LTC1624CS datasheet(14/28 Pages) LINER

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LTC1624

Boost Converters: Power MOSFET Selection

One external N-channel power MOSFET must be selected

for use with the LTC1624 for the switch. In boost applica-

tions the source of the power MOSFET is grounded along

with the SW pin. The peak-to-peak gate drive levels are set

by the INTVCC voltage. The gate drive voltage is equal to

approximately 5V for VIN > 5.6V and a logic level MOSFET

can be used. At VIN voltages below 5V the gate drive

voltage is equal to VIN – 0.6V and a sublogic level MOSFET

should be used.

Selection criteria for the power MOSFET include the “ON”

resistance RDS(ON), reverse transfer capacitance CRSS,

input voltage and maximum output current. When the

LTC1624 is operating in continuous mode the duty cycle

for the MOSFET is given by:

Main Switch Duty Cycle = 1

−

OUT

The MOSFET power dissipation at maximum output cur-

rent is given by:

APPLICATIONS INFORMATION

Boost Converter: Inductor Selection

For most applications the inductor will fall in the range of

µH to 100µH. Higher values reduce the input ripple

voltage and reduce core loss. Lower inductor values are

chosen to reduce physical size.

The input current of the boost converter is calculated at full

load current. Peak inductor current can be significantly

higher than output current, especially with smaller induc-

tors and lighter loads. The following formula assumes

continuous mode operation and calculates maximum peak

inductor current at minimum VIN:

L PEAK

OUT MAX

OUT

IN MIN

L MAX

()

( ) ()

()













∆

The ripple current in the inductor (

∆IL) is typically 20% to

30% of the peak inductor current occuring at VIN(MIN) and

IOUT(MAX).

∆I

kHz L V

OUT

P-P

()=

+−

()

()( ) +

()

200

with

∆IL(MAX) = ∆IL(P-P) at VIN = VIN(MIN).

Remember boost converters are not short-circuit pro-

tected, and that under output short conditions, inductor

current is limited only by the available current of the input

supply, IOUT(OVERLOAD). Specify the maximum inductor

current to safely handle the greater of IL(PEAK) or

IOUT(OVERLOAD). Make sure the inductor’s saturation cur-

rent rating (current when inductance begins to fall)

exceeds the maximum current rating set by RSENSE.

Boost Converter: RSENSE Selection for Maximum

Output Current

RSENSE is chosen based on the required output current.

Remember the LTC1624 current comparator has a maxi-

mum threshold of 160mV/RSENSE. The current compara-

tor threshold sets the peak of the inductor current, yielding

a maximum average output current IOUT(MAX) equal to

IL(PEAK) less half the peak-to-peak ripple current (∆IL),

divided by the output-input voltage ratio (see equation for

IL(PEAK)).

k V

kHz

where

MAIN

IN MAX

IN MIN

OUT

DS ON

OUT

IN MAX

RSS

IN MAX

OUT MAX

OUT

IN MIN

= 





−













()

() 



 ()(

)













()

185

200

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

constant inversely related to the gate drive current.

MOSFETs have I2R losses, plus the PMAIN equation

includes an additional term for transition losses that are

highest at high output voltages. For VOUT < 20V the high

current efficiency generally improves with larger MOSFETs,

while for VOUT > 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. For addi-

tional information refer to Step-Down Converter: Power

MOSFET Selection in the Applications Information

section.

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