2.ꢀkΩ.
Parameter
Conditions
Min
Typ
ꢀ35
ꢀ00
Max
Units
ns
Minimum Switch On-time
Minimum Switch Off-time
Switch Leakage Current
Minimum Bootstrap Voltage
BST Pin Current
ꢀ50
ꢀ0
ns
µA
V
ISW = -2.6A
ISW = -2.6A
ꢀ.8
60
2.3
95
mA
Oscillator
ROSC = ꢀ2.ꢀkΩ
ROSC = 93.ꢀkΩ
ꢀ.04
240
ꢀꢀ0
50
ꢀ.3
300
230
ꢀ00
ꢀ.56
360
350
ꢀ70
MHz
kHz
Switching Frequency
Foldback Frequency
ROSC = ꢀ2.ꢀkΩ, VFB = 0
ROSC = 93.ꢀkΩ, VFB = 0
kHz
Soft Start and Overload Protection
SS/EN Shutdown Threshold
0.2
ꢀ.0
0.3
ꢀ.ꢀ3
ꢀ.7
0.4
ꢀ.3
V
V
SS/EN Switching Threshold
VFB = 0 V
VSS/EN = 0 V
VSS/EN = ꢀ.5 V
Soft-start Charging Current
µA
ꢀ.2
2.0
2.8
Soft-start Discharging Current
Hiccup Arming SS/EN Voltage
Hiccup SS/EN Overload Threshold
Hiccup Retry SS/EN Voltage
ꢀ.5
µA
V
2.ꢀ5
ꢀ.9
VSS/EN Rising
VSS/EN Falling
VSS/EN Falling
V
0.6
ꢀ.0
ꢀ.2
V
Over Temperature Protection
Thermal Shutdown Temperature
Thermal Shutdown Hysteresis
ꢀ65
ꢀ0
°C
°C
Note ꢀ: Switch current limit does not vary with duty cycle.
4
SC4524A
Pin Descriptions
SO-8
Pin Name
Pin Function
Emitter of the internal NPN power transistor. Connect this pin to the inductor, the freewheeling diode and the
bootstrap capacitor.
ꢀ
SW
Power supply to the regulator. It is also the collector of the internal NPN power transistor. It must be closely by-
passed to the ground plane.
2
IN
3
4
ROSC
GND
An external resistor from this pin to ground sets the oscillator frequency.
Ground pin
Soft-start and regulator enable pin. A capacitor from this pin to ground provides soft-start and overload hiccup
functions. Hiccup can be disabled by overcoming the internal soft-start discharging current with an external pull-
up resistor connected between the SS/EN and the IN pins. Pulling the SS/EN pin below 0.2V completely shuts off
the regulator to low current state.
5
SS/EN
The output of the internal error amplifier. The voltage at this pin controls the peak switch current. A RC compensa-
tion network at this pin stabilizes the regulator.
6
7
8
9
COMP
FB
The inverting input of the error amplifier. If VFB falls below 0.8V, then the switching frequency will be reduced to
improve short-circuit robustness (see Applications Information for details).
Supply pin to the power transistor driver. Tie to an external diode-capacitor bootstrap circuit to generate drive
voltage higher than VIN in order to fully enhance the internal NPN power transistor.
BST
The exposed pad serves as a thermal contact to the circuit board. It is to be soldered to the ground plane of the
PC board.
