LTC4355CS datasheet(10/20 Pages) LINER

10 / 20 page

LTC4355

4355fe

APPLICATIONS INFORMATION

Loop Stability

The servo loop is compensated by the parasitic capaci-

tance of the power N-channel MOSFET. No further

compensation components are normally required. In

the case when a MOSFET with less than 1000pF gate

capacitance is chosen, a 1000pF compensation capacitor

connected across the gate and source pins might be

required.

Design Example

Thefollowingdesignexampledemonstratesthecalculations

involved for selecting components in a 36V to 72V system

with 5A maximum load current (see Figure 2).

First, choose the N-channel MOSFET. The 100V, FDS3672

in the SO-8 package with RDS(ON) = 22mΩ(max) offers a

good solution. The maximum voltage drop across it is:

ΔV = 5A • 22mΩ = 110mV

The maximum power dissipation in the MOSFET is a mere:

P = 5A • 110mV = 0.55W

Next, select the resistive dividers that guarantee the

PWRFLT pins will not assert when the input supplies are

above 36V. The maximum VMONx(TH) is 1.245V and the

maximum IMONx(IN) is 1μA. Choose a 1% tolerance resistor

R1 = 12.7k. Then,

R2 =

MONx(TH)

R1(MIN)

MONx (TH)(MAX)

1.245V

12.7k

Ω(−1%)

+ 1μA = 100μA

Use IR2 to choose R2.

36V

− 1.245V

100

μA

= 348kΩ

Adjust R2 down by 1% to 344k to account for its tolerance.

The next lower standard resistor value is R2 = 340k.

The LED D1, a Panasonic Green LN1351C, requires at least

1mA of current to fully turn on. Therefore, R5 is set to 33k

to accommodate the lowest input supply voltage of 36V.

4355 F02

LTC4355

GND

GATE1

GATE2

IN1

OUT

IN2

MON1

MON2

SET

12.7k

340k

12.7k

340k

33k

GND

FDS3672

LOAD

VIN1 = +48V

VIN2 = +48V

VDSFLT

FUSEFLT1

FUSEFLT2

PWRFLT1

PWRFLT2

GREEN LEDs

PANASONIC LN1351C

Figure 2. 36V to 72V/5A Design Example

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