LTC4444 Linear Technology, LTC4444 Datasheet - Page 8

• Manufacturer Part Number: LTC4444
• Description: High Voltage Synchronous N-Channel MOSFET Driver
• Manufacturer: Linear Technology

OPERATION
LTC4444
Output Stage
A simplifi ed version of the LTC4444’s output stage is shown
in Figure 1. The pull-up devices on the BG and TG outputs
are NPN bipolar junction transistors (Q1 and Q2). The BG
and TG outputs are pulled up to within an NPN V
of their positive rails (V
BG and TG have N-channel MOSFET pull-down devices
(M1 and M2) which pull BG and TG down to their nega-
tive rails, GND and TS. The large voltage swing of the BG
and TG output pins is important in driving external power
MOSFETs, whose R
gate overdrive voltage (V
Rise/Fall Time
The LTC4444’s rise and fall times are determined by the
peak current capabilities of Q1 and M1. The predriver that
drives Q1 and M1 uses a nonoverlapping transition scheme
to minimize cross-conduction currents. M1 is fully turned
off before Q1 is turned on and vice versa.
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. An additional benefi t of a strong
pull-down on the driver outputs is the prevention of cross-
conduction current. For example, when BG turns the low
side (synchronous) power MOSFET off and TG turns the
high side power MOSFET on, the voltage on the TS pin
will rise to V
voltage transient will couple through the C
of the low side power MOSFET to the BG pin. If there is
an insuffi cient pull-down on the BG pin, the voltage on
the BG pin can rise above the threshold voltage of the low
side power MOSFET, momentarily turning it back on. With
both the high side and low side MOSFETs conducting,
signifi cant cross-conduction current will fl ow through the
MOSFETs from V
power loss. A similar effect occurs on TG due to the C
and C
The powerful output driver of the LTC4444 reduces the
switching losses of the power MOSFET, which increase
with transition time. The LTC4444’s high side driver is
8
GD
capacitances of the high side MOSFET.
IN
very rapidly. This high frequency positive
IN
DS(ON)
to ground and will cause substantial
CC
GS
and BOOST, respectively). Both
is inversely proportional to the
− V
TH
).
GD
capacitance
BE
(~0.7V)
GS
capable of driving a 1nF load with 8ns rise and 5ns fall
times using a bootstrapped supply voltage V
12V while its low side driver is capable of driving a 1nF
load with 6ns rise and 3ns fall times using a supply volt-
age V
Undervoltage Lockout (UVLO)
The LTC4444 contains an undervoltage lockout detector
that monitors V
the output pins BG and TG are pulled down to GND and
TS, respectively. This turns off both external MOSFETs.
When V
will resume.
Adaptive Shoot-Through Protection
Internal adaptive shoot-through protection circuitry moni-
tors the voltages on the external MOSFETs to ensure that
they do not conduct simultaneously. This feature improves
effi ciency by eliminating cross-conduction current from
fl owing from the V
to ground during a switch transition. The adaptive shoot-
through protection circuitry also monitors the level of the
TS pin. If the TS pin stays high, BG will be turned on 150ns
after TG is turned off.
Figure 1. Capacitance Seen by BG and TG During Switching
CC
CC
of 12V.
LTC4444
has adequate supply voltage, normal operation
BOOST
CC
V
6
CC
3
Q1
Q2
IN
M1
M2
supply. When V
supply through both of the MOSFETs
GND
TG
TS
BG
7
8
4
9
www.DataSheet4U.com
C
C
C
C
GD
GS
GD
GS
CC
UP TO 100V
falls below 6.15V,
V
IN
HIGH SIDE
POWER
MOSFET
LOW SIDE
POWER
MOSFET
INDUCTOR
LOAD
BOOST-TS
4444f
of