0.5A, 300kHz, SO-8 PWM/PFM Step Down Converter with Shutdown

Impala Linear Corporation

5

(408) 574-3939

www.impalalinear.com

June 1999

ILC6377 1.3

+

V

OUT

ILC6377

(TOP VIE W)

3

45

1

2

6

7

8

+

C

L

V

IN

S/D

C

SS

SD1

L

*C

IN

Fig 1. Typical step-down DC-DC

converter application

SD1 : MA735 Schottky Diode ( MATSUSHITA )

22

µH

47

µF

10

µF

External component selection

Proper selection of external components is important for

achieving high performance. The output inductor selected

should have low DC resistance on the order of 0.2W or less

and

saturation

current

rating

of

1A

or

higher.

Recommended inductors are Sumida CD54 (22mH, 0.18W

max DC resistance) or Coilcraft DO3308P-223 (22mH,

0.18W max DC resistance) or equivalent.

The catch diode should be a schottky diode with low for-

ward drop and rated at 1A or greater current, MA735 or it’s

equivalent is recommended.

Input and output capacitors should be tantalum capacitors

with low equivalent series resistance (ESR) and voltage rat-

ing higher than the actual application.

Soft-start

Pin 4 of ILC6377 functions as the soft-start pin as well as the

shutdown pin. A soft-start capacitor (from pin 4 to ground)

controls the rate at which the power supply starts up; thus

preventing large overshoots at the output as well as large in-

rush current. The value for CSS should be 100pF or greater.

Shutdown

The ILC6377 is placed in shutdown mode by taking pin 4 to

ground. In shutdown, the quiescent current of the device is

under 2mA. When using the shutdown feature, pin 4 must

be driven from an open collector or open drain output with-

out employing an external pull-up resistor, as shown in fig-

ure 2. Since pin 4 is also used to charge an external capac-

itor for soft-start, this pin should not be driven from a push-

pull CMOS type output.

Over-current and short-circuit protection

In the event of an over-current or short-circuit condition, the

ILC6377 cycles the soft-start pin in a hiccup mode to provide

fault protection. When the output voltage decreases due to

overload, the ILC6377 will operate continuously at the max-

imum duty cycle. If the period of maximum duty cycle oper-

ation exceeds TPRO (typically 5 msec), pin 4 will be pulled

low; thus discharging the external soft-start capacitor CSS.

This action inhibits the regulator’s PWM action. Next, the

ILC6377’s soft-start circuitry starts recharging CSS and initi-

ates a controlled start-up. If the overload condition continues

to exist, the above sequence of events will repeat; thus con-

tinuing to cycle the soft-start function.

Note that very little power is dissipated with this method of

fault protection versus constant current limit protection.

Even though the internal power MOSFET is pulsed on and

off at high peak current, the DC current is low; thus leading

to low power dissipation even under short-circuit conditions.

Keep in mind that the duration of maximum duty cycle

condition is used to trigger the

differential voltage may trigger the device’s fault protec-

tion circuitry even at low output current.

Undervoltage Lockout

The undervoltage lockout feature prevents faulty operation

by disabling the operation of the regulator when input volt-

age is below the minimum operating voltage, VUVLO. When

the input voltage is lower than VUVLO the device disables

the internal P-channel MOSFET and provides “high” output

at both EXT1 and EXT2 outputs.

Figure 2 shows a typical fixed output voltage step-down DC-DC

converter application circuit for ILC6377SO-XX

Fig. 1: Typical step-down wn DC-DC

converter application