SSL1750_1

© NXP B.V. 2008. All rights reserved.

Product data sheet

Rev. 01 — 15 September 2008

16 of 29

NXP Semiconductors

SSL1750

SMPS control IC for LED drivers

The driver output is latched in the logic, preventing multiple switch-on.

7.3.4 Demagnetization (pin FBAUX)

The system is always in quasi-resonant or discontinuous conduction mode. The internal

oscillator does not start a new primary stroke until the previous secondary stroke has

ended.

Demagnetization features a cycle-by-cycle output short circuit protection by immediately

lowering the frequency (longer off-time), thereby reducing the power level.

This suppression may be necessary at low output voltages and at start-up and in

applications where the transformer has a large leakage inductance.

If pin FBAUX is open-circuit or not connected, a fault condition is assumed and the

converter stops operating immediately. Operation restarts as soon as the fault condition is

removed.

7.3.5 Flyback control / time-out (pin FBCTRL)

Pin FBCTRL is connected to an internal voltage source of 3.5 V via an internal resistor

(typical resistance is 3 k

Ω). As soon as the voltage on this pin is above 2.5 V (typ), this

connection is disabled. Above 2.5 V the pin is biased with a small current. When the

voltage on this pin rises above 4.5 V (typ), a fault is assumed and switching is inhibited.

When a small capacitor is connected to this pin, a time-out function can be created to

protect against an open control loop situation (see Figure 12 and Figure 13). The time-out

function can be disabled by connecting a resistor (100 k

Ω) to ground on pin FBCTRL.

If the pin is shorted to ground, switching of the flyback controller is inhibited.

In normal operating conditions, when the converter is regulating the output, the voltage on

pin FBCTRL is between 1.4 V and 2.0 V (typical values) from minimum to maximum

output power.

Fig 11. Frequency control of flyback part

1.5 V

2.0 V

0.52 V

flyback

frequency

reduction

PFC burst mode

FBSENSE peak voltage

014aaa028

PFC on

flyback

discontinuous

or QR

flyback

cycle skip

mode

1.4 V

0.13 V