10

SCD5031 Rev B

If desired, the user can synchronize the oscillator to an external frequency source by coupling a pulse train to the Cset

pin:

FIGURE 5 – PWM can be synchronized to external source

with just two additional components.

Operation is similar to the free running case. Cset is alternately charged and discharged by the same current mirrors and

the same comparator and thresholds are used. The only difference is that when a sync pulse is received, the capacitor

voltage is level shifted up and reaches the comparator’s upper threshold voltage before it normally would in the free

running case. If a series of pulses are received with shorter period than that of the free running oscillator, the comparator

will trip in response to the sync pulse and the oscillator will be synchronized. (NOTE: The user must ensure that the sync

simple diode clamp to the positive rail should be used to prevent damage to the PWM)

ERROR AMPLIFIER

The main error amplifier is a N-type input folded cascode configuration with a few interesting additions. The positive

input is internally tied to 2.5V derived from the on chip reference. The negative input typically draws less than 1µA and

has a voltage offset of less than 2mV. At 20µA bias current, the amplifier exceeds 2MHz bandwidth and 120dB open

loop gain (see Figure 7).

The amplifier is designed to limit at whatever voltage is applied to the SOFT pin. As mentioned previously, this function

will allow the user to implement a softstart circuit, a controlled turn-on delay, or any number of other useful functions.

Sync Pulse

2nF

24

Ω

Cset

To PWM

Pr

el

im

in

ary