Exposed Pad
5
SC4524A
Block Diagram
IN
2
SLOPE
COMP
COMP
6
+
+
S
ISEN
-
+
6.1mW
FB
7
-
+
+
+
ILIM
OC
20mV
EA
-
BST
8
V1
+
S
R
PWM
-
Q
POWER
TRANSISTOR
FREQUENCY
FOLDBACK
CLK
ROSC
3
OSCILLATOR
1.23V
+
1
A1
SW
-
R
R
SS/EN
5
SOFT-START
AND
OVERLOAD
HICCUP
GND
4
1V
1.9V
REFERENCE
& THERMAL
SHUTDOWN
FAULT
CONTROL
Figure 2. SC4524A Block Diagram
-
1.9V
B4
+
S
R
I
C
2mA
Q
OVERLOAD
B1
SS/EN
B2
1V/2.15V
FAULT
S
R
OC
I
_
Q
D
3.5mA
PWM
B3
Figure 3. Soft-start and Overload Hiccup Control Circuit
6
SC4524A
Typical Characteristics
Feedback Voltage vs Temperature
Efficiency
Efficiency
90
85
80
75
70
65
60
55
50
45
40
90
1.02
1.01
1.00
0.99
0.98
0.97
VO=5V
VIN =12V
85
VO=3.3V
VO=5V
80
VO=2.5V
VO=3.3V
VO=2.5V
75
70
VO=1.5V
65
60
55
1MHz, VIN =12V
1MHz, VIN =24V
D2 =20BQ030
50
D2=20BQ030
45
40
0
0.5
1
1.5
2
-50 -25
0
25
50 75 100 125
0
0.5
1
1.5
2
Temperature (oC)
Load Current (A)
Load Current (A)
Frequency Setting Resistor
vs Frequency
Foldback Frequency vs VFB
Frequency vs Temperature
1000
100
10
1.25
1
1.2
1.1
1.0
0.9
0.8
VIN =12V
ROSC=93.1k
ROSC=93.1k
0.75
0.5
0.25
0
ROSC=12.1k
TA =25oC
ROSC=12.1k
1
0
0.5
1
1.5
2
2.5
0.0
0.2
0.4
0.6
0.8
1.0
-50 -25
0
25 50
75 100 125
Temperature (oC)
VFB (V)
Frequency (MHz)
Switch Saturation Voltage
vs Switch Current
Switch Current Limit vs Temperature
BST Pin Current vs Switch Current
100.0
75.0
50.0
25.0
0.0
4.5
300
250
200
150
100
50
VIN =12V
VBST =15V
4.0
3.5
3.0
2.5
-40oC
-40oC
125oC
125oC
25oC
-50 -25
0
25 50
75 100 125
0
0.5
1
1.5
2
2.5
3
0.0
0.5
1.0
1.5
2.0
2.5
Temperature (oC)
Switch Current (A)
Switch Current (A)
7
SC4524A
Typical Characteristics (Cont.)
VIN Supply Current
VIN Thresholds vs Temperature
VIN Shutdown Current vs VIN
vs Soft-Start Voltage
100
80
60
40
20
0
3.0
2.5
2.0
1.5
1.0
0.5
0.0
VSS = 0
125oC
-40oC
Start
2.9
2.8
2.7
2.6
2.5
2.4
-40oC
125oC
UVLO
25
0
5
10
15
VIN (V)
20
25
30
0
0.5
1
1.5
2
-50 -25
0
50
75 100 125
Temperature (oC)
VSS (V)
Soft-Start Charging Current
vs Soft-Start Voltage
SS Shutdown Threshold
vs Temperature
VIN Quiescent Current vs VIN
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
0.40
0.35
0.30
0.25
0.20
2.5
2.0
1.5
1.0
0.5
0.0
125oC
-40oC
125oC
-40oC
VCOMP = 0
-50 -25
0
25
50
75 100 125
0
0.5
1
1.5
2
0
5
10
15
20
25
30
Temperature (oC)
VSS (V)
VIN (V)
8
SC4524A
Applications Information
Operation
When the SS/EN pin is released, the soft-start capacitor
is charged with an internal ꢀ.6µA current source (not
shown in Figure 3). As the SS/EN voltage exceeds 0.4V,
the internal bias circuit of the SC4524A turns on and the
SC4524A draws 2mA from VIN. The ꢀ.6µA charging current
turns off and the 2µA current source IC in Figure 3 slowly
charges the soft-start capacitor.
The SC4524A is a constant-frequency, peak current-
mode, step-down switching regulator with an integrated
28V, 2.6A power NPN transistor. Programmable switching
frequency makes the regulator design more flexible. With
the peak current-mode control, the double reactive poles
of the output LC filter are reduced to a single real pole by
the inner current loop. This simplifies loop compensation
and achieves fast transient response with a simple Type-2
compensation network.
