SC2620 datasheet(14/26 Pages) SEMTECH

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SC2620

POWER MANAGEMENT

Applications Information

input power supply will be

SW T

(the base charge of the

switch). The energy loss due to base charge per cycle is

for a power loss of

≈

OUT

If D

BST is tied to the output, then the charge drawn from

the output capacitor will still be

SW T

. The energy loss

due to base charge per cycle is

OUT

for a power

loss of

OUT

SW V

Since V

OUT < VIN, DBST should always be tied to VOUT (if >2.5V)

to maximize efficiency. In general efficiency penalty

increases as D decreases.

Figure 7 summarizes various ways of bootstrapping the

SC2620. A fast switching PN diode (such as 1N4148 or

1N914) and a small (0.1µF – 0.47µF) ceramic capacitor

can be used. In Figure 7(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 BOOST pin. The maximum BOOST pin voltage is

about

OUT

IN V

. If the output is below 2.8V, then D

BST will

preferably be a small Schottky diode (such as BAT-54) to

maximize bootstrap voltage. A 0.33-0.47µF bootstrap

capacitor may be needed to reduce droop. Bench

measurement shows that using Schottky bootstrapping

diode has no noticeable efficiency benefit.

The SC2620 can also be bootstrapped from the input

(Figure 7(b)). This configuration is not as efficient as Figure

7(a). However this may be 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 BOOST pin can be

somewhat higher than 2V

IN. The Zener diode in Figure 7(c)

reduces the maximum BOOST pin voltage. The BOOST pin

voltage should not exceed its absolute maximum rating of

42V.

Figures 7(d) and (e) show how to bootstrap the SC2620

from a second power supply V

S with voltage > 2.5V. VS in

Figure 7(d) can be the output of the other channel. Figures

1(a), 17(a) and 18(a) show this bootstrapping method. If

Channel 1 fails in these converters, Channel 2 will be shut

off (See Sequencing the Outputs). Proper bootstrapping

of Channel 2 therefore depends on the readiness of V

OUT1.

This may be a drawback in some applications. D

BST in Figure

7(e) prevents start up difficulty if V

IN comes up before VS.

Figure 8. Minimum Input Voltage Required to Start and to Maintain Bootstrap.(T

A = 25°C).

Minimum Starting and

Sustaining VIN vs Load Current

4.5

5.0

5.5

6.0

6.5

7.0

7.5

110

100

1000

Load Current (mA)

BST TIED

TO OUTPUT

BST TIED

TO INPUT

OUT = 5V

STARTING

SUSTAINING

MA729

(a)

(b)

Minimum Starting and

Sustaining VIN vs Load Current

3.5

4.0

4.5

5.0

5.5

0.1

1.0

10.0

100.0

1000.0

Load Current (mA)

BST TIED

TO OUTPUT

BST TIED

TO INPUT

OUT = 3.3V

STARTING

SUSTAINING

MA729

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