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LTC4412HV

sn4412hv 4412hvfs

APPLICATIO S I FOR ATIO

Introduction

The system designer will find the LTC4412HV useful in a

variety of cost and space sensitive power control applica-

tions that include low loss diode OR’ing, fully automatic

switchover from a primary to an auxiliary source of power,

microcontroller controlled switchover from a primary to

an auxiliary source of power, load sharing between two or

more batteries, charging of multiple batteries from a

single charger and high side power switching.

External P-Channel MOSFET Transistor Selection

Important parameters for the selection of MOSFETs are

must be high enough to withstand the maximum drain-

source voltage seen in the application.

The maximum gate drive voltage for the primary MOSFET

monly used, but if a low supply voltage limits the gate

voltage, a sub-logic level threshold MOSFET should be

considered. The maximum gate drive voltage for the

auxiliary MOSFET, if used, is determined by the external

resistor connected to the STAT pin and the STAT pin sink

current.

As a general rule, select a MOSFET with a low enough

operates in the linear region and acts like a voltage

controlled resistor. If the MOSFET is grossly undersized,

result. However, the drain-source diode of the MOSFET, if

the load current, will likely result in excessively high

MOSFET power dissipation. Keep in mind that the

LTC4412HV will regulate the forward voltage drop across

by the load current in amps. Achieving forward regulation

will minimize power loss and heat dissipation, but it is not

a necessity. If a forward voltage drop of more than 20mV

is acceptable then a smaller MOSFET can be used, but

must be sized compatible with the higher power dissipa-

tion. Care should be taken to ensure that the power

dissipated is never allowed to rise above the manufacturer’s

recommended maximum level. The auxiliary MOSFET

power switch, if used, has similar considerations, but its

the resistor value consider the full range of STAT pin

Many types of capacitors, ranging from 0.1

µF to 10µF and

bypassing if needed. Voltage droop can occur at the load

during a supply switchover because some time is required

to turn on the MOSFET power switch. Factors that deter-

mine the magnitude of the voltage droop include the

supply rise and fall times, the MOSFET’s characteristics,

is inversely proportional to the capacitance. Bypass ca-

pacitance for the load also depends on the application’s

dynamic load requirements and typically ranges from 1

µF

to 47

µF. In all cases, the maximum droop is limited to the

drain source diode forward drop inside the MOSFET.

Caution must be exercised when using multilayer ceramic

capacitors. Because of the self resonance and high Q

characteristics of some types of ceramic capacitors, high

voltage transients can be generated under some start-up

conditions such as connecting a supply input to a hot

power source. To reduce the Q and prevent these tran-

sients from exceeding the LTC4412HV’s absolute maxi-

mum voltage rating, the capacitor’s ESR can be increased

by adding up to several ohms of resistance in series with

the ceramic capacitor. Refer to Application Note 88.

The selected capacitance value and capacitor’s ESR can be

age transitions during dynamic conditions over the full

load current range. This should be checked with each

power source as well. Ringing may indicate an incorrect

bypass capacitor value and/or too low an ESR.