The error amplifier EA in Figure 2 has two non-inverting
inputs. The non-inverting input with the lower voltage
predominates. One of the non-inverting inputs is biased
to a precision ꢀV reference and the other non-inverting
input is tied to the output of the amplifier Aꢀ. Amplifier Aꢀ
producesanoutputVꢀ=2(VSS/EN -ꢀ.23V).Vꢀ iszeroandCOMP
is forced low when VSS/EN is below ꢀ.23V. During start up,
the effective non-inverting input of EA stays at zero until
the soft-start capacitor is charged above ꢀ.23V. Once VSS/EN
exceeds ꢀ.23V, COMP is released. The regulator starts to
switch whenVCOMP rises above 0.4V. If the soft-start interval
is made sufficiently long, then the FB voltage (hence the
output voltage) will track Vꢀ during start up. VSS/EN must be
at least ꢀ.83V for the output to achieve regulation. Proper
soft-start prevents output overshoot. Current drawn from
the input supply is also well controlled.
As shown in Figure 2, the switch collector current is
sensed with an integrated 6.ꢀmW sense resistor. The
sensed current is summed with a slope-compensating
ramp before it is compared with the transconductance
error amplifier (EA) output. The PWM comparator trip
point determines the switch turn-on pulse width. The
current-limit comparator ILIM turns off the power switch
when the sensed signal exceeds the 20mV current-limit
threshold.
Driving the base of the power transistor above the
input power supply rail minimizes the power transistor
saturation voltage and maximizes efficiency. An external
bootstrap circuit (formed by the capacitor Cꢀ and the
diode Dꢀ in Figure ꢀ) generates such a voltage at the BST
pin for driving the power transistor.
Overload / Short-Circuit Protection
Table
2 lists various fault conditions and their
corresponding protection schemes in the SC4524A.
Shutdown and Soft-Start
Table 2: Fault conditions and protections
The SS/EN pin is a multiple-function pin. An external
capacitor (4.7nF to 22nF) connected from the SS pin to
ground sets the soft-start and overload shutoff times of
the regulator (Figure 3). The effect of VSS/EN on the SC4524A
is summarized in Table ꢀ.
Condition
Fault
Protective Action
Cycle-by-cycle limit at
IL>ILimit, VFB>0.8V
Over current
programmed frequency
Cycle-by-cycle limit with
IL>ILimit, VFB<0.8V
Over current
frequency foldback
Shutdown, then retry
VSS/EN Falling
SS/EN<1.9V
Persistent over current
or short circuit
(Hiccup)
Table 1: SS/EN operation modes
Tj>160C
Over temperature
Shutdown
SS/EN
<0.2V
Mode
Supply Current
18uA @ 5Vin
2mA
Shutdown
As summarized in Table ꢀ, overload shutdown is disabled
during soft-start (VSS/EN<2.ꢀV). In Figure 3, the reset input of
the overload latch B2 will remain high if the SS/EN voltage
is below 2.ꢀV. Once the soft-start capacitor is charged
above 2.ꢀV, the output of the Schmitt trigger Bꢀ goes high,
the reset input of B2 goes low and hiccup becomes armed.
0.4V to 1.23V
1.23V to 2.1V
>2.1V
Not switching
Switching & hiccup disabled
Switching & hiccup armed
Load dependent
Pulling the SS/EN pin below 0.2V shuts off the regulator
and reduces the input supply current to ꢀ8µA (VIN = 5V).
9
SC4524A
Applications Information (Cont.)
As the load draws more current from the regulator, the
current-limit comparator ILIM (Figure 2) will eventually
limit the switch current on a cycle-by-cycle basis. The
over-current signal OC goes high, setting the latch B3. The
soft-start capacitor is discharged with (ID - IC) (Figure 3). If
the inductor current falls below the current limit and the
PWM comparator instead turns off the switch, then latch
B3 will be reset and IC will recharge the soft-start capacitor.
