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LTC3776

3776f

The typical LTC3776 application circuit is shown in

Figure 11. External component selection for each of the

LTC3776’s controllers is driven by the load requirement

and begins with the selection of the inductor (L) and the

power MOSFETs (MP and MN).

Power MOSFET Selection

Each of the LTC3776’s two controllers requires two exter-

nal power MOSFETs: a P-channel MOSFET for the topside

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

(synchronous) switch. Important parameters for the power

MOSFETs are the breakdown voltage VBR(DSS) , threshold

voltage VGS(TH), on-resistance RDS(ON) , reverse transfer

capacitance CRSS, turn-off delay tD(OFF) and the total gate

charge QG.

The gate drive voltage is the input supply voltage. Since the

LTC3776 is designed for operation down to low input

voltages, a sublogic level MOSFET (RDS(ON) guaranteed at

VGS = 2.5V) is required for applications that work close to

this voltage. When these MOSFETs are used, make sure

that the input supply to the LTC3776 is less than the abso-

lute maximum MOSFET VGS rating, which is typically 8V.

The P-channel MOSFET’s on-resistance is chosen based

on the required load current. The maximum average

output load current IOUT(MAX) is equal to the peak inductor

current minus half the peak-to-peak ripple current IRIPPLE.

The LTC3776’s current comparator monitors the drain-to-

source voltage VDS of the P-channel MOSFET, which is

sensed between the SENSE+ and SW pins. The peak

inductor current is limited by the current threshold, set by

the voltage on the ITH pin of the current comparator. The

voltage on the ITH pin is internally clamped, which limits

the maximum current sense threshold ∆VSENSE(MAX).

The output current that the LTC3776 can provide is given

by:

OUT MAX

SENSE MAX

DS ON

RIPPLE

()

–

∆

A reasonable starting point is setting ripple current IRIPPLE

to be 40% of IOUT(MAX). Rearranging the above equation

yields:

DS ON MAX

SENSE MAX

OUT MAX

()(

)

()

•

∆

for Duty Cycle < 20%.

However, for operation above 20% duty cycle, slope

compensation has to be taken into consideration to select

the appropriate value of RDS(ON) to provide the required

amount of load current:

RSF

DS ON MAX

SENSE MAX

OUT MAX

()(

)

()

••

∆

where SF is a scale factor whose value is obtained from the

curve in Figure 2.

These must be further derated to take into account the

significant variation in on-resistance with temperature.

The following equation is a good guide for determining the

required RDS(ON)MAX at 25°C (manufacturer’s specifica-

tion), allowing some margin for variations in the LTC3776

and external component values:

RSF

DS ON MAX

SENSE MAX

OUT MAX

()(

)

()

•. •

•

∆

The ρT is a normalizing term accounting for the tempera-

ture variation in on-resistance, which is typically about

0.4%/°C, as shown in Figure 5. Junction to case tempera-

ture TJC is about 10°C in most applications. For a maxi-

mum ambient temperature of 70°C, using ρ80°C ~ 1.3 in

the above equation is a reasonable choice.

APPLICATIO S I FOR ATIO

INPUT VOLTAGE (V)

0.2

0.6

0.8

1.0

2.0

1.4

3776 F04

0.4

1.6

1.8

1.2

SINGLE PHASE

DUAL CONTROLLER

2-PHASE

DUAL CONTROLLER

VOUT1 = 2.5V/2A

VOUT2 = 1.8V/2A

Figure 4. RMS Input Current Comparison

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