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LTC3703

3703f

output remains heavy. To avoid this, the auxiliary output

voltage can be divided down with a conventional feedback

resistor string with the divided auxiliary output voltage fed

back to the MODE/SYNC pin. The MODE/SYNC threshold

is trimmed to 800mV with 20mV of hysteresis, allowing

precise control of the auxiliary voltage and is set as

follows:

SEC MIN

()

≈+









where R1 and R2 are shown in Figure 10c.

If the LTC3703 is operating in Pulse Skip Mode and the

auxiliary output voltage drops below VSEC(MIN), the MODE/

SYNC pin will trip and the LTC3703 will resume continu-

ous operation regardless of the load on the main output.

Thus, the MODE/SYNC pin removes the requirement that

power must be drawn from the inductor primary in order

to extract power from the auxiliary winding. With the loop

in continuous mode (MODE/SYNC < 0.8V), the auxiliary

outputs may nominally be loaded without regard to the

primary output load.

The following table summarizes the possible states avail-

able on the MODE/SYNC pin:

Table 1.

MODE/SYNC Pin

Condition

DC Voltage: 0V to 0.75V

Forced Continuous

Current Reversal Enabled

DC Voltage:

≥ 0.87V

Pulse Skip Mode Operation

No Current Reversal

Feedback Resistors

Regulating a Secondary Winding

Ext. Clock: 0V to

≥ 2V

Forced Continuous

No Current Reversal

MODE/SYNC Pin (External Synchronization)

The internal LTC3703 oscillator can be synchronized to an

external oscillator by applying and clocking the MODE/

SYNC pin with a signal above 2VP-P. The internal oscillator

locks to the external clock after the second clock transition

is received. When external synchronization is detected,

LTC3703 will operate in forced continuous mode. If an

external clock transition is not detected for three succes-

sive periods, the internal oscillator will revert to the

frequency programmed by the RSET resistor. The internal

If RUN/SS starts at 0V, the delay before starting is approxi-

mately:

F C

DELAY START

)

=µ

025

plus an additional delay, before the output will reach its

regulated value, of:

F C

DELAY REG

–

)

≥

=µ

The start delay can be reduced by using diode D1 in

Figure 18.

Figure 18. RUN/SS Pin Interfacing

MODE/SYNC Pin (Operating Mode and Secondary

Winding Control)

The MODE/SYNC pin is a dual function pin that can be used

for enabling or disabling Pulse Skip Mode operation and

also as an external clock input for synchronizing the

internal oscillator (see next section). Pulse Skip Mode is

enabled when the MODE/SYNC pin is above 0.8V and is

disabled, i.e. forced continuous, when the pin is below

0.8V.

In addition to providing a logic input to force continuous

operation and external synchronization, the MODE/SYNC

pin provides a means to regulate a flyback winding output

as shown in Figure 10c. The auxiliary output is taken from

a second winding on the core of the inductor, converting

it to a transformer. The auxiliary output voltage is set by

the main output voltage and the turns ratio of the extra

winding to the primary winding as follows:

VSEC ≈ (N + 1)VOUT

Since the secondary winding only draws current when the

synchronous switch is on, load regulation at the auxiliary

output will be relatively good as long as the main output is

running in continuous mode. As the load on the primary

output drops and the LTC3703 switches to Pulse Skip

Mode operation, the auxiliary output may not be able to

maintain regulation, especially if the load on the auxiliary

APPLICATIO S I FOR ATIO

3.3V

OR 5V

RUN/SS

CSS

3703 F18

RUN/SS

CSS

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