If over-current condition persists or OC becomes asserted
more often than PWM over a period of time, then the
soft-start capacitor will be discharged below ꢀ.9V. At this
juncture, comparator B4 sets the overload latch B2. The
soft-start capacitor will be continuously discharged with
(ID - IC). The COMP pin is immediately pulled to ground. The
switching regulator is shut off until the soft-start capacitor
is discharged below ꢀ.0V. At this moment, the overload
latch is reset. The soft-start capacitor is recharged and
the converter again undergoes soft-start. The regulator
will go through soft-start, overload shutdown and restart
until it is no longer overloaded.
down switching regulator in continuous-conduction
mode (CCM) is given by
VO + VD
V + VD − VCESAT
D =
(2)
IN
where VCESAT is the switch saturation voltage and VD is
voltage drop across the rectifying diode.
In peak current-mode control, the PWM modulating
ramp is the sensed current ramp of the power switch.
This current ramp is absent unless the switch is turned
on. The intersection of this ramp with the output of the
voltage feedback error amplifier determines the switch
pulse width. The propagation delay time required to
immediately turn off the switch after it is turned on is the
minimum controllable switch on time (TON(MIN)).
Closed-loop measurement shows that the SC4524A
minimum on time is about ꢀ35ns at room temperature
(Figure 4). If the required switch on time is shorter than
the minimum on time, the regulator will either skip cycles
or it will jitter.
If the FB voltage falls below 0.8V because of output
overload, then the switching frequency will be reduced.
Frequency foldback helps to limit the inductor current
when the output is hard shorted to ground.
Minimum On Time vs Temperature
200
190
180
170
160
150
140
130
120
110
100
VO =1.5V
1MHz
During normal operation, the soft-start capacitor is
charged to 2.4V.
Setting the Output Voltage
The regulator output voltage is set with an external
resistive divider (Figure ꢀ) with its center tap tied to the
FB pin. For a given R6 value, R4 can be found by
VO
6
-50 -25
0
25
50
75 100 125
R4 = R
−1
(ꢀ)
1.0V
Temperature (OC)
Setting the Switching Frequency
Figure 4. Variation of Minimum On Time
with Ambient Temperature
The switching frequency of the SC4524A is set with an
external resistor from the ROSC pin to ground.
To allow for transient headroom, the minimum operating
switch on time should be at least 20% to 30% higher than
the worst-case minimum on time.
Minimum On Time Consideration
The operating duty cycle of a non-synchronous step-
ꢀ0
SC4524A
Applications Information (Cont.)
Minimum Off Time Limitation
The input capacitance must also be high enough to keep
input ripple voltage within specification. This is important
in reducing the conductive EMI from the regulator. The
input capacitance can be estimated from
The PWM latch in Figure 2 is reset every cycle by the
clock. The clock also turns off the power transistor to
refresh the bootstrap capacitor. This minimum off time
limits the attainable duty cycle of the regulator at a given
switching frequency. The measured minimum off time is
ꢀ00ns typically. If the required duty cycle is higher than
the attainable maximum, then the output voltage will not
be able to reach its set value in continuous-conduction
mode.
IO
CIN
>
(6)
4 ⋅ DV ⋅FSW
IN
where DVIN is the allowable input ripple voltage.
Multi-layer ceramic capacitors, which have very low ESR
(a few mW) and can easily handle high RMS ripple current,
are the ideal choice for input filtering. A single 4.7µF
X5R ceramic capacitor is adequate for 500kHz or higher
switching frequency applications, and ꢀ0µF is adequate
for 200kHz to 500kHz switching frequency. For high
voltage applications, a small ceramic (ꢀµF or 2.2µF) can be
placed in parallel with a low ESR electrolytic capacitor to
satisfy both the ESR and bulk capacitance requirements.
Inductor Selection
The inductor ripple current for a non-synchronous step-
down converter in continuous-conduction mode is
(VO + VD ) ⋅(1 − D)
DIL =
(3)
FSW ⋅L1
where FSW is the switching frequency and Lꢀ is the
inductance.
Output Capacitor
The output ripple voltage DVO of a buck converter can be
An inductor ripple current between 20% to 50% of the
maximum load current gives a good compromise among
efficiency, cost and size. Re-arranging Equation (3) and
assuming 35% inductor ripple current, the inductor is
given by
expressed as
1
DVO = DIL ⋅ ESR +
(7)
8 ⋅FSW ⋅CO
where CO is the output capacitance.
