SC339 datasheet(7/15 Pages) SEMTECH

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SC339

POWER MANAGEMENT

Applications Information (Cont.)

Note: R

DS(ON) must be met at all temperatures and at the mini-

mum V

GS condition.

Setting The Output Voltage: the adjust pin connects

directly to the inverting input of the error ampliﬁer, and

the output voltage is set using external resistors (please

refer to the Typical Application Circuit on Page 1).

Using output 1 as an example, the output voltage can be

calculated as follows:

OUT = 0.5 • ( 1 + —— )

The input bias current for the adjust pin is so low that

it can be safely ignored. To avoid picking up noise, it is

recommended that the total resistance of the feedback

chain be less than 100k

Ω. With ceramic capacitors, a

recommended divider current of >100μA is recommended

to keep the FET conducting during light load conditions to

improve transient response.

SC339 Supply Comes Up Before MOSFET Drain Supply

MOSFET Drain Supply Comes Up Before SC339 Supply

2.8V

2.7V

Power Supply Sequencing

Input Capacitors: placement of low ESR capacitors such

as Sanyo POSCAPs or Panasonic SP-caps at the input to

the MOSFET (V

DRAIN) will help to hold up the power supply

during fast load changes, thus improving overall transient

response. If V

DRAIN is located at the bulk capacitors for

the upstream voltage regulator, additional capacitance

may not be required. In this case a 0.1μF ceramic

capacitor will sufﬁce. The 5V bias supply to the SC339

should be bypassed with a 0.1μF ceramic capacitor.

MOSFETs: very low or low threshold N-channel

MOSFETs are required. Select FETs rated for V

of 2.7V or lower. For the device to work under all

operating conditions, a maximum R

DS(ON) must be met to

ensure that the output will never go into dropout:

IN(MIN) ― VOUT(MAX)

DSON(MAX) =

———————

OUT(MAX)

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