SC419 datasheet(12/24 Pages) SEMTECH

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SC419

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 SC419 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 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:

VFB falls below the 500mV reference

The Zero Cross Detector trips if power-save is

active

OUT Voltage Selection

The output voltage is regulated by comparing V

OUT as seen

through a resistor divider at the FB pin to the internal

500mV reference (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.

•

RIPPLE

OUT

Forced Continuous Mode Operation

The SC419 operates in FCM (Forced Continuous Mode) by

connecting the PSV pin to VDDA. (The PSV pin should not

exceed the VDDA supply.) See Figure 4 for FCM waveforms.

In this mode one of the power MOSFETs is always on, with

no intentional dead time other than to avoid cross-con-

duction. This results in uniform frequency across the full

load range with the trade-off being reduced efficiency at

light loads due to the high-frequency switching of the

MOSFETs.

FB Ripple

Voltage (VFB)

FB threshold

Inductor

Current

DC Load Current

DH on-time is triggered when

VFB reaches the FB Threshold.

(500mV)

On-time

(TON)

DL drives high when on-time is completed.

DL remains high until VFB falls to the FB threshold.

Figure 4 — Forced Continuous Mode Operation

The switcher output voltage can be programmed higher

than 5V with careful design. In this case the VOUT pin

cannot connect directly to the switcher output due to its

the maximum voltage rating. An additional resistor

divider network is required to connect from the switcher

output to the VOUT pin. The voltage at the VOUT pin

should be at least 500mV lower than the VDDA supply, to

prevent the VLDO switch-over function. For example, the

voltage at the VOUT pin can be 4V if VDDA is set for 5V.

When the SC418 operates from an external power source

and the LDO is disabled by grounding the ENL pin, the

voltage at the VOUT pin can be as high as shown in

Recommended Operating Conditions. Note that RTON

must be adjusted higher by the same divider ratio to

Applications Information (continued)

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