sc4525c Semtech Corporation, sc4525c Datasheet - Page 12

• Manufacturer Part Number: sc4525c
• Description: 28v 3a Step-down Switching Regulator
• Manufacturer: Semtech Corporation

Applications Information (Cont.)
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.
Freewheeling Diode
Use of Schottky barrier diodes as freewheeling rectifiers
reduces diode reverse recovery input current spikes,
easing high-side current sensing in the SC4525C. These
diodes should have an average forward current rating
at least 3A 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 SC4525C 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 SC4525C 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 (Equation (10), page 14), using a lower voltage
to power the bootstrap circuit reduces driving loss and
improves efficiency.
O
>2.5V as shown in Figure 6(a). Since the bootstrap
Fig 5
Fig 5
SS270 REV 6-7
SS270 REV 6-7
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
reduce the maximum BST voltage by connecting a Zener
diode (D
> 8V, then a protection diode D
the BST pins will be required as shown in Figure 6 (c). D
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.
Figure 5. Typical Minimum Bootstrap Voltage required
Figure 6(a). Bootstrapping the SC4525C from the
1
. When bootstrapping from high input voltages (>20V),
3
VIN
) in series with D
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
Converter Output
-50
-50
IN
Minimum Bootstrap Voltage
Minimum Bootstrap Voltage
I
I
SW
SW
-25
-25
SC4525C
=-3.9A
=-3.9A
BST
GND
vs Temperature
vs Temperature
Temperature (
Temperature (
0
0
SW
(a)
(a)
1
25
25
as shown in Figure 6 (b). If V
50
50
D 2
C1
o
o
4
C)
C)
75
75
D1
between the SW and
100 125
100 125
SW
VOUT
= -3.9A)
OUT
12
4