SC338IMSTRT datasheet(6/14 Pages) SEMTECH

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 2004 Semtech Corp.

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SC338(A)

PRELIMINARY

POWER MANAGEMENT

Theory Of Operation

The SC338(A) dual linear FET controller provides a simple

way to drive two N-channel MOSFETs to produce tightly

regulated output voltages from one or two available,

higher, supply voltages. It takes its power from either a

5V or 12V supply, drawing typically 500µA while

operating.

It contains an internal bandgap reference which is

compared to the output voltages via resistor dividers.

These resistor dividers are external and user selectable .

Depending upon the input voltage used for the device,

the drive pin (DRV1, DRV2) can pull up to a guaranteed

minimum of 6.6V (from 12V supply) or 4.7V (from 5V

supply). Thus the device can be used to regulate a large

range of output voltages by careful selection of the

external MOSFETs (see component selection, below).

The SC338(A) includes an active high enable control (EN1,

EN2) for each output. If this pin is pulled low, the related

drive pin is pulled low, turning off the N-channel MOSFET.

If the pin is pulled up to 1.8V

≤ V

EN ≤ VIN, the drive pin will

be enabled. This pin should not be allowed to float.

Each output has a power good output (PGD1, PGD2)

which are open drain outputs that pull low if the related

output is below the power good threshold (-12% of the

programmed output voltage typical, -6% typical at start-

up). The power good circuitry is active if the device is

enabled, regardless of the state of the over current latch.

The power good circuitry is not active if that

particular output is disabled.

Also included for each output is an overcurrent

protection circuit that monitors the output voltage. If the

output voltage drops below 50% (typ.) of nominal, as

would occur during an overcurrent or short condition, the

device will pull the drive pin low and latch off. The device

will need to have the power supply or enable pin toggled

to reset the latch condition. Each output latches

independently (i.e. if one output latches off, the other

output will function normally).

Drive Outputs and Soft Start

The drive outputs for each output are source and sink

capable. The sink current is typically 0.8mA at 5V in (1mA

at 12V in). The source current is typically 2mA at 5V in

Applications Infomation

and 3.75mA at 12V in during normal operation. The high

side drive voltage is generated from V

IN by a 7V (nominal)

low dropout regulator, thus at 12V in, 6.9V is available

and at 5V in, 4.85V is available (since the LDO will be in

dropout).

At start-up, the source current available from the drive

pins is limited to 10µA (typical) until the power good

threshold is reached, at approximately 6% below

nominal output voltage. At this point the full drive

capability is enabled. With this constant current source

at start-up, it is a simple matter to use a small capacitor

on the drive pin to slow this rate of rise. The rate of rise

of the drive pin voltage will be:

DRV

A 10nF soft start capacitor will give a 1ms output rise

time for V

IN = 12V and VOUT = 1.05V, for example. The

output rise time will of course depend upon the gate

threshold of the MOSFET being used. Please refer to

the Output Rise Time chart on Page 13 showing typical

output rise times. For very low ESR output capacitors

(<5m

Ω) and very high soft start capacitance (>100nF),

it may be necessary to add a resistor in series with the

soft start capacitor to ensure stability. Generally,

however, this resistor is not required, as this is a very

unlikely situation.

The soft start capacitance does not adversely affect

transient response since the drive current capability is

200 times higher once the device has started.

OCP and Power Supply Sequencing

The SC338(A) has output undervoltage protection that

looks at a particular output to see if it is a) less than

50% (typical) of it’s nominal value and b) V

DRV for that

output is within 350mV (typical) of maximum. If both of

these criteria are met, there is a 50µs (typical) delay and

then the output is shut down. This provides inherent

immunity to UV shutdown at start-up (which may occur

while the output capacitors are being charged) since V

DRV

has a very slow rate of rise with I

DRV limited to 10µA.

At start-up, it is necessary to ensure that the power

supplies and enables are sequenced correctly to avoid

erroneous latch-off. For UV latch-off not to occur at start-

up due to sequencing issues, the key is that the voltage

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