AN1049 datasheet(3/24 Pages) STMICROELECTRONICS

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The L5991 can be used in off-line SMPS’ with any single-ended topology. However, its features make

the device particularly useful for power supplies based on flyback topology for office equipment that

must comply with standards concerning energy saving. Monitor displays, printers, photocopiers, scan-

ners and fax machines are the most noticeable examples.

Standby function description

The L5991 automatically detects a light load

condition for the converter and decreases the

oscillator frequency on that occurrence. The nor-

mal (higher) oscillation frequency is automat-

ically resumed when the output load builds up

and exceeds a defined threshold. This function-

ality is called "Standby function".

Like in every "peak" current mode controller, the

output voltage (VCOMP) of the Error Amplifier

(E/A) of the device moves depending on the

power drawn from the mains (see Appendix

"Peak Current Mode Control Basics"). The basic

principle of the Standby is then monitoring the

E/A output .

If the peak primary current decreases as a result of a decrease of the power demanded by the load

VCOMP will decrease as well. If this falls below a fixed threshold (VT1), the oscillator, which was working

at fosc, will be forced to work at a lower value (fSB). The frequency drop, however, implies a sudden in-

crease of the peak primary current and, therefore, of VCOMP. Some hysteresis will be necessary to pre-

vent the frequency from switching back to fosc. In fact, VCOMP will have to exceed a second threshold

(VT2 > VT1) in order for the oscillator frequency to be reset at fosc. The hysteresis (VT2-VT1) will be large

enough to prevent the above mentioned undesired phenomenon. This operation is shown in fig. 2.

Fig. 3 shows how the function is implemented internally. Only one pin (ST-BY, 16) is required and is

used to connect an external resistor (RB) to the oscillator pin (RCT, 2). In this way, both the normal and

the standby frequency are externally programmable.

The capacitor CT and the resistor RB, along with RA, set the operating frequency of the oscillator in nor-

mal operation (fosc). In fact, as long as the STANDBY signal is high, the pin is internally connected to

the reference voltage VREF by a N-channel FET (see fig. 3), thus the timing capacitor CT is charged

through RA and RB. When the STANDBY signal goes low the N-channel FET is turned off and the pin

becomes floating. RB is now disconnected and CT is charged through RA only. In this way the oscillator

frequency (fSB) will be lower.

The oscillation frequency can be estimated with the following approximate relationships:

fosc

≈

⋅ (0.693 ⋅ (RA ⁄ ⁄ RB) + KT)

(1),

234

VCOMP

fosc

fSB

Stand-by

Normal operation

VT1

PNO

PSB

VT2

Figure 2.Standby function dynamic operation.

2.5

2.5/4

66K

Ω

STANDBY

10V

LEVEL SHIFT

COMP

VREF

ST-BY

RCT

LOW

HIGH

STANDBY

D97IN752A

VT1

2.6V

VT2

VCOMP

ISEN

DRIVER

CUT

STANDBY BLOCK

Figure 3. Standby function internal schematic and operation

AN1049 APPLICATION NOTE

3/24

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