LTC3832 datasheet(13/24 Pages) LINER

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LTC3832/LTC3832-1

sn3832 3832fs

Diagram). This increases the G2 on-time and allows the

charge pump capacitors to be refreshed.

For applications using an external supply to PVCC1, this

supply must also be higher than VCC by at least 2.5V to

ensure normal operation.

For applications with a 5V or higher VIN supply, PVCC2 can

be tied to VIN if a logic level MOSFET is used. PVCC1 can be

supplied using a doubling charge pump as shown in

Figure 9. This circuit provides 2VIN – VF to PVCC1 while Q1

is ON.

enhance standard power MOSFETs. Under this condition,

the effective MOSFET RDS(ON) may be quite high, raising

the dissipation in the FETs and reducing efficiency. Logic

level FETs are the recommended choice for 5V or lower

voltage systems. Logic level FETs can be fully enhanced

with a doubler/tripling charge pump and will operate at

maximum efficiency.

After the MOSFET threshold voltage is selected, choose the

RDS(ON) based on the input voltage, the output voltage,

allowable power dissipation and maximum output current.

In a typical LTC3832 circuit, operating in continuous mode,

the average inductor current is equal to the output load

current. This current flows through either Q1 or Q2 with the

power dissipation split up according to the duty cycle:

DC Q

OUT

()

–

The RDS(ON) required for a given conduction loss can now

be calculated by rearranging the relation P = I2R.

DC Q

DS ON Q

MAX Q

LOAD

MAX Q

OUT

LOAD

DS ON Q

MAX Q

LOAD

MAX Q

OUT

LOAD

()

() • (

)

•

•(

)

() • (

)

•

(–

) • (

)

PMAX should be calculated based primarily on required

efficiency or allowable thermal dissipation. A typical high

efficiency circuit designed for 3.3V input and 2.5V at 10A

output might allow no more than 3% efficiency loss at full

load for each MOSFET. Assuming roughly 90% efficiency

at this current level, this gives a PMAX value of:

(2.5V)(10A/0.9)(0.03) = 0.83W per FET

and a required RDS(ON) of:

DS ON Q

()

) • (.

)

( .

)(

)

) • (.

)

( .

– .

)(

)

083

0 011

083

0 034

Ω

APPLICATIO S I FOR ATIO

LTC3832

3832 F09

12V

1N5242

COUT

VOUT

0.1

µF

PVCC2

OPTIONAL

USE FOR VIN ≥ 7V

MBR0530T1

PVCC1

VIN

Figure 9. Doubling Charge Pump

Power MOSFETs

Two N-channel power MOSFETs are required for most

LTC3832 circuits. These should be selected based

primarily on threshold voltage and on-resistance consid-

erations. Thermal dissipation is often a secondary con-

cern in high efficiency designs. The required MOSFET

threshold should be determined based on the available

power supply voltages and/or the complexity of the gate

drive charge pump scheme. In 3.3V input designs where

an auxiliary 12V supply is available to power PVCC1 and

PVCC2, standard MOSFETs with RDS(ON) specified at VGS

= 5V or 6V can be used with good results. The current

drawn from this supply varies with the MOSFETs used

and the LTC3832’s operating frequency, but is generally

less than 50mA.

LTC3832 applications that use 5V or lower VIN voltage and

a doubling/tripling charge pump to generate PVCC1 and

PVCC2, do not provide enough gate drive voltage to fully

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