sc4525d Semtech Corporation, sc4525d Datasheet - Page 12

• Manufacturer Part Number: sc4525d
• Description: 18v, 3a, 350khz Step-down Switching Regulator
• Manufacturer: Semtech Corporation

Applications Information (Cont.)
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 SC4525D to minimize ringing due to trace
inductance. 20BQ030 (International Rectifier), B320A,
B330A (Diodes Inc.), SS33 (Vishay), CMSH3-20MA and
CMSH3-40MA (Central-Semi.) are all suitable.
The freewheeling diode should be placed close to the SW
pin of the SC4525D 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
2V 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(a), 6(b) and 6(c)). To maximize
efficiency, tie the bootstrap diode to the converter output
if V
supply current is proportional to the converter load
current, using a lower voltage to power the bootstrap
circuit reduces driving loss (Equation (11), page 14) and
improves efficiency.
Figure 5. Typical Minimum Bootstrap Voltage required
© 2011 Semtech Corp.
SS270 REV 6-7
SS270 REV 6-7
O
>2.5V as shown in Figure 6 (a). Since the bootstrap
Fig.6: Methods of BST
Fig.6: Methods of BST
to Saturate the 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
Minimum Bootstrap Voltage
Minimum Bootstrap Voltage
I
I
SW
SW
-25
-25
=-3.9A
=-3.9A
VIN
VIN
vs Temperature
vs Temperature
Temperature (
Temperature (
0
0
25
25
IN
IN
50
50
SC4525D
SC4525D
BST
BST
GND
GND
o
o
C)
C)
75
75
SW
SW
100 125
100 125
SW
(a)
(a)
= -3.9A)
D 2
D 2
C1
C1
D1
D1
VOUT
VOUT
For the bootstrap circuit, a fast switching PN diode
(such as 1N4148 or 1N914) and a small (0.33µF – 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
D
SW and the BST pins will be required as shown in Figure 6
(c). D
the operating temperature does not exceed 85 ºC. Use a
small Schottky diode (BAT54 or similar) if the converter is
to operate up to 125 ºC.
1
. If V
Figure 6. Methods of Bootstrapping the SC4525D
4
can be a small PN diode such as 1N4148 or 1N914 if
OUT
VIN
VIN
VIN
VIN
12
12
12
12
> 8V, then a protection diode D
D4 is either a pn juntion diode or a Schottky diode
IN
IN
IN
D1
D1
IN
depending on the operating temperature.
SC4525D
SC4525D
SC4525D
SC4525D
BST
BST
BST
GND
GND
GND
GND
BST
SW
SW
SW
SW
(b)
(b)
(a)
(a)
(C)
D4
D 2
D 2
D 2
D2
C1
C1
www.semtech.com
C1
C1
D1
D1
SC4525D
4
between the
VOUT
VOUT
VOUT
VOUT>8V
12