SC411 datasheet(9/27 Pages) SEMTECH

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SC411

POWER MANAGEMENT

+5V Bias Supplies

The SC411 requires an external +5V bias supply in addi-

tion to the battery. If stand-alone capability is required,

the +5V supply can be generated with an external linear

regulator such as the Semtech LP2951.

For optimal operation, the controller has its own ground

reference, VSSA, which should be tied along with PGND

directly to the thermal pad under the part, which in turn

should connect to the ground plane using multiple vias.

All external components referenced to VSSA in the Typical

Application Circuit on Page 1 located near their respec-

tive pins. Supply decoupling capacitors should be located

adjacent to their respective pins. A 10Ω resistor should

be used to decouple VCCA from the main VDDP supply. All

ground connections are connected directly to the ground

plane as mentioned above. VSSA and PGND should be

starred at the thermal pad. The VDDP input provides

power to the upper and lower gate drivers; a decoupling

capacitor is required. No series resistor between VDDP

and 5V is required. See Layout Guidelines on page 17 for

more details.

Pseudo-Fixed Frequency Constant On-Time PWM

Controller

The PWM control architecture consists of a constant on-

time, pseudo ﬁxed frequency PWM controller (Block Dia-

gram, Page 6). The output ripple voltage developed across

the output ﬁlter capacitor’s ESR provides the PWM ramp

signal eliminating the need for a current sense resistor.

The high-side switch on-time is determined by a one-shot

whose period is directly proportional to output voltage and

inversely proportional to input voltage. A second one-shot

sets the minimum off-time which is typically 400ns.

On-Time One-Shot (t

ON)

The on-time one-shot comparator has two inputs. One

input looks at the output voltage, while the other input

samples the input voltage and converts it to a current.

This input voltage-proportional current is used to charge

an internal on-time capacitor. The on-time is the time re-

quired for the voltage on this capacitor to charge from zero

volts to VOUT, thereby making the on-time of the high-side

switch directly proportional to output voltage and inversely

proportional to input voltage. This implementation results

in a nearly constant switching frequency without the need

for a clock generator.

For VOUT < 3.3V:

)

(

BAT

OUT

TON

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

•

−

For 3.3V ≤ VOUT ≤ 5V:

TON is a resistor connected from the input supply (VBAT)

to the TON pin. Due to the high impedance of this resistor,

the TON pin should always be bypassed to VSSA using a

1nF ceramic capacitor.

EN/PSV: Enable, PSAVE and Soft Discharge

The EN/PSV pin enables the supply. When EN/PSV is

tied to VCCA the controller is enabled and power save will

also be enabled. When the EN/PSV pin is tri-stated, an

internal pull-up will activate the controller and power save

will be disabled. If PSAVE is enabled, the SC411 PSAVE

comparator will look for the inductor current to cross zero

on eight consecutive switching cycles by comparing the

phase node (LX) to PGND. Once observed, the controller

will enter power save and turn off the low side MOSFET

when the current crosses zero. To improve light-load ef-

ﬁciency and add hysteresis, the on-time is increased by

50% in power save. The efﬁciency improvement at light-

loads more than offsets the disadvantage of slightly high-

er output ripple. If the inductor current does not cross

zero on any switching cycle, the controller will immediately

exit power save. Since the controller counts zero cross-

ings, the converter can sink current as long as the cur-

rent does not cross zero on eight consecutive cycles. This

allows the output voltage to recover quickly in response

to negative load steps even when PSAVE is enabled. If

the EN/PSV pin is pulled low, the related output will be

shut down and discharged using a switch with a nominal

resistance of 22 Ohms. This will ensure that the output is

in a deﬁned state next time it is enabled and also ensure,

since this is a soft discharge, that there are no danger-

ous negative voltage excursions to be concerned about. In

order for the soft discharge circuitry to function correctly,

the chip supply must be present.

)

(

BAT

OUT

TON

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

•

−

Application Information

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