SC286 datasheet(14/20 Pages) SEMTECH

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SC286

Applications Information (continued)

Under this condition, the output voltage will be the product

of I

CL_HOLD and the load resistance. When the load presented

falls below the current limit holding level, the output will

charge to the upper PSAVE voltage threshold and return to

normal operation. The SC286 is capable of sustaining an

indefinite short circuit without damage. During soft start, if

current limit has occurred before the SS voltage has reached

400mV, the part enters foldback current limit mode.

Foldback current limit mode will be disabled during soft-

start after the SS voltage is higher than 400mV.

Over-Voltage Protection

In the event of a 15% over-voltage on each independent

output, the PWM drive is disabled with the LX pin floating.

Switching does not resume until the output voltage falls

below the nominal V

OUT regulation voltage.

Programmable Output Voltage

The SC286 has fifteen pre-determined output voltage

values which can be individually selected for each channel

by programming the CTL input pins (see Table 1 — Output

Voltage Settings). Each CTL pin has an active 500kΩ internal

pull-down resistor. The 500kΩ resistor is switched in circuit

whenever the CTL input voltage is below the input thresh-

old, 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 AVIN 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 program-

mable output voltages as this would disable the device.

SC286 is also capable of regulating a different (higher)

output voltage, which is not shown in the Table 1, via an

external resistor divider for each channel. There will be a

typical 2μA current flowing into the VOUTA/B pin. The

typical schematic for an adjustable output voltage option

from the standard 1.0V with CTL

XA/B = [0010], is shown in

Figure 2. RFB1A/B and RFB2A/B are used to adjust the

desired output voltage. If the RFB2A/B current is such that

the 2μA VOUTA/B pin current can be ignored, then RFB1A/

B can be found using the next equation. RFB2A/B needs to

be low enough in value for the current through the resis-

tor chain to be at least 20μA in order to ignore theVOUTA/B

pin current.

SC286

AVINA

VINA

CAVINA

0.1µF

RAVINA1Ω

PVINA

AVINB

VINB

CAVINB

0.1µF

RAVINB1Ω

PVINB

PGOODB

PGOODA

CINA

22µF

CINB

22µF

RPGOODA

100kΩ

RPGOODA

100kΩ

CTL0B

CTL1B

CTL2B

CTL3B

Enable B

CTL0A

CTL1A

CTL2A

CTL3A

Enable A

PGNDA

AGNDA

PGNDB

AGNDB

SSA

SSB

CSS

1nF

CSS

1nF

VOUTA

COUTA

LXA

VOUTA

RFB2A

10kΩ

CFFA

RFB1A

RFB1A = (VOUTA-1)

x RFB2A

for CTLAX = 0010

(1.0V)

VOUTB

COUTB

LXB

VOUTB

RFB2B

10kΩ

CFFB

RFB1B

RFB1B = (VOUTB-1)

x RFB2B

for CTLBX = 0010

(1.0V)

Figure 2 — Output Voltage Programming

OSTD

OUT

where V

OSTD is the pre-determined output voltage via the

CTL pins.

FF is needed to maintain good transient response perfor-

mance. The correct value of C

FF can be found using the

following equation.

)

(

]

[

]

[

OSTD

OUT

To simplify the design, it is recommended to program the

desired output voltage from a standard 1.0V as shown in

Figure 2 with a proper C

FF calculated from Equation 2. For

programming the output voltage from other standard

voltages, R

FB1, RFB2 and CFF need to be adjusted to conform

to the previous equations.

Maximum Power Dissipation

Each channel of SC286 has its own Θ

JA of 32.5°C/W when

only one channel is in operation. Since both channels

are within the same package, there is about 50% of the

heat generated which will be transferred to the adjacent

channel. The equivalent total thermal impedance will be

higher when the neighboring channel is also in operation.

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