SC4524A_08 Datasheet, PDF (12/19 Pages) Semtech Corporation – 28V 2A Step-Down Switching Regulator

Fig.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.
SS270 REV 6-7
2.2
Minimum Bootstrap Voltage
vs Temperature
2.1
2.0
1.9
1.8
1.7
I_SW = -2.6A
The freewheeling diode should be placed close to the
Fig.6 ^{SW pin of the SC4524A to minimize ringing due to trace}
inductance. 20BQ030 (International Rectifier), B230A
1.6
-50 -25
0 25 50 75 100 125
Temperature (^oC)
(Diodes Inc.), SS13, SS23 (Vishay), CMSH1-40M, CMSH1-
40ML and CMSH2-40M (Central-Semi.) are all suitable.
Figure 5. Typical Minimum Bootstrap Voltage required
to Saturate Transistor (I_SW= -2.6A).
The freewheeling diode should be placed close to the SW
D1
D3
pin of the SC4524A on the PCB to minimize ringing due to
trace inductance.
BST
C1
Bootstrapping the Power Transistor
Fig.6
The typical minimum BST-SW voltage required to fully
saturate the power transistor is shown in Figure 5, which
is about 1.96V at room temperature.
VIN
IN
SW
SC4524A
D2
GND
(a)
VOUT
VIN
The BST-SW voltage is supplied by a bootstrap ^Dc¹ircuit
D3 D1
powered from either the input or the output of the
converter (Figure 6). To maxim^BSi^Tze efficiency^C, ¹ tie the
BST
C1
bootstrap
diode
to^VINthe
converter
IN
output
SW
if
V_O>2.5V. ^VOUT
VIN
IN
SW
VOUT
Since the bootstrap supply current is proportional to the
SC4524A
SC4524A
converter load current, using a lower voltag^De2 to power
D2
GND
GND
the bootstrap circuit reduces driving loss and improves
efficiency.
(a)
(b)
For the bootstrap circuit, a fast switching PN diode (such
Figure 6. Methods of Bootstrapping the SC4524A
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 Loop Compensation
low forward drop Schottky diode (BAT-54 or similar) for
D₁. When bootstrapping from high input voltages (>20V),
reduce the maximum BST voltage by connecting a Zener
diode (D₃) in series with D₁.
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.
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