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LTC1876

1876fa

Low VIN Applications

In applications where the input supply is low (<5V), the

LTC1876 auxiliary regulator can be used to step-up the

input to provide the gate drive to the external MOSFETs as

shown in Figure 9.

Shown in the Typical Application section of the data sheet

is a circuit (3.3VIN Dual-Phase High Efficiency Power

Supply) with input supply of 3.3V. The boost section of the

LTC1876 is set up to generate 5V and is used to provide the

gate drive to the external MOSFETs. The circuit provides

dual outputs, a 2.5V/15A and 1.8V/15A. Both drawing

power directly from VIN.

APPLICATIO S I FOR ATIO

Figure 9. Generating the Gate Drive

for Low Input Supply Applications

Figure 10. Single Output Configuration

Single Output/High Current Applications

In applications that demand current much higher than a

single stage can supply (>20A), the LTC1876 can be

configured as a single output converter. Figure 10 shows

the block diagram of the configuration. Note that the

compensation pins (ITH1 and ITH2) of the two channels are

connected together, saving a set of passive components.

In addition, the output voltage sense pins (VOSENSE1 and

VOSENSE2) are shorted together, using only one resistor

divider to set the output voltage.

Although the output current requirement is high, the input

capacitors ripple current requirement is not much differ-

ent compared to the dual outputs circuit. This is attributed

to the fact that the current is shared between two channels

and an out-of-phase architecture is implemented for the

controllers

(See Theory and Benefits of 2-Phase

Operation).

Auxiliary Regulator’s Inductor Value Calculation

Since the current limit for the auxiliary regulator is inter-

nally set at 1A, it makes the selection of components

easier. For the boost regulator, the duty cycle is given by:

Duty Cycle

OUT

= 1–

Since energy is only transferred to the output capacitor(s)

during the off-time, the maximum output current that can

be supplied by the regulator without losing regulation is:

IOUT = 0.5(2 • IPK – ∆IL)(1 – Duty Cycle)

where IPK = peak inductor current and is internally set at

1A.

∆IL = inductor’s ripple current

With the required ripple current determined, the value of

the inductor is:

Duty Cycle

∆

(•

)

(•

)

where f = operating frequency (1.2MHz)

In most cases, a larger value of inductance is used. This is

done to account for component variation. It also lowers

the inductor ripple current and results in lower core

losses. In addition, lower ripple also translates into lower

ESR losses in the output capacitors and smaller output

voltage ripple.

1876 F09

LTC1876

BOOST

SECTION

LTC1876

STEP-DOWN

SECTION

AUXVIN

AUXSW

AUXVFB

SGND

EXTERNAL

MOSFETs

VIN

COUT

INPUT

SUPPLY

1876 F10

LTC1876

SGND

EXTERNAL

MOSFETs

VIN

VOUT

INPUT

SUPPLY

ITH1

ITH2

VOSENSE1

VOSENSE2

TO SENSE1+

AND SENSE1–

TO SENSE2+

AND SENSE2–

RS1

RS2

LTC1876 Datasheet(PDF) 26 Page - Linear Technology

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