(VO + VD ) ⋅(1 − D)
35% ⋅IO ⋅FSW
L1 =
(4)
Since the inductor ripple current DIL increases as D
decreases (Equation (3)), the output ripple voltage is
therefore the highest when VIN is at its maximum.
If the input voltage varies over a wide range, then choose
Lꢀ based on the nominal input voltage. Always verify
converter operation at the input voltage extremes.
A ꢀ0µF to 47µF X5R ceramic capacitor is found adequate
for output filtering in most applications. Ripple current
in the output capacitor is not a concern because the
inductor current of a buck converter directly feeds CO,
resulting in very low ripple current. Avoid using Z5U
and Y5V ceramic capacitors for output filtering because
these types of capacitors have high temperature and high
voltage coefficients.
The peak current limit of SC4524A power transistor is at
least 2.6A. The maximum deliverable load current for the
SC4524A is 2.6A minus one half of the inductor ripple
current.
Input Decoupling Capacitor
The input capacitor should be chosen to handle the RMS
ripple current of a buck converter. This value is given by
Freewheeling Diode
Use of Schottky barrier diodes as freewheeling rectifiers
reduces diode reverse recovery input current spikes,
easing high-side current sensing in the SC4524A. These
IRMS _ CIN = IO ⋅ D ⋅(1 − D)
(5)
ꢀꢀ
SC4524A
Applications Information (Cont.)
diodes should have an average forward current rating
at least 2A and a reverse blocking voltage of at least a
few volts higher than the input voltage. For switching
regulators operating at low duty cycles (i.e. low output
voltage to input voltage conversion ratios), it is beneficial
to use freewheeling diodes with somewhat higher
average current ratings (thus lower forward voltages). This
is because the diode conduction interval is much longer
than that of the transistor. Converter efficiency will be
improved if the voltage drop across the diode is lower.
Minimum Bootstrap Voltage
vs Temperature
2.2
2.1
2.0
1.9
1.8
1.7
1.6
I
SW = -2.6A
The freewheeling diode should be placed close to the
SW pin of the SC4524A to minimize ringing due to trace
inductance. 20BQ030 (International Rectifier), B230A
(Diodes Inc.), SSꢀ3, SS23 (Vishay), CMSHꢀ-40M, CMSHꢀ-
40ML and CMSH2-40M (Central-Semi.) are all suitable.
-50 -25
0
25 50
75 100 125
Temperature (oC)
Figure 5. Typical Minimum Bootstrap Voltage required
to Saturate Transistor (ISW= -2.6A).
The freewheeling diode should be placed close to the SW
pin of the SC4524A on the PCB to minimize ringing due to
trace inductance.
D1
BST
C1
VIN
VOUT
Bootstrapping the Power Transistor
SW
IN
SC4524A
D
2
The typical minimum BST-SW voltage required to fully
saturate the power transistor is shown in Figure 5, which
is about ꢀ.96V at room temperature.
GND
(a)
D3
D1
The BST-SW voltage is supplied by a bootstrap circuit
powered from either the input or the output of the
converter (Figure 6). To maximize efficiency, tie the
bootstrap diode to the converter output if VO>2.5V.
Since the bootstrap supply current is proportional to the
converter load current, using a lower voltage to power
the bootstrap circuit reduces driving loss and improves
efficiency.
BST
C1
VIN
VOUT
SW
IN
SC4524A
D
2
GND
(b)
Figure 6. Methods of Bootstrapping the SC4524A
For the bootstrap circuit, a fast switching PN diode (such
as ꢀN4ꢀ48 or ꢀN9ꢀ4) and a small (0.ꢀµF – 0.47µF) ceramic
capacitor is sufficient for most applications. When
bootstrapping from 2.5V to 3.0V output voltages, use a
low forward drop Schottky diode (BAT-54 or similar) for
Dꢀ. When bootstrapping from high input voltages (>20V),
reduce the maximum BST voltage by connecting a Zener
diode (D3) in series with Dꢀ.
