LED6000PHR datasheet(12/49 Pages) STMICROELECTRONICS

12 / 49 page

Functional description

LED6000

12/49

DocID027777 Rev 2

If the FSW pin is left floating, the programmed frequency is 250 kHz (typ.); if the FSW pin is

connected to an external resistor, the programmed switching frequency can be increased up

to 1.5 MHz, as shown in Figure 5. The required RFSW value (expressed in k) is estimated

by Equation 1:

Equation 1

Figure 5. Switching frequency programmability

To improve the line transient performance, keeping the PWM gain constant versus the input

voltage, the input voltage feedforward is implemented by changing the slope of the sawtooth

ramp, according to the input voltage change (Figure 6 a).

The slope of the sawtooth also changes if the oscillator frequency is programmed by the

external resistor. In this way a frequency feedforward is implemented (Figure 6 b) in order to

keep the PWM modulator gain constant versus the switching frequency.

On the SYNCH pin the synchronization signal is generated. This signal has a phase shift of

180° with respect to the clock. This delay is useful when two devices are synchronized

connecting the SYNCH pins together. When SYNCH pins are connected, the device with

a higher oscillator frequency works as a master, so the slave device switches at the

frequency of the master but with a delay of half a period. This helps reducing the RMS

current flowing through the input capacitor. Up to five LED6000s can be connected to the

same SYNCH pin; however, the clock phase shift from master switching frequency to the

slave input clock is 180°.

The LED6000 device can be synchronized to work at a higher frequency, in the range of

250 kHz - 1500 kHz, providing an external clock signal on the SYNCH pin. The

synchronization changes the sawtooth amplitude, also affecting the PWM gain (Figure 6 c).

This change must be taken into account when the loop stability is studied. In order to

FSW

kHz

12500

250

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LED6000PHR 数据表(PDF) 12 Page - STMicroelectronics