SC488MLTRT datasheet(9/24 Pages) SEMTECH

9 / 24 page

www.semtech.com

SC488

POWER MANAGEMENT

Application Information

+5V Bias Supplies

The SC488 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. To minimize crosstalk, the controller has seven

supply pins: VDDP (2 pins), PGND1 (2 pins), PGND2, VCCA

and AGND.

The controller requires its own AGND plane which should

be tied by a single trace to the negative terminal of the

output capacitor. All external components referenced to

AGND in the schematic should then be connected to the

AGND plane. The supply decoupling capacitor should be

tied between VCCA and AGND. A single 10Ω resistor should

be used to decouple the VCCA supply from the main VDDP

supply. PGND can then be a separate plane which is not

used for routing analog traces. All PGND connections

should connect directly to this plane with special attention

given to avoiding indirect connections between AGND and

PGND which will create ground loops. As mentioned above,

the AGND plane must be connected to the PGND plane at

the negative terminal of the output capacitor. The VDDP

input provides power to the upper and lower gate drivers.

A decoupling capacitor for the VDDP supply and PGND is

recommended. No series resistor between VDDP and the

5 volt bias is required.

Pseudo-Fixed Frequency Constant On-Time

PWM Controller

The PWM control method is a constant-on-time, pseudo-

ﬁxed frequency PWM controller, see Figure 1. The ripple

voltage seen across the output capacitor’s ESR provides

the PWM ramp signal, eliminating the need for a current

sense resistor. The on-time is determined by a one-shot

whose period is proportional to output voltage, and in-

versely proportional to input voltage. A separate one-shot

sets the minimum off-time (typically 425ns).

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 proportional

current. This current charges an internal on-time capaci-

tor. The TON time is the time required for 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 volt-

age. This implementation results in a nearly constant

switching frequency without the need of a clock generator.

OUT

)

37x10

(RTON

3.3x10

RTON is a resistor connected between the input supply and

the TON pin.

VDDQ/VTT Enable & Power-Save

The EN/PSV pin controls the VDDQ supply and the REF

output (1/2 of VDDQ). VTTEN enables the VTT supply. The

VTT and VDDQ supplies may be enabled independently.

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

abled in power-save mode. When the EN/PSV pin is ﬂoated,

an internal resistor divider activates the VDDQ controller

with power-save disabled. If PSAVE is enabled, the SC488

PSAVE comparator looks for inductor current to cross zero

on eight consecutive cycles. Once observed, the controller

enters power-save and turns off the low-side MOSFET when

the current crosses zero. To improve the efﬁciency and

add hysteresis, the on-time is increased by 20% in power-

save. The efﬁciency improvement at light loads more than

offsets the disadvantage of slightly higher output ripple. If

the inductor current does not cross zero on any switching

cycle, the controller immediately exits power-save. Since

the controller counts zero crossings, the converter can

sink current as long as the current does not cross zero on

eight consecutive cycles. This allows the output voltage

to recover quickly in response to negative load steps even

when power-save is enabled.

VDDQ Voltage Selection

VDDQ voltage is set using the FB pin. Grounding FB sets

VDDQ to ﬁxed 2.5V. Connecting FB to +5V sets VDDQ to

ﬁxed 1.8V. VDDQ can also be adjusted from 1.5 to 3.0V

using external resistors, see Figure 2. The voltage at FB is

then compared to the internal 1.5V reference.

To VDDQ output capacitor

To SC488 FB (pin 9)

Figure 2

SC488MLTRT Datasheet(PDF) 9 Page - Semtech Corporation

SC488MLTRT Datasheet(HTML) 9 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL