7

LTC4440

4440i

APPLICATIO S I FOR ATIO

BOOST

UP TO 100V

TS

TG

POWER

MOSFET

LOAD

INDUCTOR

4440 F03

LTC4440

Q1

N1

Figure 3. Capacitance Seen by TG During Switching

Overview

The LTC4440 receives a ground-referenced, low voltage

digital input signal to drive a high side N-channel power

MOSFET whose drain can float up to 100V above ground,

eliminating the need for a transformer between the low

voltage control signal and the high side gate driver. The

LTC4440 normally operates in applications with input

and continue to function during 100V, 100ms transients

on the input supply.

The powerful output driver of the LTC4440 reduces the

switching losses of the power MOSFET, which increase

with transition time. The LTC4440 is capable of driving a

1nF load with 10ns rise and 7ns fall times using a

Input Stage

The LTC4440 employs TTL/CMOS compatible input thresh-

olds that allow a low voltage digital signal to drive standard

power MOSFETs. The LTC4440 contains an internal volt-

age regulator that biases the input buffer, allowing the

switching transitions. However, care should be taken to

keep this pin from any noise pickup, especially in high

frequency, high voltage applications. The LTC4440 input

buffer has a high input impedance and draws negligible

input current, simplifying the drive circuitry required for

the input.

Output Stage

A simplified version of the LTC4440’s output stage is

shown in Figure 3 . The pull-down device is an N-channel

MOSFET (N1) and the pull-up device is an NPN bipolar

junction transistor (Q1). The output swings from the lower

(BOOST). This large voltage swing is important in driving

The LTC4440’s peak pull-up (Q1) current is 2.4A while the

pull-down (N1) resistance is 1.6

Ω. The low impedance of

N1 is required to discharge the power MOSFET’s gate

capacitance during high-to-low signal transitions. When

the power MOSFET’s gate is pulled low (gate shorted to

source through N1) by the LTC4440, its source (TS) is

pulled low by its load (e.g., an inductor or resistor). The

slew rate of the source/gate voltage causes current to flow

back to the MOSFET’s gate through the gate-to-drain

sufficient sink current capability (low output impedance),

mentarily pull the gate high, turning the MOSFET back on.

A similar scenario exists when the LTC4440 is used to

drive a low side MOSFET. When the low side power

MOSFET’s gate is pulled low by the LTC4440, its drain

voltage is pulled high by its load (e.g., inductor or resis-

tor). The slew rate of the drain voltage causes current to

flow back to the MOSFET’s gate through its gate-to-drain

capacitance. If the MOSFET driver does not have sufficient

sink current capability (low output impedance), the cur-

pull the gate high, turning the MOSFET back on.