SC4524B Semtech Corporation, SC4524B Datasheet - Page 12

• Manufacturer Part Number: SC4524B
• Description: 16V 2A Step-Down Switching Regulator
• Manufacturer: Semtech Corporation

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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.
The freewheeling diode should be placed close to the
SW pin of the SC4524B to minimize ringing due to trace
inductance. 10BQ015, 20BQ030 (International Rectifier),
B220A (Diodes Inc.), SS13, SS22 (Vishay), CMSH1-20M,
CMSH1-20ML and CMSH2-20M (Central-Semi.) are all
suitable.
The freewheeling diode should be placed close to the SW
pin of the SC4524B on the PCB to minimize ringing due to
trace inductance.
Bootstrapping the Power Transistor
The minimum BST-SW voltage required to fully saturate
the power transistor is shown in Figure 5, which is about
1.96V at room temperature.
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 V
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.
For the bootstrap circuit, a fast switching PN diode (such
as 1N4148 or 1N914) and a small (0.1µF – 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
D1.
VIN
VIN
IN
IN
SC4524B
SC4524B
BST
BST
GND
GND
Fig.5
Fig.5
SW
SW
(a)
(a)
D
D
2
2
C1
C1
O
D1
D1
>2.5V.
VOUT
VOUT
Figure 5. Typical Minimum Bootstrap Voltage required
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.
SS270 REV 6-7
SS270 REV 6-7
Figure 6. Methods of Bootstrapping the SC4524B
VIN
VIN
VIN
VIN
to Saturate Transistor (I
2.2
2.2
2.1
2.1
2.0
2.0
1.9
1.9
1.8
1.8
1.7
1.7
1.6
1.6
-50
-50
IN
IN
IN
IN
D1
D1
Minimum Bootstrap Voltage
Minimum Bootstrap Voltage
I
I
SW
SW
-25
-25
SC4524B
SC4524B
SC4524B
SC4524B
BST
BST
BST
BST
= -2.6A
= -2.6A
GND
GND
GND
GND
vs Temperature
vs Temperature
Temperature (
Temperature (
0
0
SW
SW
SW
SW
(a)
(a)
(b)
(b)
25
25
50
50
D
D
D
D
2
2
o
o
SW
2
2
C)
C)
75
75
C1
C1
C1
C1
= -2.6A).
D1
D1
100 125
100 125
SC4524B
VOUT
VOUT
VOUT
VOUT
12
VIN
VIN