SC3203 datasheet(13/20 Pages) SEMTECH

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SC3203

Applications Information (continued)

The output voltage is approximated by the following

equation:

Note that the Adaptive On-time control regulates the

valley of the FB ripple voltage, not the DC value. In

practice the DC value of FB and V

OUT can be slightly

higher than the value predicted by DC equations; this is

easily corrected by reducing the value of R

TOP slightly. For

recommended values of the FB components for different

VOUTs, see the Table in the Detailed Application Circuit.

Continuous Mode Operation

The SC3203 operates in CCM (Continuous Conduction

Mode) when the load current exceeds 50% of the induc-

tor ripple current (Figure 3). In this mode one of the

power mosfets is always on, with no intentional dead

time other than to avoid cross-conduction. This mode of

operation results in typically 500kHz operation.

FB Ripple

Voltage (VFB)

FB threshold

Inductor

Current

DC Load Current

LX on-tim e is triggered when VFB

reaches the FB Threshold.

(750m V)

O n-tim e

(TON)

LX drives to PG ND when on-tim e is com pleted.

LX rem ains low (PG ND) until VFB falls to the FB threshold.

Figure 3 — Continuous Mode Operation

Power-save Operation

At light loads the SC3203 enters power-save mode to

improve efficiency. During the low-side on-time, the

internal zero-cross comparator monitors inductor current

via the LX voltage across the low-side mosfet. If the

inductor current falls to zero for 8 consecutive switching

cycles, the controller enters power-save (PSave) mode.

In PSave mode, after the high-side on-time has completed

and the low-side mosfet is on, the low-side is turned off

when the inductor current reaches zero. At this time both

mosfets remain off until V

FB drops to the 750mV threshold.

While the mosfets are off, the load is supplied by the

output capacitor. Figure 4 shows power-save operation

at light loads.

FB Ripple

Voltage

(VFB)

FB threshold

Inductor

Current

Zero (0A)

LX on-tim e is triggered when

VFB reaches the FB Threshold.

(750m V)

O n-tim e (TON)

LX drives to PG ND when

on-tim e is com pleted.

Dead tim e varies

according to load

LX tri-states when inductor

current reaches zero.

LX ringing

(parasitic LC)

LX = VOUT

Figure 4 — Power-save Operation

While operating in power-save mode, after each high-side

on-time, the low-side turns on for at least the minimum

260nsec off-time. After this, if the inductor current has

not reached zero and the FB pin falls below the FB thresh-

old, power-save operation is terminated. The controller

immediately generates a high-side on-time and returns

to Continuous Mode Operation, resulting in a rapid

response to large step load increases.

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