LTC1439CG datasheet(14/32 Pages) LINER

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LTC1438/LTC1439

APPLICATIONS INFORMATION

The peak-to-peak drive levels are set by the INTVCC volt-

age. This voltage is typically 5V during start-up (see

EXTVCC Pin Connection). Consequently, logic level thresh-

old MOSFETs must be used in most LTC1438/LTC1439

applications. The only exception is applications in which

EXTVCC is powered from an external supply greater than

8V (must be less than 10V), in which standard threshold

MOSFETs (VGS(TH) < 4V) may be used. Pay close attention

to the BVDSS specification for the MOSFETs as well; many

of the logic level MOSFETs are limited to 30V or less.

Selection criteria for the power MOSFETs include the "ON"

resistance RSD(ON), reverse transfer capacitance CRSS,

input voltage and maximum output current. When the

LTC1438/LTC1439 are operating in continuous mode the

duty cycles for the top and bottom MOSFETs are given by:

Main Switch Duty Cycle

Synchronous Switch Duty Cycle

= ()

OUT

–

The MOSFET power dissipations at maximum output

current are given by:

MAIN

OUT

MAX

DS ON

RSS

SYNC

OUT

MAX

DS ON

() +

()

() (

)(

)( )

() +

()

–

1.85

MAX

where

δ is the temperature dependency of RDS(ON) and k

is a constant inversely related to the gate drive current.

Both MOSFETs have I2R losses while the topside

N-channel equation includes an additional term for transi-

tion losses, which are highest at high input voltages. For

VIN < 20V the high current efficiency generally improves

with larger MOSFETs, while for VIN > 20V the transition

losses rapidly increase to the point that the use of a higher

RDS(ON) device with lower CRSS actual provides higher

efficiency. The synchronous MOSFET losses are greatest

at high input voltage or during a short circuit when the duty

cycle in this switch is nearly 100%. Refer to the Foldback

Current Limiting section for further applications information.

The term (1 +

δ) is generally given for a MOSFET in the

form of a normalized RDS(ON) vs Temperature curve, but

δ = 0.005/°C can be used as an approximation for low

voltage MOSFETs. CRSS is usually specified in the MOSFET

characteristics. The constant k = 2.5 can be used to

estimate the contributions of the two terms in the main

switch dissipation equation.

The Schottky diode D1 shown in Figure 1 serves two

purposes. During continuous synchronous operation, D1

conducts during the dead-time between the conduction of

the two large power MOSFETs. This prevents the body

diode of the bottom MOSFET from turning on and storing

charge during the dead-time, which could cost as much as

1% in efficiency. During low current operation, D1 oper-

ates in conjunction with the small top MOSFET to provide

an efficient low current output stage. A 1A Schottky is

generally a good compromise for both regions of opera-

tion due to the relatively small average current.

CIN and COUT Selection

In continuous mode, the source current of the top

N-channel MOSFET is a square wave of duty cycle VOUT/

VIN. To prevent large voltage transients, a low ESR input

capacitor sized for the maximum RMS current must be

used. The maximum RMS capacitor current is given by:

Required I

RMS ≈

()

[]

MAX

OUT

–

This formula has a maximum at VIN = 2VOUT, where IRMS

= IOUT/2. This simple worst-case condition is commonly

used for design because even significant deviations do not

offer much relief. Note that capacitor manufacturer’s ripple

current ratings are often based on only 2000 hours of life.

This makes it advisable to further derate the capacitor or

to choose a capacitor rated at a higher temperature than

required. Several capacitors may also be paralleled to

meet size or height requirements in the design. Always

consult the manufacturer if there is any question.

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