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LTC1624

where

OUT MAX

OUT

: ISW MAX

()













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

constant inversely related to the gate drive current. The

maximum switch current occurs at VIN(MIN) and the peak

switch current is ISW(MAX) + ∆IL/2. The maximum voltage

across the switch is VIN(MAX) + VOUT.

MOSFETs have I2R losses plus the PMAIN equation

includes an additional term for transition losses that are

highest at high total input plus output voltages. For

(

VOUT+ VIN) < 20V the high current efficiency generally

improves with larger MOSFETs, while for (

VOUT+ 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. For additional informa-

tion refer to the Step-Down Converter: Power MOSFET

Selection in the Applications Information section.

Positive-to-Negative Converter: Inductor Selection

For most applications the inductor will fall in the range of

µH to 100µH. Higher values reduce the input and output

ripple voltage (although not as much as step-down con-

verters) and also reduce core loss. Lower inductor values

are chosen to reduce physical size and improve transient

response but do increase output ripple.

Like the boost converter, the input current of the positive-

to-negative converter is calculated at full load current.

Peak inductor current can be significantly higher than

output current, especially with smaller inductors (with

high

∆IL values). The following formula assumes continu-

ous mode operation and calculates maximum peak induc-

tor current at minimum VIN:

L PEAK

OUT MAX

OUT

()

( )













∆

The ripple current in the inductor (

∆IL) is typically 20% to

50% of the peak inductor current occuring at VIN(MIN) and

IOUT(MAX) to minimize output ripple. Maximum ∆IL occurs

at minimum VIN.

APPLICATIONS INFORMATION

∆I

kHz L V

OUT

P-P

() =

()

()( ) ++

()

200

Specify the maximum inductor current to safely handle

IL(PEAK). Make sure the inductor’s saturation current rat-

ing (current when inductance begins to fall) exceeds the

maximum current rating set by RSENSE.

Positive-to-Negative 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 with the

remainder divided by the duty cycle.

Allowing a margin for variations in the LTC1624 (without

considering variation in RSENSE) and assuming 30% ripple

current in the inductor, yields:

SENSE

OUT MAX

IN MIN

OUT













()

100

Positive-to-Negative Converter: Output Diode

The output diode conducts current only during the switch

off-time. Peak reverse voltage for positive-to-negative

converters is equal to

VOUT+ VIN. Average forward

current in normal operation is equal to ID(PEAK) – ∆IL/2.

Peak diode current (occurring at VIN(MIN)) is:

D PEAK

OUT MAX

OUT

()

( )

()













∆

Positive-to-Negative Converter: Input and

Output Capacitors

The output capacitor is normally chosen by its effective

series resistance (ESR), because this is what determines

output ripple voltage. Both input and output capacitors

need to be sized to handle the ripple current safely.

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