Philips Semiconductors

Product specification

NE/SE5561

Switched-mode power supply control circuit

1994 Aug 31

5

R

VCC +

V

S *

V

CC

(10 20mA)

NOTE:

See DC Electrical Characteristics for Current Fed VCC Range.

NE5561 Voltage-Current-Fed

Supply Characteristics

Current-Fed Dropping Resistor

VS

RVCC

VCC

GND

1

8

mA

20

10

0

0

10

20

30

V

SL00387

Figure 4. NE5561 Voltage-Current-Fed Supply Characteristics

NE5561 START-UP

The start-up, or initial turn-on, of this device requires some degree of

external protective duty cycle limiting to prevent the duty cycle from

initially going to the extreme maximum (

δ>90%). Either overcurrent

limit or slow-start circuitry must be employed to limit duty cycle to a

safe value during start-up. Both may be used, if desired.

To implement slow-start, the start-up circuit can be used. The divider

R1 and R2 sets a voltage, buffered by Q1, such that the output of

the error amplifier is clamped to a maximum output voltage, thereby

limiting the maximum duty cycle. The addition of capacitor C will

cause this voltage to ramp-up slowly when power is applied, causing

the duty cycle to ramp-up simultaneously.

Overcurrent limit may be used also. To limit duty cycle in this mode,

the switch current is monitored at Pin 6 and the output of the 5561 is

disabled on a cycle-by-cycle basis when current reaches the

programmed limit. With current limit control of slow-start, the duty

cycle is limited to that value, just allowing maximum switch current

to flow. (Approximately 0.50V measured at Pin 6.)

APPLICATIONS

5V, 0.5A Buck Regulator Operates from 15V

The converter design shows how simple it is to derive a TTL supply

from a system supply of 15V (see Figure 1). The NE5561 drives a

2N4920 PNP transistor directly to provide switching current to the

inductor.

Overall line regulation is excellent and covers a range of 12V to 18V

with minimal change (<10mV) in the output operating at full load.

As with all NE5561 circuits, the auxiliary slow start and

is required, as evidenced by Q1. The

by using the relationship:

R2

R1

) R2

(8.2V)

+

V

d

MAX

The maximum duty cycle is then determined from the pulse-width

modulator transfer graph, with R1 and R2 being defined from the

desired conditions.