LTC1436A datasheet(13/28 Pages) LINER

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LTC1436A

LTC1436A-PLL/LTC1437A

APPLICATIONS INFORMATION

frequency operation down to lower currents before cycle

skipping occurs.

The RDS(ON) recommended for the small MOSFET is

around 0.5

Ω. Be careful not to use a MOSFET with an

RDS(ON) that is too low; remember, we want to conserve

gate charge. (A higher RDS(ON) MOSFET has a smaller gate

capacitance and thus requires less current to charge its

gate). For cost sensitive applications the small MOSFET

can be removed. The circuit will then begin Burst Mode

operation as the load current is dropped.

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

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

EXTVCC Pin Connection). Consequently, logic level

threshold MOSFETs must be used in most LTC1436A/

LTC1437A 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

LTC1436A/LTC1437A are operating in continuous mode

the duty cycles for the top and bottom MOSFETs are

given by:

Main Switch Duty Cycle =

OUT

Synchronous Switch Duty Cycle =

VIN −

()

OUT

Kool M

µ is a registered trademark of Magnetics, Inc.

MAIN

OUT

MAX

DS ON

MAX

RSS

SYNC

OUT

MAX

DS ON

() +

()

() ( )( )( )

−

() +

()

185

Inductor Core Selection

Once the value for L is known, the type of inductor must

be selected. High efficiency converters generally cannot

afford the core loss found in low cost powdered iron

cores, forcing the use of more expensive ferrite,

molypermalloy, or Kool M

µ® cores. Actual core loss is

independent of core size for a fixed inductor value, but it

is very dependent on inductance selected. As inductance

increases, core losses go down. Unfortunately, increased

inductance requires more turns of wire and therefore

copper losses will increase.

Ferrite designs have very low core loss and are prefered at

high switching frequencies, so design goals can concen-

trate on copper loss and preventing saturation. Ferrite

core material saturates “hard,” which means that induc-

tance collapses abruptly when the peak design current is

exceeded. This results in an abrupt increase in inductor

ripple current and consequent output voltage ripple. Do

not allow the core to saturate!

Molypermalloy (from Magnetics, Inc.) is a very good, low

loss core material for toroids, but it is more expensive than

ferrite. A reasonable compromise from the same manu-

facturer is Kool M

µ. Toroids are very space efficient,

especially when you can use several layers of wire.

Because they generally lack a bobbin, mounting is more

difficult. However, designs for surface mount are available

which do not increase the height significantly.

Power MOSFET and D1 Selection

Three external power MOSFETs must be selected for use

with the LTC1436A/LTC1437A: a pair of N-channel MOS-

FETs for the top (main) switch and an N-channel MOSFET

for the bottom (synchronous) switch.

To take advantage of the Adaptive Power output stage, two

topside MOSFETs must be selected. A large (low RSD(ON))

MOSFET and a small (higher RDS(ON)) MOSFET are

required. The large MOSFET is used as the main switch

and works in conjunction with the synchronous switch.

The smaller MOSFET is only enabled under low load

current conditions. This increases midcurrent efficiencies

while continuing to operate at constant frequency. Also, by

using the small MOSFET the circuit can maintain constant

The MOSFET power dissipations at maximum output

current are given by:

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