LTC4440-5

7

44405fb

applicaTions inForMaTion

Figure 1. Capacitance Seen by TG During Switching

Overview

The LTC4440-5 receives a ground-referenced, low voltage

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

MOSFET whose drain can float up to 80V above ground,

eliminating the need for a transformer between the low

voltage control signal and the high side gate driver. The

LTC4440-5 normally operates in applications with input

and continue to function during 80V, 100ms transients

on the input supply.

The powerful output driver of the LTC4440-5 reduces the

switching losses of the power MOSFET, which increase

with transition time. The LTC4440-5 is capable of driv-

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

Input Stage

TheLTC4440-5employsTTL/CMOScompatibleinputlogic

level or thresholds that allow a low voltage digital signal to

drive standard threshold power MOSFETs. The LTC4440-5

contains an internal voltage regulator that biases the input

false triggering due to noise during switching transitions.

However, care should be taken to keep this pin from any

noise pickup, especially in high frequency, high voltage

applications. The LTC4440-5 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 LTC4440-5’s output stage is

shown in Figure 1. 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 driv-

The LTC4440-5’s peak pull-up (Q1) current is 1.1A while

the pull-down (N1) resistance is 1.85Ω, with a BOOST-

TS supply of 6V. 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 LTC4440-5, 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

MOSFET driver does not have sufficient sink current ca-

pability (low output impedance), the current through the

turning the MOSFET back on.

A similar scenario exists when the LTC4440-5 is used

to drive a low side MOSFET. When the low side power

MOSFET’s gate is pulled low by the LTC4440-5, 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

sinkcurrentcapability(lowoutputimpedance),thecurrent

the gate high, turning the MOSFET back on.

Rise/Fall Time

Since the power MOSFET generally accounts for the ma-

jority of the power loss in a converter, it is important to

quickly turn it on or off, thereby minimizing the transition

time in its linear region. The LTC4440-5 can drive a 1nF

load with a 10ns rise time and 7ns fall time.

The LTC4440-5’s rise and fall times are determined by the

peak current capabilities of Q1 and N1. The predriver that

drivesQ1andN1usesanonoverlappingtransitionscheme

to minimize cross-conduction currents. N1 is fully turned

off before Q1 is turned on and vice versa.

BOOST

UP TO 100V

TS

TG

POWER

MOSFET

LOAD

INDUCTOR

4440 F01

LTC4440-5

Q1

N1