LED7708 datasheet(25/53 Pages) STMICROELECTRONICS

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LED7708

Recommended operating conditions

Doc ID 022698 Rev 1

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In addition, when the FSW pin is tied to LDO3, the LED7708 uses a default 610 kHz fixed

switching frequency, allowing the saving of a resistor in minimum component-count

applications.

The FSW pin can also be used as synchronization input, allowing the LED7708 to operate

both as master or slave device. If a clock signal having a frequency within the capture range

is applied to this pin, the device locks synchronized. The minimum pulse width which allows

the synchronizing pulses to be detected is 270 ns (typ.). The SYNC pin is a synchronization

output and provides a 35% (typ.) duty cycle clock when the LED7708 is set as master or a

replica of the FSW pin when it is set 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 LED7708. When an external synchronization clock is applied to the FSW pin, the

internal oscillator is overdriven and the external power MOSFET is ignited on the rising edge

of the synchronization signal while the slope compensation ramp starts on the falling edge

of the same signal. For this reason, the duty cycle of the external synchronization clock

should be 30% to 40% to prevent sub-harmonic instability when the boost converter is

working in continuous-conduction mode (CCM).

8.3.3

Slope compensation

The constant frequency, peak current-mode topology has the advantage of very easy loop

compensation with output ceramic caps (reduced cost and size of the application) and fast

transient response. In addition, the intrinsic peak current measurement simplifies the current

limit protection, avoiding undesired saturation of the inductor. On the other side, this

topology has a drawback: there is inherent open loop instability when operating with a duty

cycle greater than 0.5. This phenomenon is known as “sub-harmonic instability” and can be

avoided by adding an external ramp to the one coming from the sensed current. This

compensating technique, based on the additional ramp, is called “slope compensation”. As

seen in Figure 6, when the switching duty cycle is higher than 0.5, the small perturbation

ΔIL

dies away in subsequent cycles thanks to the slope compensation and the system reverts to

a stable situation.

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