SC4525FEVB datasheet(12/23 Pages) SEMTECH

12 / 23 page

SC4525F

Applications Information (Cont.)

Bootstrapping the Power Transistor

To maximize efficiency, the turn-on voltage across the

internal power NPN transistors should be minimized. If

these transistors are to be driven into saturation, then

their bases will have to be driven from a power supply

higher in voltage than V

IN. The required driver supply volt-

age (at least 2.3V higher than the SW voltage) is gener-

ated with a bootstrap circuit (the diode D

and the capaci-

tor C

in Figure 6). The bootstrapped output (the common

node between D

and C) is connected to the BST pin of

the SC4525F.

The minimum BST to SW voltage required to fully satu-

rate the power transistor is shown in Figure 5. The mini-

mum required V

C increases as temperature decreases.

The bootstrap circuit reaches equilibrium when the base

charge drawn from C

during transistor on time is equal to

the charge replenished during the off interval.

Figure 5. Typical Minimum Bootstrap Voltage required

to Saturate the Transistor (I

SW= -3.9A)

Figure 6 summarizes various ways of bootstrapping the

SC4525F. A fast switching PN diode (such as N448 or

N94) and a small (0.33μF – 0.47μF) ceramic capacitor

can be used.

In Figure 6(a) the power switch is bootstrapped from the

output. This is the most efficient configuration and it also

results in the least voltage stress at the BST pin. The maxi-

mum BST pin voltage is about V

IN + VOUT. The minimum

OUT required for this bootstrap configuration is 2.5V. If the

output voltage is between 2.5V and 3V, then use a small

Schottky diode (such as BAT54) for D

to maximize the

bootstrap voltage.

The SC4525F can also be bootstrapped from the input

[Figure 6(b)]. This configuration is not as efficient as Figure

6(a). However this may be the only option if the output

voltage is less than 2.5V and there is no other supply with

voltage higher than 2.5V. Voltage stress at the BST pin can

be somewhat higher than 2V

IN.

Figures 6(c) and (d) show how to bootstrap the SC4525F

from a second independent power supply V

The minimum bootstrap capacitance C

can be estimated

as:

where V

S is the voltage applied to the anode of D.

The inductor current charges the bootstrap capacitor

when it pulls the SW node low during the switch off time.

If D

is connected to the converter input, then C will be

charged as soon as V

IN is applied.

If the bootstrap diode is tied to the converter output [Fig

ures 6(a)], then C

can only be charged from the regulator

output through the inductor. Before the converter starts,

there is no output voltage or inductor current. Hence it

is necessary for the regulator to deliver some inductor

current to the output before C

can be charged. If VIN is

not much higher than the programmed V

OUT and it ramps

up very slowly, then the inductor current will not be high

enough for the bootstrap circuit to run, especially at light

loads. In order to have some inductor current to charge C

the converter output needs to be loaded or V

IN needs to

be increased. Using larger soft-start capacitor C

SS will also

help in starting bootstrap because there will be current in

the inductor over a longer period of time. Figures 7(a) and

7(b) show the minimum input voltage required to start

bootstrap and to run before dropping out as a function of

the load current. The minimum start-up V

IN decreases with

higher dV

IN/dt or larger soft-start capacitor CSS. The lines

labeled “dropout”in these graphs show that once started,

the bootstrap circuit is able to sustain itself down to zero

load.

Minimum Bootstrap Voltage

vs Temperature

1.6

1.7

1.8

1.9

2.0

2.1

2.2

-50

-25

100 125

Temperature (oC)

ISW =-3.9A

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