SC4524
POWER MANAGEMENT
Applications Information
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 diodes should be placed close to the
SW pins of the SC4524 to minimize ringing due to trace
inductance. 10BQ015, 20BQ030 (International Rectifier),
MBRM120LT3 (ON Semi), UPS120 and UPS140 (Micro-
Semi) are all suitable.
Bootstrapping the Power Transistors
To maximize efficiency, the turn-on voltage across the
internal power NPN transistor should be minimized. If
the transistor is to be driven into saturation, then its
base will have to be driven from a power supply higher in
voltage than V
IN
. The required driver supply voltage (at
least 2.5V higher than the SW voltage over the industrial
temperature range) is generated with a bootstrap circuit
(the diode D
BST
and the capacitor C
BST
in Figure 7). The
bootstrapped output (the common node between D
BST
and C
BST
) is connected to the BST pin of the SC4524.
The power transistor in the SC4524 is first switched on
to build up current in the inductor. When the transistor
is switched off, the inductor current pulls the SW node
low, allowing C
BST
to be charged through D
BST
. When the
power switch is again turned on, the SW voltage goes
high. This brings the BST voltage to
9
6:
+
9
&
%67
, thus back-
biasing D
BST
. C
BST
voltage increases with each subsequent
switching cycle, as does the bootstrapped voltage at the
BST pin. After a number of switching cycles, C
BST
will be
fully charged to a voltage approximately equal to that
applied to the anode of D
BST
. Figure 6 shows the typical
minimum BST to SW voltage required to fully saturate
the power transistor. This differential voltage (
=
9
&
%67
)
must be at least 1.8V at room temperature. This is also
specified in the Electrical Characteristics as Minimum
Bootstrap Voltage. The minimum required V
CBST
increases
as temperature decreases. The bootstrap circuit reaches
equilibrium when the base charge drawn from C
BST
during
transistor on time is equal to the charge replenished
ã
2006 Semtech Corp.
12
during the off interval.
,
6:
,
6:
, where I
SW
and
b
+
are the switch emitter current and current gain
respectively, is drawn from the bootstrap capacitor C
BST
.
The switch base current
=
, 7
Charge
6: 21
is drawn from C
BST
during the switch on
time, resulting in a voltage droop of
,
6:
7
21
. If I
SW
= 2A,
&
%67
T
ON
= 1ms,
b =
35 and C
BST
= 0.1mF, then the
9
&
%67
droop
will be 0.57V. C
BST
is refreshed to
9
$
-
9
'
%67
+
9
'
5(&7
every
cycle, where
9
$
is the applied D
BST
anode voltage. Switch
base current discharges the bootstrap capacitor to
9
$
-
9
'
%67
+
9
'
5(&7
-
,
6:
7
21
at the end of conduction. This
b
&
%67
voltage must be higher than the minimum shown in Figure
6 to ensure full switch enhancement. D
BST
can be tied
either to the input or to the output of the DC/DC
converter.
If
D
BST
is tied to the input, then the charge drawn from
,
6:
7
21
the input power supply will be
(the base charge
b
of the switch). The energy loss due to base charge per
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Figure 6. Typical Minimum Bootstrap Voltage Re-
quired to Maintain Saturation at I
SW
= 2A.
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