LED7707 datasheet(16/48 Pages) STMICROELECTRONICS

16 / 48 page

Operation description

LED7707

16/48

5.1.5

Switching frequency selection and synchronization

The switching frequency of the boost converter can be set in the 250 kHz-1 MHz range by

connecting the FSW pin to ground through a resistor. Calculation of the setting resistor is

made using equation 4 and should not exceed the 100 k

Ω-400 kΩ range.

Equation 4

In addition, when the FSW pin is tied to AVCC, the LED7707 uses a default 660 kHz fixed

switching frequency, allowing to save a resistor in minimum component-count applications.

Figure 10.

Multiple device synchronization

The FSW pin can also be used as synchronization input, allowing the LED7707 to operate

both as master or slave device. If a clock signal with a 210 kHz minimum frequency is

applied to this pin, the device locks synchronized. The signal provided to the FSW pin must

cross the 270 mV threshold in order to be recognized. The minimum pulse width which

allows the synchronizing pulses to be detected is 270 ns. An Internal time-out allows

synchronization as long as the external clock frequency is greater than 220 kHz.

Keeping the FSW pin voltage lower than 270 mV for more than 4.5 µs results in device

turned off. Normal operation is resumed as soon as FSW rises above the mentioned

threshold and the soft-start sequence is repeated.

The SYNC pin is a synchronization output and provides a 35 % (typ.) duty-cycle clock when

the LED7707 is used as master or a replica of the FSW pin when used as slave. It is used to

connect multiple devices in a daisy-chain configuration or to synchronize other switching

converters running in the system with the LED7707 (master operation). When an external

synchronization clock is applied to the FSW pin, the internal oscillator is over-driven: each

switching cycle begins at the rising edge of clock, while the slope compensation (Figure 11)

ramp starts at the falling edge of the same signal. Thus, to prevent sub-harmonic instability

(see Section 5.1.6), the external synchronization clock is required to have a 40 % maximum

duty-cycle when the boost converter is working in continuous-conduction mode (CCM) in

order to assure that the slope compensation is effective (starts with duty-cycle lower than

40%)

FSW =

SYNC

SGND

LED7707

AVCC

RFSW

FSW

SYNC

SGND

LED7707

SLAVE

FSW

MASTER

Sync Out

SYNC

AM00587v1

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