SC284P datasheet(14/22 Pages) SEMTECH

14 / 22 page

SC284P

Applications Information

Detailed Description

The SC284P is a two channel synchronous step-down

converter. Both channels of this device are designed

to operate at a fixed-frequency of 2.5MHz in CCM and

provide the same current capacity of up to 2A. The

switching frequency is chosen to minimize the size of the

external inductor and capacitors while maintaining high

efficiency. Both channels of SC284P are independent.

Operation

During normal operation, the PMOS FET is activated on

each rising edge of the internal oscillator. The voltage

feedback loop uses an internal feedback resistor divider.

The period is set by the internal oscillator. The device has

an internal synchronous NMOS rectifier and does not

require a Schottky diode on the LXpin.

Programmable Output Voltage

Both channels on SC284P have seven pre-determined

output voltage values which can be individually selected

by programming the CTL input pins (see Table — Output

Voltage Settings). Each CTL pin has an active MΩ internal

pull-down resistor. The MΩ resistor is switched in circuit

whenever the CTL input voltage is below the input

threshold, or when the part is in under-voltage lockout. It is

recommended to tie all high CTL pins together and use an

external pull-up resistor to V

IN if there is no enable signal, or

if the enable input is an open drain/collector signal.The CTL

pins may be driven by a microprocessor to allow dynamic

voltage adjustment for systems that reduce the supply

voltage when entering sleep states. Avoid all zeros being

present on the CTL pins when changing programmable

output voltages as this would momentarily disable the

device.

SC284P is also capable of regulating a different (higher)

output voltage, which is not shown in the Table , via an

external resistor divider.There will be a typical 2µA current

flowing into the V

OUT pin. The typical schematic for an ad-

justableoutputvoltageoptionfromthestandard.0Vwith

CTL

X=[00], is shown in Figure . RFBA/B and RFB2A/B are used

to adjust the desired output voltage. If the R

FB2A/B current

is such that the 2µAV

OUT pin current can be ignored, then RFBA/B

can be found by the next equation. R

FB2A/B need to be low

enough in value for the current through the resistor chain

to be at least 20µA in order to ignore the V

OUT pin current.

where V

OSTD is the pre-determined output voltage via the

CTL pins.

FF is needed to maintain good transient response

performance. The correct value of C

FF can be found using

the following equation.

To simplify the design, it is recommended to program the

desired output voltage from a standard .0V as shown in

Figure with the correct C

FF calculated from Equation 2.

For programming the output voltage from other standard

voltages, R

FB, RFB2 and CFF need to be adjusted to meet

Equations and 2.

SC284P

PGOODB

CTL3B

CTL2B

CTL1B

Enable B

PGOODA

CTL3A

CTL2A

CTL1A

Enable A

PGNDA

PGNDB

RFB1B = (VOUTB-1)

x RFB2B

for CTLBX = 0010

(1.0V)

AVINB

VINB

CAVINB

0.1µF

RAVINB 1Ω

PVINB

CINB

10µF

AGNDB

AVINA

VINA

CAVINA

0.1µF

RAVINA 1Ω

PVINA

CINA

10µF

AGNDA

VOUTB

COUTB

LXB

VOUTB

RFB2B

10kΩ

CFFB

RFB1B

VOUTA

COUTA

LXA

VOUTA

RFB2A

10kΩ

CFFA

RFB1A

RFB1A = (VOUTA-1)

x RFB2A

for CTLAX = 0010

(1.0V)

PGOODA

PGOODB

Figure 1 — Output Voltage Programming

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