SC414 datasheet(15/29 Pages) SEMTECH

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SC414/SC424

This method automatically produces an on-time that is

proportional to V

OUT

and inversely proportional to V

Under steady-state conditions, the switching frequency

can be determined from the on-time by the following

equation.

OUT

The SC414/SC424 uses an external resistor to set the on-

time which indirectly sets the frequency. The on-time can

be programmed to provide an operating frequency from

200kHz to 1MHz using a resistor between the TON pin and

ground. The resistor value is

selected by the following

equation.

OUT

TON

400

The maximum R

TON

value allowed is shown by the follow-

ing equation.

MIN

MAX

TON

Immediately after the on-time, the DL (drive signal for the

low side FET) output drives high to turn on the low-side

MOSFET. DL has a minimum high time of ~320ns, after

which DL continues to stay high until one of the following

occurs:

VFB falls below the 750mV reference

The Zero Cross Detector senses that the voltage

on the LX node is below ground. Power Save is

activated when a zero crossing is detected.

TON limitations and V5V Supply Voltage

For V5V below 4.5V, the TON accuracy may be limited by

the input voltage.

The original RTON equation is accurate if V

satisfies the

below relation over the entire V

range:

IN < (V5V - 1.6V) x 10

If V

exceeds (V5V - 1.6V) x 10, for all or part of the V

range, the RTON equation is not accurate. In all cases

where V

> (V5V - 1.6V) x 10, the RTON equation must be

modified as follows.

•

OUT

TON

1.6V)

(V5V

400

25pF

Note that when V

> (V5V - 1.6V) x 10 , the actual on-time

is fixed and does not vary with V

. When operating in this

condition, the switching frequency will vary inversely with

rather than approximating a fixed frequency.

OUT

Voltage Selection

The switcher output voltage is regulated by comparing

OUT

as seen through a resistor divider at the FB pin to the

internal 750mV reference voltage (see Figure 3).

VOUT

To FB pin

Figure 3 — Output Voltage Selection

Note that this control method regulates the valley of the

output ripple voltage, not the DC value. The DC output

voltage V

OUT

is offset by the output ripple according to the

following equation.

RIPPLE

OUT

When a large capacitor is placed in parallel with R1 (C

TOP

)

OUT

is shown by the following equation.

TOP

RIPPLE

OUT

)

(

Where

ω is the angular switching frequency.

Enable and Power-save Inputs

The EN/PSV and ENL inputs are used to enable or disable

the switching regulator and the LDO. When EN/PSV is low

(grounded), the switching regulator is off and in its lowest

power state. When off, the output of the switching regula-

tor soft-discharges the output into a 10Ω internal resistor

via the VOUT pin. When EN/PSV is allowed to float, the pin

voltage will float to 33% of the voltage at V5V. The switch-

ing regulator turns on with power-save disabled and all

switching is in forced continuous mode. For V5V < 4.5V, it

Applications Information (continued)

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