LT1500CS datasheet(7/16 Pages) LINER

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LT1500/LT1501

PIN FUNCTIONS

NFB/SELECT (LT1500 Only): NFB is a second feedback

node used to regulate a negative output voltage. Negative

output voltages can be generated by using a transformer

flyback circuit, a Cuk converter or a capacitor charge pump

added to a boost converter. The regulating point for NFB

is 1.265V and the internal resistance to ground is 100k

Ω.

External divider current should be 300

µA or greater to

avoid negative output voltage variations due to production

variations in the internal resistor value. FB should be left

open when using NFB.

On fixed voltage parts, NFB is replaced with Select. The

Select pin is used to set output voltage at either 3.3V or 5V.

VC (LT1500 Only): This is the output of the error amplifier

and the input to the current comparator. The VC pin voltage

is about 700mV at very light loads and about 1.2V at full

load. An internal comparator detects when the VC voltage

drops below about 750mV and shuts down the current

comparator and the power switch biasing to reduce quies-

cent current. This forces the regulator to operate in Burst

Mode operation.

SYNC (LT1500 Only): This is a logic level input used to

synchronize switching frequency to an external clock. The

sync signal overrides the internal current comparator and

turns the switch on. Minimum sync pulse width should be

50ns and maximum width should be 300ns. A continuous

high sync signal will force the power switch to stay on

indefinitely and current will increase without limit. Don’t

do this!

SS (LT1500 Only): This is the soft start function using the

base of a PNP transistor whose emitter is tied to the VC pin.

Grounding SS will turn off switching by pulling VC low. A

capacitor tied from SS to ground will force VC to ramp up

slowly during start-up at a rate set by the capacitor value

and the internal 4

µA pull-up current. An external resistor

must be used to reset the capacitor voltage completely to

0V at power down.

SHDN: Logic Level Shutdown Pin.

This pin must be held

high (> 1.1V) for the regulator to run. SHDN can be tied

directly to VIN, even with VIN = 18V. The low-battery

detector remains active in shutdown, but all other circuitry

is turned off.

VIN: This pin supplies power to the regulator and is

connected to one side of the inductor sense resistor. It

should be bypassed close to the chip with a low ESR

capacitor.

ISENSE: This is one end of the internal inductor-current

sense resistor. With most applications, only the external

inductor is tied to this pin.

GND: This pin carries only low level current in the LT1500,

but it carries full switch current in the LT1501. The

negative end of the input bypass capacitor should be

connected close to this pin and the pin should go directly

to the ground plane with the LT1501.

PGND (LT1500 Only): This pin is the emitter of the internal

NPN power switch. Connect it directly to the ground plane.

SW: This is the collector of the internal NPN power switch.

To avoid EMI and overvoltage spikes, keep connections to

this pin very short.

LBI: This is the input to the low-battery detector with a

threshold of 1.24V. Maximum pin voltage is 5V. Bypass

LBI with a small filter capacitor when used. If unused, tie

LBI to ground. The low-battery detector remains active in

shutdown.

LBO : This is the open collector output of the low-battery

detector. It will sink up to 2mA. Leave open if not used.

FB/VOUT: FB is the inverting input to the error amplifier with

a regulating point of 1.265V and a typical bias current of

30nA. Bias current is reduced with a canceling circuit, so

bias current could flow in either direction. FB is replaced

with VOUT on fixed voltage parts. VOUT is the top of an

internal divider that is connected to the internal FB node.

A switch disconnects the divider in shutdown so that the

divider current does not load VIN through the inductor and

catch diode.

LT1500CS Datasheet(PDF) 7 Page - Linear Technology

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