Loop Compensation
The goal of compensation is to shape the frequency
response of the converter so as to achieve high DC
accuracy and fast transient response while maintaining
loop stability.
ꢀ2
SC4524A
Applications Information (Cont.)
CONTROLLER AND SCHOTTKY DIODE
Including the voltage divider (R4 and R6), the control to
feedback transfer function is found and plotted in Figure
8 as the converter gain.
Io
Rs
CA
REF
+
Vc
PWM
MODULATOR
EA
FB
-
L1
Vo
Since the converter gain has only one dominant pole at
low frequency, a simple Type-2 compensation network
is sufficient for voltage loop compensation. As shown in
Figure 8, the voltage compensator has a low frequency
integrator pole, a zero at FZꢀ, and a high frequency pole
at FPꢀ. The integrator is used to boost the gain at low
frequency. The zero is introduced to compensate the
excessive phase lag at the loop gain crossover due to the
integrator pole (-90deg) and the dominant pole (-90deg).
The high frequency pole nulls the ESR zero and attenuates
high frequency noise.
Vramp
SW
COMP
R4
R6
Co
C5
C8
R7
Resr
Figure 7. Block diagram of control loops
The block diagram in Figure 7 shows the control loops of a
buck converter with the SC4524A. The inner loop (current
loop) consists of a current sensing resistor (Rs=6.ꢀmW)
and a current amplifier (CA) with gain (GCA=28). The outer
loop (voltage loop) consists of an error amplifier (EA), a
PWM modulator, and a LC filter.
Since the current loop is internally closed, the remaining
task for the loop compensation is to design the voltage
compensator (C5, R7, and C8).
For a converter with switching frequency FSW, output
inductance Lꢀ, output capacitance CO and loading R, the
control (VC) to output (VO) transfer function in Figure 7 is
given by:
GPWM (1 + sRESRCO )
Vo
=
(8)
Figure 8. Bode plots for voltage loop design
Vc (1 + s /ωp )(1 + s /ωn Q + s2 /ωn2 )
This transfer function has a finite DC gain
Therefore, the procedure of the voltage loop design for
the SC4524A can be summarized as:
R
GPWM
≈
,
GCA ⋅RS
(ꢀ) Plot the converter gain, i.e. control to feedback transfer
function.
an ESR zero FZ at
(2) Select the open loop crossover frequency, FC, between
ꢀ0% and 20% of the switching frequency. At FC, find the
required compensator gain, AC. In typical applications with
ceramic output capacitors, the ESR zero is neglected and
the required compensator gain at FC can be estimated by
1
ωZ =
,
RESRCO
a dominant low-frequency pole FP at
1
ωp ≈
,
RCO
VFB
1
1
(9)
AC = − 20 ⋅log
⋅
⋅
GCARS 2πFCCO VO
and double poles at half the switching frequency.
ꢀ3
SC4524A
Applications Information (Cont.)
capacitor, the main power switch and the freewheeling
diode carry pulse current (Figure 9). For jitter-free
operation,thesizeoftheloopformedbythesecomponents
should be minimized. Since the power switch is already
integrated within the SC4524A, connecting the anode of
the freewheeling diode close to the negative terminal of
the input bypass capacitor minimizes size of the switched
current loop. The input bypass capacitor should be placed
close to the IN pin. Shortening the traces of the SW and
BST nodes reduces the parasitic trace inductance at these
nodes. This not only reduces EMI but also decreases
switching voltage spikes at these nodes.
(3) Place the compensator zero, FZꢀ, between ꢀ0% and
20% of the crossover frequency, FC.
(4) Use the compensator pole, FPꢀ, to cancel the ESR zero,
FZ.
(5) Then, the parameters of the compensation network
can be calculated by
A
C
20
10
R7 =
C5 =
C8 =
gm
1
2πFZ1 R7
1
The exposed pad should be soldered to a large ground
plane as the ground copper acts as a heat sink for the
device. To ensure proper adhesion to the ground plane,
avoid using vias directly under the device.
2πFP1 R7
where gm=0.28mA/V is the EA gain of the SC4524A.
