SC461 datasheet(15/31 Pages) SEMTECH

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SC461

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

The resistors must be chosen so that the VOUT does not

exceed the VDDA supply. Note that the VOUT pin has an

internal 500kW resistor connected to AGND. To minimize

the effect of this resistor on the resistor divider ratio, the

maximum recommend value for resistor R

V2 in Figure 4 is

10kW.

In addition to the resistor divider, the RTON resistor value

must be adjusted. The on-time is calculated according to

the voltage at the VOUT pin. In order to select the desired

on-time and operating frequency, the RTON resistor

should be adjusted to a higher value to compensate for

the reduced voltage at the VOUT pin. For output voltages

exceeding 5V, the required RTON value can be determined

by the following equation.

For applications where V

OUT exceeds 5V, FCM operation is

recommended.

Forced Continuous Mode Operation

The SC461 operates the switcher in Forced Continuous

Mode (FCM) by connecting the PSV pin to VDDA. The PSV

pin should never exceed the VDDA supply. See Figure 5

for FCM waveforms. In this mode one of the power

MOSFETs is always on, with no intentional dead time other

than to avoid cross-conduction. This results in more

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.

The PSV pin contains a 5μA current sink to prevent stray

leakage current from pulling the PSV pin up to the VDDA

supply when the PSV pin is floated to select Power-Save

operation. To select Forced Continuous Mode operation,

the maximum recommended resistance between the

VDDA supply and the PSV pin is 40kW.

FB Ripple

Voltage (VFB)

FB threshold

Inductor

Current

DC Load Current

DH on-time is triggered when

VFB reaches the FB Threshold.

On-time

(TON)

DL drives high when on-time is completed.

DL remains high until VFB falls to the FB threshold.

Figure 5 — Forced Continuous Mode Operation

Power-Save Mode Operation

The SC461 provides power-save operation at light loads

with no minimum operating frequency, selected by float-

ing the PSV pin (no connection). In this mode of opera-

tion, the zero cross comparator monitors inductor current

via the voltage across the low-side MOSFET during the

off-time. If the inductor current falls to zero for 8 consecu-

tive switching cycles, the controller enters power-save

operation. It will then turn off the low-side MOSFET on

each subsequent cycle, provided that the current falls to

zero. After the low-side MOSFET is off, both high-side and

low-sides MOSFETs remain off until V

FB drops to the 600mV

threshold. While the MOSFETs are off the load is supplied

Applications Information (continued)

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