SC4525D View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC4525D' PDF : 21 Pages View PDF
SC4525D
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.
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
D1. If VOUT > 8V, then a protection diode D4 between the
SW and the BST pins will be required as shown in Figure 6
(c). D4 can be a small PN diode such as 1N4148 or 1N914 if
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.
D1
Bootstrapping the Power Transistor
The mFiingi.m6u:mMBeStTh-SoWdsvooltafgBeSreTquired 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 Do1f the
converter (Figure 6(a), 6(b) and 6(c)). To maximize
efficiency, tie the bootstrap diodeBStoT the convertCe1r output
isfuVpOp>ly2.5cVurarsensthoiswnpVriINonpFoigrtuiorIeNna6l
(a).
to
Since the bootstrap
tShWe converter loadVOUT
vs Tempcurrent, using a lower voltageSCto452p5oDwer the bootstrap
circuit reduces driving loss (EquaGtNioDn (11), paDg2e 14) and
iSmS2p7r0oRvEeVs6e-7fficiency.
Minimum Bootstrap Voltage
(a)
vs Temperature
2.2
BST
VIN
12
IN
SW
SC4525D
GND
C1
D2
(a)
D1
BST
C1
VIN
IN
SW
SC4525D
GND
D2
VOUT
VOUT
(b)
D4
D1
2.1
BST
C1
2.0
VIN
VOUT>8V
IN
SW
1.9
SC4525D
D2
GND
1.8
1.7
ISW =-3.9A
1.6
-50 -25
0 25 50 75 100 125
Temperature (oC)
D4 is either a pn juntion diode or a Schottky diode
depending on the operating temperature.
(C)
Figure 6. Methods of Bootstrapping the SC4525D
Figure 5. Typical Minimum Bootstrap Voltage required
to Saturate the Transistor (ISW= -3.9A)
© 2011 Semtech Corp.
www.semtech.com 12
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