Example: Determine the voltage compensator for an
800kHz, ꢀ2V to 3.3V/2A converter with 22uF ceramic
output capacitor.
V
IN
Choose a loop gain crossover frequency of 80kHz, and
place voltage compensator zero and pole at FZꢀ=ꢀ6kHz
(20% of FC), and FPꢀ=600kHz. From Equation (9), the
required compensator gain at FC is
V
OUT
1
1
1.0
3.3
AC = − 20 ⋅log
⋅
⋅
=15.9dB
28 ⋅ 6.1 ⋅10−3 2π ⋅ 80 ⋅103 ⋅ 22 ⋅10−6
Z
L
Then the compensator parameters are
1015.9
20
R7 =
C5 =
= 22.3k
0.28 ⋅10−3
1
Figure 9. Heavy lines indicate the critical pulse
current loop. The inductance of this
loop should be minimized.
= 0.45nF
=12pF
2π ⋅16 ⋅103 ⋅ 22.1 ⋅103
1
C8 =
2π⋅600 ⋅103 ⋅22.1 ⋅103
Select R7=22.ꢀk, C5=0.47nF, and C8=ꢀ0pF for the design.
Compensator parameters for various typical applications
are listed in Table 4. A MathCAD program is also available
upon request for detailed calculation of the compensator
parameters.
PCB Layout Considerations
In a step-down switching regulator, the input bypass
ꢀ4
SC4524A
Recommended Component Parameters in Typical Applications
Table 4 lists the recommended inductance (Lꢀ) and compensation network (R7, C5, C8) for common input and output
voltages. The inductance is determined by assuming that the ripple current is 35% of load current IO. The compensator
parameters are calculated by assuming a 22mF low ESR ceramic output capacitor and a loop gain crossover frequency
of FSW/ꢀ0.
Table 4. Recommended inductance (L1) and compensator (R7, C5, C8)
Typical Applications
Recommended Parameters
Vin(V)
Vo(V)
Io(A)
Fsw(kHz)
C2(uF)
L1(uH)
15
8.2
15
8.2
22
10
R7(k)
C5(nF)
C8(pF)
1
2
1
2
1
2
1.5
2.5
3.3
300
500
4.32
2.2
12.4
15
1.0
0.82
500
1000
500
1000
500
1000
500
1000
500
22
15
15
6.8
33
15
15
8.2
33
22
20
43.2
20
0.68
0.47
0.68
0.47
0.68
0.47
0.68
0.47
0.68
0.47
0.68
0.47
1
2
1
2
1
2
5
24
22
10
43.2
35.7
63.4
35.7
63.4
43.2
84.5
43.2
84.5
7.5
10
1000
500
1000
15
10
ꢀ5
SC4524A
Typical Application Schematics
D1
D3
24V
V
IN
18V Zener
1N4148
C4
4.7mF
C1
0.33mF
BST
SW
IN
L1
OUT
8.2mH
1.5V/2A
R4
SC4524A
SS/EN
33.2k
FB
GND
COMP
ROSC
D2
20BQ030
R6
66.5k
C2
22mF
C7
10nF
R7
4.32k
R5
90.9k
C8
22pF
C5
2.2nF
L1: Coiltronics DR73-8R2
C2: Murata GRM31CR60J226K
C4: Murata GRM32ER71H475K
Figure 10. 300kHz 24V to 1.5V/2A Step-down Converter
D1
10V – 26V
V
IN
1N4148
C1
0.1mF
C4
4.7mF
BST
SW
IN
L1
OUT
8.2mH
3.3V/2A
R4
3.65k
SC4524A
SS/EN
FB
GND
COMP
ROSC
D2
SS23
R6
1.58k
C2
22mF
C7
10nF
R7
R5
23.7k
C8
22pF
13.0k
C5
2.2nF
L1: Coiltronics DR73-8R2
C2: Murata GRM31CR60J226M
C4: Murata GRM32ER71H475K
Figure 11. 800kHz 10V-26V to 3.3V/2A Step-down Converter
