SC508 datasheet(14/32 Pages) SEMTECH

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SC508/SC508A

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 SC508 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

Note that the on-time variation increases to typically 15%

at input voltages from 4.5 to 10V. In most applications this

does not significantly affect overall performance.

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 CTOP is placed in

parallel with R1 to provide a larger ripple signal from VOUT

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

VOUT 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

to the switcher output due to its maximum voltage rating.

An additional resistor divider network is required to

connect from the switcher output to the VOUT pin as

shown in Figure 4.

To VOUT pin

VOUT > 5V

RV1

COUT

RV2

Figure 4 — Resistor Divider For V

OUT Exceeding 5V

Applications Information (continued)

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