LTC4440A-5
7
4440a5f
For more information www.linear.com/LTC4440A-5
applicaTions inForMaTion
Figure 1. Capacitance Seen by TG During Switching
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 (V
IN
) 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 V
BOOST–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
(V
IH
= 1.6V, V
IL
= 1.25V) to be relatively independent of
variations in V
CC
. The 350mV hysteresis between V
IH
and
V
IL
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 V
BE
(~0.7V) of the positive rail
(BOOST). This large voltage swing is important in driv-
ing external power MOSFETs, whose R
DS(ON)
is inversely
proportional to its gate overdrive voltage (V
GS
– V
TH
).
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
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 (C
GD
). If the
MOSFET driver does not have sufficient sink current ca-
pability (low output impedance), the current through the
power
MOSFET
’s C
GD
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 C
GD
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.
IN
UP TO 100V
–
TG
C
GD
POWER
MOSFET
LOAD
INDUCTOR
C
GS
4440A5 F01
LTC4440A-5
Q1
N1
