Datasheet

LTC4442/LTC4442-1
9
4442fb
OPERATION
8
BOOST
LTC4442
TG
N1
Q1
1
TS
C
GS
C
GD
HIGH SIDE
POWER
MOSFET
V
IN
UP TO 38V
2
7
V
CC
BG
N2
Q2
Q3
3
GND
C
GS
4442 F02
C
GD
LOW SIDE
POWER
MOSFET
4
LOAD
INDUCTOR
Figure 2. Capacitance Seen by BG and TG During Switching
eliminating cross-conduction current from fl owing from
the V
IN
supply through the MOSFETs to ground during a
switch transition.
Output Stage
A simplifi ed version of the LTC4442’s output stage is
shown in Figure 2. The pull-up device on both the BG and
TG outputs is an NPN bipolar junction transistor (Q1 and
Q2). The BG and TG outputs are pulled up to within an
NPN V
BE
(~0.7V) of their positive rails (V
CC
and BOOST,
respectively). Both BG and TG have N-channel MOSFET pull-
down devices (N1 and N2) which pull BG and TG down to
their negative rails, GND and TS. An additional NPN bipolar
junction transistor (Q3) is present on BG to increase its
pull-down drive current capacity. The large voltage swing of
the BG and TG output pins is important in driving external
power MOSFETs, whose R
DS(ON)
is inversely proportional
to its gate overdrive voltage (V
GS
– V
TH
).
Rise/Fall Time
Since the power MOSFET generally accounts for the major-
ity of power loss in a converter, it is important to quickly
turn it on and off, thereby minimizing the transition time
and power loss. The LTC4442’s peak pull-up current of
2.4A for both BG and TG (Q1 and Q2) produces a rapid
turn-on transition for the MOSFETs. This high current is
capable of driving a 3nF load with a 12ns rise time.
It is also important to turn the power MOSFETs off quickly
to minimize power loss due to transition time; however,
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 power MOSFET off
and TG turns the high-side power MOSFET on, the volt-
age on the TS pin will rise to V
IN
very rapidly. This high
frequency positive voltage transient will couple through
the C
GD
capacitance of the low side power MOSFET to
the BG pin. If the BG pin is not held down suffi ciently, the
voltage on the BG pin will rise above the threshold volt-
age of the low side power MOSFET, momentarily turning
it back on. As a result, both the high side and low side
MOSFETs will be conducting, which will cause signifi cant
cross-conduction current to fl ow through the MOSFETs
from V
IN
to ground, thereby introducing substantial power
loss. A similar effect occurs on TG due to the C
GS
and C
GD
capacitances of the high side MOSFET.
The LTC4442’s powerful parallel combination of the
N-channel MOSFET (N2) and NPN (Q3) on the BG pull-down
generates a phenomenal 5ns fall time on BG while driving
a 3nF load. Similarly, the 1Ω pull-down MOSFET (N1) on
TG results in a rapid 8ns fall time with a 3nF load. These
powerful pull-down devices minimize the power loss as-
sociated with MOSFET turn-off time and cross-conduction
current.