LTC3245 datasheet(10/16 Pages) LINER

10 / 16 page

LTC3245

3245f

For more information www.linear.com/LTC3245

applicaTions inForMaTion

2:1 Step-Down Charge Pump Operation

When the input supply is greater than about two times

the output voltage, the LTC3245 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

half of the total output current. The efficiency (

η) and chip

power dissipation (PD) in 2:1 are approximately:

η ≅

POUT

VOUT •IOUT

VIN• 12 IOUT

2VOUT

VIN

PD=

VIN

– VOUT









IOUT

1:1 Step-Down Charge Pump Operation

When the input supply is less than about two times the

output voltage but more than the programmed output

voltage, the LTC3245 will operate in 1:1 step-down mode.

This method of regulation is very similar to a linear regula-

tor. Charge is delivered directly from VIN to VOUT through

most of the oscillator period. The charge transfer is briefly

interrupted at the end of the period. The interruption in

charge transfer improves stability and transient response.

When in 1:1 step-down mode the input current will be

approximately equal to the total output current. Thus

efficiency (

η) and chip power dissipation (PD) in 1:1 are

approximately:

η ≅

POUT

VOUT •IOUT

VIN• IOUT

VOUT

VIN

PD= VIN– VOUT

(

) IOUT

1:2 Step-Up Charge Pump Operation

When the input supply is less than the output voltage the

LTC3245 will operate in 1:2 step-up mode. Charge trans-

fer happens in two phases. On the first phase the flying

capacitor (CFLY) is connected between VIN and GND. On

this phase CFLY is charged up. On the second phase the

flying capacitor (CFLY) is connected between VIN and VOUT

and the charge stored on CFLY during the first phase is

transferred to VOUT. When in 1:2 step-up mode the input

current will be approximately twice the total output cur-

rent. Thus efficiency (

η) and chip power dissipation (PD)

in 1:2 are approximately:

η ≅

POUT

VOUT •IOUT

VIN• 2IOUT

VOUT

2VIN

PD= 2VIN– VOUT

(

) IOUT

Due to the limited drive in 1:2 step-up mode the device

always operates in Burst Mode operation when operating

at this conversion ratio. This is done to delay the onset of

dropout at the expense of more output ripple.

PGOOD Output Operation

TheLTC3245includesanopen-drainpowergood(PGOOD)

output pin. If the chip is in shutdown or under UVLO con-

ditions (VIN < 2.2V typical), PGOOD is low impedance to

ground.PGOODbecomeshighimpedancewhenVOUTrises

to 95% (typical) of its regulation voltage. PGOOD stays

high impedance until VOUT is shut down or drops below

the PGOOD threshold (91% typical) due to an overload

condition. A pull-up resistor can be inserted between

PGOOD and a low voltage positive logic supply (such as

VOUT) to signal a valid power good condition. The use of

a large pull-up resistor on PGOOD and a capacitor placed

betweenPGOODandGNDcanbeusedtodelaythePGOOD

signal if desired.

VOUT Ripple and Capacitor Selection

The type and value of capacitors used with the LTC3245

determine several important parameters such as regula-

tor control loop stability, output ripple and charge pump

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