Operation description - boost section

LED7706

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6.5

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 5 and should not exceed the 100 k

Ω-400 kΩ range.

Equation 5

In addition, when the FSW pin is tied to AVCC, the LED7706 uses a default 660 kHz fixed

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

The FSW pin can also be used as synchronization input, allowing the LED7706 to operate

both as master or slave device. If a clock signal with a 220 kHz minimum frequency is

applied to this pin, the device locks synchronized (270 mV threshold). An Internal time-out

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

Keeping the FSW pin voltage lower than 270 mV for more than 6

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 LED7706 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 LED7706

(master operation).

Figure 8.

Multiple device synchronization

When an external synchronization clock is applied to the FSW pin, the internal oscillator is

overdriven: each switching cycle begins at the rising edge of clock, while the slope

compensation ramp starts at the falling edge of the same signal. Thus, the external

synchronization clock is required to have a 40 % maximum duty-cycle when the boost

converter is working in Continuous-Conduction Mode (CCM). The minimum pulse width

which allows the synchronizing pulses to be detected is 270 ns.

5

.

2

F

R

SW

SYNC

SGND

LED7706

AVCC

FSW

SYNC

SGND

LED7706

SLAVE

FSW

MASTER

Sync Out

SYNC