SSL152X_2

© NXP B.V. 2009. All rights reserved.

Product data sheet

Rev. 02 — 10 March 2009

6 of 17

NXP Semiconductors

SSL152x

SMPS ICs for low power systems

8.4 Valley switching

A new cycle is started when the primary switch is switched on (see Figure 4). After a

certain time (determined by the oscillator voltage RC and the internal regulation level), the

switch is turned off and the secondary stroke starts. The internal regulation level is

determined by the voltage on pin REG.

After the secondary stroke, the drain voltage shows an oscillation with a frequency of

approximately:

(1)

where:

As soon as the oscillator voltage becomes high again and after the secondary stroke has

ended, the circuit waits for a low drain voltage before starting a new primary stroke.

Figure 4 shows the drain voltage together with the valley signal, the signal indicating the

secondary stroke and the RC voltage.

The primary stroke starts some time before the actual valley at low ringing frequencies,

and some time after the actual valley at high ringing frequencies.

A: Start of new cycle with valley switching.

B: Start of new cycle in a classical system.

Fig 4.

Signals for valley switching

1

2

π

×

L

p

C

p

×

()

×

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mgt423

drain

valley

secondary

ringing

secondary

stroke

primary

stroke

secondary

stroke

RC

oscillator

regulation level

A

B