SC1486ITSTRT datasheet(9/17 Pages) SEMTECH

9 / 17 page

 2002 Semtech Corp.

www.semtech.com

SC1486

POWER MANAGEMENT

+5V Bias Supplies

The SC1486 requires an external +5V bias supply in ad-

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

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

regulator such as the Semtech LP2951A. To minimize

channel to channel crosstalk, each controller has 4 sup-

ply pins, VDDP, PGND, VCCA and AGND.

To avoid ground loops, separate AGND planes are rec-

ommended. Each contoller requires its own AGND plane

which should be tied by a single trace to the negative

terminal of that controller’s output capacitor. All external

components referenced to AGND in the schematic should

then be connected to the appropriate AGND plane. The

supply decoupling capacitor for controller 1 should be

tied between VCCA1 and AGND1. Likewise, the supply

decoupling capacitor for controller 2 should be tied be-

tween VCCA2 and AGND2. A single 10 ohms resistor

should be used to decouple the VCCA supplies from the

main VDDP supplies. PGND can then be a separate plane

which is not used for routing traces. All PGND connec-

tions are connected directly to this plane with special

attention given to avoiding indirect connections which

may create ground loops. As mentioned above, the two

AGND planes must be connected to the PGND plane at

the negative terminal of the respective output capaci-

tors. The VDDP1 and VDDP2 input provides power to

the upper and lower gate drivers. A decoupling capacitor

for each supply is recommended. No series resistor be-

tween VDDP and the 5 volt bias is required.

Pseudo-fixed Frequency Constant On-Time PWM

Controller

The PWM control architecture consists of a constant-on-

time, pseudo fixed frequency PWM controller, (Figure 1).

The output ripple voltage developed across the output

filter capacitor’s ESR provides the PWM ramp signal elimi-

nating 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 in-

versely 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 TON time is the time

required 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 imple-

mentation results in a nearly constant switching frequency

without the need of a clock generator.









•

−

OUT

TON

)

(

+50ns

RTON is a resistor connected from the input supply to the

TON pin.

Enable & Psave

The EN/PSV pin enables the VDDQ (2.5 volt) supply.

REFIN and VDDP2 enable the VTT (1.25 volt) supply. The

VTT and VDDQ supplies may be enabled independently.

When EN/PSV is tied to VCCA the VDDQ controller is

enabled and power save will also be enabled. When the

EN/PSV pin is tristated, an internal pulled-up will acti-

vate the VDDQ controller and power save will be disabled.

If PSAVE is enabled, the SC1486 PSAVE comparator will

look for the inductor current to cross zero on eight con-

secutive cycles. Once observed, the controller will enter

power save and turn off the low side MOSFET when the

current crosses zero. To improve the efficiency 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 slighlty higher output ripple.

If the inductor current does not cross zero on any switch-

ing cycle, the controller will immediately exit power save.

Since the controller counts crossings, the converter can

sink current as long as the current does not cross zero

on eight consecutive cycles. This allows the output volt-

age to recover quickly in response to negative load steps

even when psave is enabled.Since the VTT supply must

sink current, this controller does not have a power save

option. If REFIN is low, the VTT controller shuts down to

a low bias current. If Refin is greater than 1 volt, and

VDDP2 is low, the reference buffer is active, but the VTT

buck converter is disabled (S3 state). If REFIN is greater

than 1 volt and VDDP is greater than 4 volts, the VTT

supply is active.

Output Voltage Selection

The output voltage selection is set by the feedback re-

sistors R2 & R3 of Figure 3. The internal reference is

1.5V. The internal feedback pin is multiplied by three to

match the 1.5V reference. Therefore the output can

beselected to a minimum of 0.5V. The equation for set-

ting he output voltage based on Figure 3 is:

Application Information

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