LTC4442/LTC4442-1
8
4442fb
OPERATION
Overview
The LTC4442 receives a ground-referenced, low voltage
digital input signal to drive two N-channel power MOSFETs
in a synchronous buck power supply confi guration. The
gate of the low side MOSFET is driven either to V
CC
or GND,
depending on the state of the input. Similarly, the gate of
the high side MOSFET is driven to either BOOST or TS by
a supply bootstrapped off of the switch node (TS).
Input Stage
The LTC4442 employs a unique three-state input stage with
transition thresholds that are proportional to the V
LOGIC
supply. The V
LOGIC
supply can be tied to the controller
IC’s power supply so that the input thresholds will match
those of the controller’s output signal. Alternatively, V
LOGIC
can be tied to V
CC
to simplify routing. An internal voltage
regulator in the LTC4442 limits the input threshold values
for V
LOGIC
supply voltages greater than 5V.
The relationship between the transition thresholds and the
three input states of the LTC4442 is illustrated in Figure 1.
When the voltage on IN is greater than the threshold V
IH(TG)
,
TG is pulled up to BOOST, turning the high side MOSFET
on. This MOSFET will stay on until IN falls below V
IL(TG)
.
Similarly, when IN is less than V
IH(BG)
, BG is pulled up
to V
CC
, turning the low side (synchronous) MOSFET on.
BG will stay high until IN increases above the threshold
V
IL(BG)
.
TG HIGH
TG HIGH
V
IH(TG)
V
IL(BG)
V
IL(TG)
V
IH(BG)
IN
TG LOW
TG LOW
BG LOW
BG HIGH
4442 F01
BG LOW
BG HIGH
Figure 1. Three-State Input Operation
The thresholds are positioned to allow for a region in which
both BG and TG are low. An internal resistive divider will
pull IN into this region if the signal driving the IN pin goes
into a high impedance state.
One application of this three-state input is to keep both of
the power MOSFETs off while an undervoltage condition
exists on the controller IC power supply. This can be ac-
complished by driving the IN pin with a logic buffer that
has an enable pin. With the enable pin of the buffer tied
to the power good pin of the controller IC, the logic buf-
fer output will remain in a high impedance state until the
controller confi rms that its supply is not in an undervoltage
state. The three-state input of the LTC4442 will therefore
pull IN into the region where TG and BG are low until the
controller has enough voltage to operate predictably.
The hysteresis between the corresponding V
IH
and V
IL
voltage levels eliminates false triggering due to noise
during switch transitions; however, care should be taken
to keep noise from coupling into the IN pin, particularly
in high frequency, high voltage applications.
Undervoltage Lockout
The LTC4442 contains undervoltage lockout detectors that
monitor both the V
CC
and V
LOGIC
supplies. When V
CC
falls
below 3.04V or V
LOGIC
falls below 2.65V, the output pins
BG and TG are pulled to GND and TS, respectively. This
turns off both of the external MOSFETs. When V
CC
and
V
LOGIC
have adequate supply voltage for the LTC4442 to
operate reliably, normal operation will resume.
Adaptive Shoot-Through Protection
Internal adaptive shoot-through protection circuitry
monitors the voltages on the external MOSFETs to ensure
that they do not conduct simultaneously. The LTC4442
does not allow the bottom MOSFET to turn on until the
gate-source voltage on the top MOSFET is suffi ciently
low, and vice-versa. This feature improves effi ciency by
