SC403 datasheet(16/32 Pages) SEMTECH

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SC403

One-Shot Timer and Operating Frequency

One-shot timer operation is shown in Figure 2. The FB

Comparator output goes high when V

FB is less than the

internal 750mV reference. This feeds into the gate drive

and turns on the high-side MOSFET, and starts the one-

shot timer. The one-shot timer uses an internal compara-

tor and a capacitor. One comparator input is connected to

OUT, the other input is connected to the capacitor. When

the on-time begins, the internal capacitor charges from

zero volts through a current which is proportional to V

IN.

When the capacitor voltage reaches V

OUT, the on-time is

completed and the high-side MOSFET turns off.

Gate

Drives

FB Comparator

One-Shot

Timer

On-time = K x RTON x (VOUT/VIN)

VOUT

VIN

750mV

COUT

VIN

ESR

VOUT

VLX

RTON

Figure 2 — On-Time Generation

This method automatically produces an on-time that is

proportional to V

OUT and inversely proportional to VIN.

Under steady-state conditions, the switching frequency

can be determined from the on-time by the following

equation.

OUT

The SC403 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

)

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 eight cycles after a zero crossing is

detected.

TON limitations and V

DD Supply Voltage

For V

DD below 4.5V, the TON accuracy may be limited by

the input voltage.

The original R

TON equation is accurate if VIN satisfies the

relationship over the entire V

IN range, as follows.

IN < (VDD - 1.6V) x 10

If V

IN exceeds (V

DD - 1.6V) x 10, for all or part of the VIN

range, the R

TON equation is not accurate. In all cases where

IN > (V

DD - 1.6V) x 10, the RTON equation must be modified,

as follows.

OUT

TON

25pF

1.6V)

10ns)

Note that when V

IN > (V

DD - 1.6V) x 10 , the actual on-time

is fixed and does not vary with V

IN. When operating in this

condition, the switching frequency will vary inversely with

IN 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.

To FB pin

VOUT

Figure 3 — Output Voltage Selection

•

Applications Information (continued)

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