SC461 datasheet(14/31 Pages) SEMTECH

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SC461

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 SC461 uses an external resistor to set the on-time which

indirectly sets the frequency. The on-time can be pro-

grammed to provide an operating frequency of up to 1MHz

using a resistor between the TON pin and ground. The resis-

tor value is

selected by the following equation.

The maximum recommended R

TON value is shown by the

following equation.

Immediately after the on-time, the DL output drives high

to energize the low-side MOSFET. DL has a minimum high

time of ~250ns, after which DL continues to stay high until

one of the following occurs:

The FB input falls below the 600mV reference

The Zero Cross Detector trips if power-save is

active

TON Limitations and VDDA Supply Voltage

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

IN. The previous RTON equation is accurate if VIN satisfies

the below relation over the entire V

IN range:

IN < (VDDA - 1.6V) x 10

If V

IN exceeds ((VDDA - 1.6V) x 10) for all or part of the VIN

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

cases where VIN > ((VDDA - 1.6V) x 10), the RTON equation

must be modified as follows.

•

Note that when V

IN is greater than ((VDDA - 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 V

IN rather than approximating 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 600mV reference voltage (see Figure 3).

To FB pin

VOUT

Figure 3 — Output Voltage Selection

Note that this control method regulates the valley of the

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

OUT is offset by the output ripple according to the follow-

ing equation.

0.6

RIPPLE

OUT

In some applications a small capacitor C

TOP is placed in

parallel with R1 to provide a larger ripple signal from V

OUT

to the FB pin. In these applications, the output voltage

OUT is calculated according to the following equation in

which w represents the switching frequency.

Configuring V

OUT Greater Than 5V

The switcher output voltage can be programmed higher

than 5V with careful attention to the VOUT and RTON pins.

In these applications the VOUT pin cannot connect directly

Applications Information (continued)

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