LM1572 datasheet(2/17 Pages) NSC

2 / 17 page

Typical Applications (Fixed/Adjustable Voltage Parts) (Continued)

20033314

Connection Diagram

20033315

16-Lead TSSOP

NS Package Number MTC16

Pin Description

BOOT (Pin 1) - Bootstrap pin. It provides the upper rail for

the floating driver stage of the internal MOSFET switch, the

lower rail being the switching node (Pins 5 and 6). A small

decoupling capacitor (typically 0.1µF-0.22µF) is therefore

connected between the Bootstrap pin and the switching

node. This capacitor should be 0.18µF-0.22µF for applica-

tions with an output voltage greater than 3.3V, if the mini-

mum load (including the current drawn by internal/external

feedback resistor divider) is less than 1mA. Additional drive

voltage is provided by connecting this pin directly to the 5V

output rail via a diode as shown in the Typical Application

Circuit for the fixed voltage part. The same method can be

used for an adjustable part provided the part is adjusted for

an output of 5V. For other output voltages (between 2.42V to

5V) a more general method of providing this external drive

voltage is illustrated in the Typical Application Circuit for the

adjustable part. Note that the NPN signal transistor shown,

must have a guaranteed hfe greater than 400. Damage can

occur to Pin 1 if it is connected (via a diode) to any external

voltage source greater than 6V.

IN (Pin 2) - This is the Analog VIN and provides the supply

to the internal control circuitry, the (Power) V

IN pins (Pins

3,4) providing the supply to the internal power stage. In the

simplest layout scheme, the Analog V

IN pin can simply be

connected to the V

IN pins directly on the pads where the IC

is mounted. But for better noise rejection the trace to Pin 2

can be routed separately from the (Power) V

IN trace, starting

from the positive terminal of the input capacitor. A simple RC

filter solution can also be used for better results, particularly

at low input voltages. This consists of a 10

Ω resistor con-

nected between Analog V

IN and VIN, and a 0.47µF capacitor

between Analog V

IN and Ground. Note that if this RC filter is

used, a 1M

Ω resistor between Pin 1 and Ground is also

required.

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