NCP1546

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8

BOOST Pin

The BOOST pin provides base driving current for the

headroom to turn on the power switch. This in turn reduces

IC power dissipation and improves overall system

efficiency. The BOOST pin can be connected to an external

boost−strapping circuit which typically uses a 0.1

mF capacitor

and a 1N914 or 1N4148 diode, as shown in Figure 1. When the

power switch is turned on, the voltage on the BOOST pin is

equal to

VBOOST + VIN ) VO * VF

where:

The anode of the diode can be connected to any DC

voltage as well as the regulated output voltage (Figure 1).

However, the maximum voltage on the BOOST pin shall not

exceed 40 V.

As shown in Figure 7, the BOOST pin current includes a

constant 7.0 mA pre−driver current and base current

proportional to switch conducting current. A detailed

discussion of this current is conducted in Thermal

Consideration section. A 0.1

mF capacitor is usually

adequate for maintaining the Boost pin voltage during the on

time.

Figure 7. The Boost Pin Current Includes 7.0 mA

Pre−Driver Current and Base Current when the

Switch is Turned On. The Beta Decline of the

Power Switch Further Increases the Base

Current at High Switching Current

0

0.5

1.0

1.5

SWITCHING CURRENT (A)

0

5

10

15

20

25

30

Shutdown

pin rises above the Startup Voltage. This ensures no

switching will occur until adequate supply voltage is

provided to the IC.

The IC enters a sleep mode when the SHDNB pin is pulled

below the Shutdown Threshold Voltage. In sleep mode, the

power switch is kept open and the supply current reduces to

Shutdown Quiescent Current ( 1

mA typically). This pin has

an internal pull−down current. When not in use, pull this pin

Startup

During power up, the regulator tends to quickly charge up

the output capacitors to reach voltage regulation. This gives

rise to an excessive in−rush current which can be detrimental

compensation capacitor provides Soft−Start with no need

for extra pin or circuitry. During the power up, the Output

Source Current of the error amplifier charges the

voltage ramp up gradually. The Soft−Start duration can be

calculated by

TSS +

VC

CCOMP

ISOURCE

where:

equal to error amplifier’s reference voltage.

Using a 0.1

5.0 ms which is adequate to avoid any current stresses.

after the output voltage reaches the regulation. If the supply

over−shoot.

Figure 8. The Power Up Transition of NCP1546

Regulator