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LTC4440IMS8E

High Speed, High Voltage High Side Gate Driver

Linear Technology 

LTC4440

APPLICATIO S I FOR ATIO

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

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 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

bootstrapped supply voltage VBOOST–TS of 12V.

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 input

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

variations in VCC. The 350mV hysteresis between VIH and

VIL eliminates false triggering due to noise during switch-

ing 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, sim-

plifying 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

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

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

external power MOSFETs, whose RDS(ON) is inversely

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

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

pull-down (N1) resistance is 1.5Ω. The low impedance of

N1 is required to discharge the power MOSFET’s gate

capacitance during high-to-low signal transitions. When

BOOST

VIN

UP TO 100V

LTC4440

Q1

TG

N1

TS

CGD

POWER

MOSFET

CGS

LOAD

INDUCTOR

V 4440 F03

–

Figure 3. Capacitance Seen by TG During Switching

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

capacitance (CGD). If the MOSFET driver does not have

sufficient sink current capability (low output impedance),

the current through the power MOSFET’s CGD can mo-

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-

rent 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

majority 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 can drive a 1nF load

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

The LTC4440’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.

4440f

7

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