LTC4440AI-5 Просмотр технического описания (PDF) - Linear Technology

Номер в каталоге

Компоненты Описание

производитель

LTC4440AI-5

High Speed, High Voltage, High Side Gate Driver

Linear Technology 

LTC4440A-5

Applications Information

Overview

The LTC4440A-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

LTC4440A-5 normally operates in applications with input

supply voltages (VIN) up to 80V, but is able to withstand

and continue to function during 100V, 100ms transients

on the input supply.

The powerful output driver of the LTC4440A-5 reduces the

switching losses of the power MOSFET, which increase

with transition time. The LTC4440A-5 is capable of driv-

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

bootstrapped supply voltage VBOOST–TS of 6V.

Input Stage

The LTC4440A-5 employs TTL/CMOS compatible input

logic level or thresholds that allow a low voltage digital

signal to drive standard threshold power MOSFETs. The

LTC4440A-5 contains an internal voltage regulator that

biases the input buffer, allowing the input thresholds

(VIH = 1.6V, VIL = 1.25V) to be relatively independent of

variations in VCC. The 350mV hysteresis between VIH and

VIL eliminates 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 LTC4440A-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 LTC4440A-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

rail (TS) to within an NPN VBE (~0.7V) of the positive rail

(BOOST). This large voltage swing is important in driv-

ing external power MOSFETs, whose RDS(ON) is inversely

proportional to its gate overdrive voltage (VGS – VTH).

The LTC4440A-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

BOOST

VIN

UP TO 100V

LTC4440A-5

Q1

TG

N1

TS

CGD

POWER

MOSFET

CGS

LOAD

INDUCTOR

V 4440A5 F01

–

Figure 1. Capacitance Seen by TG During Switching

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 LTC4440A-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

gate through the gate-to-drain capacitance (CGD). If the

MOSFET driver does not have sufficient sink current ca-

pability (low output impedance), the current through the

power MOSFET’s CGD can momentarily pull the gate high,

turning the MOSFET back on.

A similar scenario exists when the LTC4440A-5 is used

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

MOSFET’s gate is pulled low by the LTC4440A-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

sink current capability (low output impedance), the current

through the power MOSFET’s CGD can momentarily pull

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 LTC4440A-5 can drive a 1nF

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

The LTC4440A-5’s rise and fall times are determined by

the peak current capabilities of Q1 and N1. The predriver

that drives Q1 and N1 uses a nonoverlapping transition

scheme to minimize cross-conduction currents. N1 is fully

turned off before Q1 is turned on and vice versa.

4440a5f

For more information www.linear.com/LTC4440A-5

7

Share Link: