LTC3255 datasheet(11/20 Pages) AD

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LTC3246

3246fa

For more information www.linear.com/LTC3246

Short-Circuit/Thermal Protection

The LTC3246 has built-in short-circuit current limiting

on both the VOUT and BIAS outputs to protect the part in

the event of a short. During short-circuit conditions, the

device will automatically limit the output current from

both outputs.

The LTC3246 has thermal protection that will shut

down the device if the junction temperature exceeds the

overtemperature threshold (typically 175°C). Thermal

shutdown is included to protect the IC in cases of exces-

sively high ambient temperatures, or in cases of excessive

power dissipation inside the IC. The charge transfer will

reactivate once the junction temperature drops back to

approximately 165°C.

Whenthethermalprotectionisactive,thejunctiontempera-

ture is beyond the specified operating range. The thermal

and short-circuit protection are intended for momentary

overload conditions outside normal operation. Continu-

ous operation above the specified maximum operating

conditions may impair device reliability.

Programming the Output Voltage (OUTS/ADJ Pin)

The LTC3246 output voltage programming is very flexible

offering a fixed 3.3V output, fixed 5V output as well as

adjustable output that is programmed through an external

resistor divider. The desired output regulation method is

selected through the SEL pins.

For a fixed output simply short OUTS (OUTS/ADJ pin) to

VOUT as shown in Figure 1. Fixed 3.3V operation is enabled

by driving both SEL1 and SEL2 pins high, while fixed 5V

operating is selected by driving SEL2 high with SEL1 low.

Driving both SEL1 and SEL2 low shuts down the device

causing VOUT to be pulled low by an internal impedance

of about 80k.

LTC3246

COUT

3246 F01

VOUT

FIXED 3.3V OR

FIXED 5.0V

VOUT

OUTS/ADJ

GND

Figure1. Fixed Output Operation

APPLICATIONS INFORMATION

Adjustable output programming is accomplished by con-

necting ADJ (OUTS/ADJ pin) to a resistor divider between

VOUT and GND as shown in Figure 2. Adjustable operation

isenabledbydrivingSEL1highandSEL2low.Drivingboth

SEL1 and SEL2 low shuts down the device, causing VOUT

to be pulled low by an internal impedance of about 80k.

LTC3246

COUT

3246 F02

VOUT

1.1V 1

VOUT

OUTS/ADJ

GND

Figure2. Adjustable Output Operation

Using adjustable operation, the output (VOUT) can be

programmed to regulate from 2.5V to 5V. The limited

programming range provides the required VOUT operating

voltage without overstressing the VOUT pin.

The desired adjustable output voltage is programmed by

solving the following equation for RA and RB:

VOUT

1.11V

– 1

Select a value for RB in the range of 1k to 1M and solve

for RA. Note that the resistor divider current adds to the

total no load operating current. Thus, a larger value for

RB will result in lower operating current.

2:1 Step-Down Charge Pump Operation

When the input supply is greater than about two times

the output voltage, the LTC3246 will operate in 2:1 step-

down mode. Charge transfer happens in two phases. On

the first phase, the flying capacitor (CFLY) is connected

between VIN and VOUT. On this phase, CFLY is charged up

and current is delivered to VOUT. On the second phase,

the flying capacitor (CFLY) is connected between VOUT and

GND. The charge stored on CFLY during the first phase is

transferred to VOUT on the second phase. When in 2:1

step-down mode, the input current will be approximately

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