SC4530 datasheet(13/23 Pages) SEMTECH

13 / 23 page

SC4530

S, the bootstrap voltage can be varied. Figure 5(a) shows

the switching waveforms of a correctly bootstrapped 0V

to 5V regulator with V

S = 2.5V. All three traces share the

same ground level. When the power transistor is turned

on, V

SW should come within a few hundred millivolts of VIN

Applications Information (Continued)

and V

BST should have at least 2V of headroom above VSW.

As V

S is reduced to .9V, excessive VBST droop decreases

transistor driver headroom, as shown in Figure 5(b). The

power transistor can no longer be fully saturated (as

evidenced by the round V

SW turn-off corners), resulting in

high power dissipation. When bootstrapping from a low-

voltage output or supply, checking the bootstrap voltage

is a good precaution.

Since the inductor current charges C

3, the bootstrap

circuit requires some minimum load current to function.

Figures 6(a) and 6(b) show the minimum input voltage

required to saturate the power transistor and to produce

a regulated output as a function of the load current. Once

started, the bootstrap circuit is able to sustain itself down

to zero load.

VSW

VBST

VIN

All Traces

2V/div

(a)

400ns/div

VBST

VIN

VSW

400ns/div

All Traces

2V/div

(b)

Minimum Input Voltage

5.0

5.5

6.0

6.5

7.0

100

1000

Load Current (mA)

VOUT = 5V

To Start

Dropout

(a)

Minimum Input Voltage

3.0

3.5

4.0

4.5

5.0

5.5

100

1000

Load Current (mA)

VOUT = 3.3V

To Start

Dropout

(b)

The Minimum InputVoltage Required to Start

and to Operate Before Dropout

(a) V

OUT =5V

(b) V

OUT = 3.3V

Figure 6.

Figure 5. Switching Waveforms of a 10V to 5V

Regulator

(a) Sufficient Bootstrap Voltage Drives the

Power Transistor into Saturation, Minimizing

Power Loss.

(b) Excessive Droop in Bootstrap Capacitor

Voltage fails to keep the Power Transistor

Saturated near the End of its Conduction

Cycles, Causing Jitters and Low Efficiency.

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