SC508 datasheet(15/32 Pages) SEMTECH

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SC508/SC508A

If the internal LDO is used for bias power, the LDO switch-

over function must be inhibited by selecting the resistor

divider so that the voltage at the VOUT pin does not

exceed 4V; this will inhibit the VLDO switch-over func-

tion. If the SC508 bias power is from an external 5V supply

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

voltage at the VOUT pin is not limited to 4V and can be as

high as the VDDA supply voltage.

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 SC508 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

Programmable Ultrasonic Power-Save Operation

The device provides programmable ultrasonic power-save

operation at light loads; the minimum operating fre-

quency is programmed by connecting a resistor from PSV

to AGND. The SC508 uses the PSV resistor to set an inter-

nal timer that monitors the time between consecutive

high-side gate pulses. If the time exceeds the programmed

timer, DL drives high to turn the low-side MOSFET on. This

draws current from V

OUT through the inductor, forcing both

OUT and VFB to fall. When VFB drops to the 600mV thresh-

old, the next DH on-time is triggered. After the on-time is

completed the high-side MOSFET is turned off and the

low-side MOSFET turns on, and the internal timer is

restarted. The low-side MOSFET remains on until the

inductor current ramps down to zero, at which point the

low-side MOSFET is turned off. This ends the cycle until V

again falls below the 600mV threshold, or the internal

timer forces another DL turn-on.

Applications Information (continued)

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