Datasheet

LTC3775
13
3775fa
APPLICATIONS INFORMATION
To prevent discharging a pre-biased V
OUT
, the LTC3775
always starts switching in pulse-skipping mode up to SS =
0.54V, regardless of the mode selected by the MODE/SYNC
pin. Thus if V
OUT
> 0V during power-up, V
OUT
will remain
at the pre-biased voltage (if there is no load) until the SS
voltage catches up with V
OUT
, after which V
OUT
will track
the SS ramp. The LTC3775 reverts to the selected mode
once SS > 0.54V.
Constant Switching Frequency
The internal oscillator can be programmed from 250kHz
to 1MHz with an external resistor from the FREQ pin to
ground, in order to optimize component size, effi ciency
and noise for the specifi c application. The internal oscillator
can also be synchronized to an external clock connected
to the MODE/SYNC pin and can lock to a range of ±20%
of the programmed free-running frequency. When locked
to an external clock, pulse-skipping mode operation is
automatically disabled. Constant frequency operation of-
fers a number of benefi ts: inductor and capacitor values
can be chosen for a precise operating frequency and the
feedback loop can be similarly tightly specifi ed. Noise
generated by the circuit will always be at known frequen-
cies. Subharmonic oscillation and slope compensation,
common headaches with constant frequency current
mode switchers, are absent in voltage mode designs like
the LTC3775.
Thermal Shutdown
The LTC3775 has a thermal detector that pulls the driver
outputs low if the junction temperature of the chip ex-
ceeds 165°C. The thermal shutdown circuit has 25°C of
hysteresis.
Current Limit
The LTC3775 includes an onboard cycle-by-cycle current
limit circuit that limits the maximum output current to a
user-programmed level. The current limit circuit consists
of two comparators, CTLIM and CBLIM that monitor the
voltage drop across the top and bottom MOSFETs respec-
tively. Since the MOSFET ’s effective resistance, R
DS(ON)
,
is low during its on-time, the voltage drop from the drain
to source is proportional to the current fl ow. Alternatively,
for better accuracy, the topside current may be monitored
with a sense resistor.
The benefi t of having two comparators is to allow continu-
ous monitoring and cycle-by-cycle control of the inductor
current regardless of the operating duty cycle. In high
duty cycle operation the top MOSFET, Q
T
, is on most of
the time. Thus, a high side comparator is necessary to
limit the output current during high duty cycle operation.
Architectures that contain only one comparator to monitor
the low side MOSFET will not effectively limit the output
current during high duty cycle operation. Conversely, during
low duty cycle operation, a low side comparator is neces-
sary to limit the output current. Another common current
sensing scheme uses a sense resistor in series with the
inductor to allow continuous monitoring. However, this
scheme restricts the range of V
OUT
due to the common
mode range of the current limit comparator. The LTC3775
does not have this V
OUT
restriction.
Figure 4 shows the current limit circuitry. The top current
limit comparator, CTLIM monitors the current through the
top MOSFET, Q
T
, when TG is high. If the inductor current
exceeds the current limit threshold when Q
T
is on, Q
T
turns
off immediately and the bottom MOSFET, Q
B
, turns on. The
SENSE pin is the input for CTLIM. For applications where
Figure 4. LTC3775 Current Limit Circuit
+
+
–
100μA
R
ILIMB
(OPT)
10μA
0.2 • V
ILIMB
R
ILIMT
V
IN
LTC3775
R
SENSE
SENSE
CTLIM
TURN OFF TG
+
–
CBLIM
EXTEND BG
I
LIMB
SW
I
LIMT
TG Q
T
V
IN
BG Q
B
V
OUT
3775 F04
PGND
SGND