ꢀ6
SC4524A
Typical Performance Characteristics
(For A 24V to 5V/2A Step-down Converter with 1MHz Switching Frequency)
6
5
24V Input (10V/DIV)
4
3
2
1
0
5V Output (2V/DIV)
SS Voltage (1V/DIV)
0
0.5
1
1.5
2
2.5
3
10ms/DIV
Load Current (A)
Figure 12(b). VIN Start up Transient (IO=2A)
Figure 12(a). Load Characteristic
5V Output Short (5V/DIV)
5V Output Response (500mV/DIV, AC Coupling)
Inductor Current (1A/DIV)
Retry Inductor Current (2A/DIV)
SS Voltage (2V/DIV)
40us/DIV
20ms/DIV
Figure 12(c). Load Transient Response
(IO= 0.3A to 2A)
Figure 12(d). Output Short Circuit (Hiccup)
ꢀ7
SC4524A
Outline Drawing - SOIC-8 EDP
A
D
E
e
DIMENSIONS
INCHES MILLIMETERS
N
DIM
A
MIN
MAX MIN
.069 1.35
.005 0.00
.065 1.25
.020 0.31
.010 0.17
MAX
1.75
0.13
1.65
0.51
0.25
NOM
NOM
-
-
-
-
-
-
-
-
-
-
.053
A1 .000
2X E/2
A2 .049
E1
b
c
.012
.007
D
.189 .193 .197 4.80 4.90 5.00
1
2
E1 .150 .154 .157 3.80 3.90 4.00
E
e
F
H
.236 BSC
.050 BSC
6.00 BSC
1.27 BSC
ccc
2X N/2 TIPS
C
e/2
.116 .120 .130 2.95 3.05 3.30
.085 .095 .099 2.15 2.41 2.51
B
-
-
h
.010
.020 0.25
0.50
L
L1
N
01
aaa
bbb
ccc
.016 .028 .041 0.40 0.72 1.04
D
(.041)
(1.05)
8
-
8
-
aaa
C
0°
8°
0°
8°
A2
A
.004
.010
.008
0.10
0.25
0.20
SEATING
PLANE
C
A1
bxN
bbb
C
A-B D
h
F
EXPOSED PAD
h
H
H
c
GAGE
PLANE
0.25
L
01
(L1)
DETAIL
A
SEE DETAIL
A
SIDE VIEW
NOTES:
1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).
2. DATUMS -A- AND -B- TO BE DETERMINED AT DATUM PLANE -H-
3. DIMENSIONS "E1" AND "D" DO NOT INCLUDE MOLD FLASH, PROTRUSIONS
OR GATE BURRS.
REFERENCE JEDEC STD MS-012, VARIATION BA.
4.
Land Pattern - SOIC-8 EDP
E
SOLDER MASK
D
DIMENSIONS
DIM
C
INCHES
(.205)
MILLIMETERS
(5.20)
Z
(C)
D
E
F
G
P
X
Y
Z
.134
.201
.101
.118
.050
.024
.087
.291
3.40
5.10
2.56
3.00
1.27
0.60
2.20
7.40
G
Y
F
THERMAL VIA
Ø 0.36mm
P
X
NOTES:
1. THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY.
CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR
COMPANY'S MANUFACTURING GUIDELINES ARE MET.
2. REFERENCE IPC-SM-782A, RLP NO. 300A.
3. THERMAL VIAS IN THE LAND PATTERN OF THE EXPOSED PAD
SHALL BE CONNECTED TO A SYSTEM GROUND PLANE.
FAILURE TO DO SO MAY COMPROMISE THE THERMAL AND/OR
FUNCTIONAL PERFORMANCE OF THE DEVICE.
Contact Information
Semtech Corporation
Power Mangement Products Division
200 Flynn Road, Camarillo, CA 930ꢀ2
Phone: (805) 498-2ꢀꢀꢀ Fax: (805) 498-3804
ꢀ8
相关型号：
SC4524DSETRT
Switching Regulator, Current-mode, 4.3A, 1560kHz Switching Freq-Max, PDSO8, HALOGEN FREE AND ROHS COMPLIANT, MS-012BA, SOIC-8
SEMTECH
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