SSL1623PH datasheet(5/15 Pages) NXP

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SSL1623PH_1

Product data sheet

Rev. 01 — 15 September 2008

5 of 15

NXP Semiconductors

SSL1623PH

case, it operates at a constant frequency, which can be adjusted with external

components RRC and CRC. Furthermore, a primary stroke is started only in a valley of the

secondary ringing. This valley switching principle minimizes capacitive switch-on losses.

8.1 Start-up and undervoltage lockout

Initially, the IC is self-supplying from the rectiﬁed mains voltage. The IC starts switching as

soon as the voltage on pin VCC passes the VCC(startup) level. The supply is taken over by

the auxiliary winding of the transformer as soon as VCC is high enough and the supply

from the line is stopped for high efﬁciency operation.

As soon as the voltage on pin VCC drops below the VCC(stop) level, the IC stops switching

and restarts from the rectiﬁed mains voltage.

8.2 Oscillator

The frequency of the oscillator is set by the external resistor and capacitor on pin RC. The

external capacitor is charged rapidly to the VRC(max) level and, starting from a new primary

stroke, it discharges to the VRC(min) level. Because the discharge is exponential, the

relative sensitivity of the duty factor to the regulation voltage at low duty factor is almost

equal to the sensitivity at high duty factors. This results in a more constant gain over the

duty factor range compared to systems with a linear sawtooth oscillator. Stable operation

at low duty factors is easily realized. For high efﬁciency, the frequency is reduced as soon

as the duty factor drops below a certain value. This is accomplished by increasing the

oscillator charge time.

To ensure that the capacitor can be charged within the charge time, the value of the

oscillator capacitor should be limited to approximately 1 nF.

8.3 Duty factor control

The duty factor is controlled by the internal regulation voltage and the oscillator signal on

pin RC. The internal regulation voltage is equal to the external regulation voltage (minus

2.5 V) multiplied by the gain of the error ampliﬁer (typical 20 dB).

The minimum duty factor of the switched mode power supply is 0 %. The maximum duty

factor is set to 75 % (typical value at 100 kHz oscillation frequency).

8.4 Valley switching

A new cycle is started at the primary stroke when the switch is switched on (see Figure 3).

After a certain time (determined by the RC oscillator voltage 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, see Equation 1.

(1)

Where:

Lp is the primary self-inductance on the drain node.

Cp is the parasitic capacitance on the drain node.

pC p

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