Datasheet

6
LTC3413
3413fb
OPERATIO
U
Main Control Loop
The LTC3413 is a monolithic, constant frequency, current
mode step-down DC/DC converter that is capable of sourc-
ing and sinking current at the output. During normal op-
eration, the internal top power switch (P-channel MOSFET)
is turned on at the beginning of each clock cycle. Current
in the inductor increases until the current comparator trips
and turns off the top power MOSFET. The peak inductor
current at which the current comparator shuts off the top
power switch is controlled by the voltage on the I
TH
pin.
The error amplifier adjusts the voltage on the I
TH
pin by
comparing the feedback signal on the V
FB
pin with a refer-
ence voltage that is equal to one-half of the voltage on the
V
REF
pin. When the load current increases, it causes a
reduction in the feedback voltage relative to the reference.
The error amplifier raises the I
TH
voltage until the average
inductor current matches the new load current. When the
top power MOSFET shuts off, the synchronous power
switch (N-channel MOSFET) turns on until either the bot-
tom current limit is reached or the beginning of the next
clock cycle. The bottom current limit is set at –7A.
The operating frequency is set by an external resistor
connected between the R
T
pin and ground. The switching
frequency can range from 300kHz to 2MHz.
Overvoltage and undervoltage comparators will pull the
PGOOD output low if the output voltage comes out of
regulation by ±10%. In an overvoltage condition, the top
power MOSFET is turned off and the bottom power MOSFET
is switched on until either the overvoltage condition clears
or the bottom MOSFET’s current limit is reached.
Dropout Operation
When the input supply voltage decreases toward the
output voltage, the duty cycle increases toward the maxi-
mum on-time. Further reduction of the supply voltage
forces the main switch to remain on for more than one
cycle until it reaches 100% duty cycle. The output voltage
will then be determined by the input voltage minus the
voltage drop across the internal P-channel MOSFET and
the inductor.
Low Supply Operation
The LTC3413 is designed to operate down to an SV
IN
input
supply voltage of 2.25V. One important consideration at
low input supply voltages is that the R
DS(ON)
of the P-
channel and N-channel power switches increases. The
user should calculate the power dissipation when the
LTC3413 is used at 100% duty cycle with low input
voltages to ensure that thermal limits are not exceeded.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant fre-
quency architectures by preventing subharmonic oscilla-
tions at duty cycles greater than 50%. It is accomplished
internally by adding a compensating ramp to the inductor
current signal at duty cycles in excess of 40%. Normally,
the maximum inductor peak current is reduced when slope
compensation is added. In the LTC3413, however, slope
compensation recovery is implemented to keep the maxi-
mum inductor peak current constant throughout the range
of duty cycles.
Short-Circuit Protection
When the output is shorted to ground, the inductor current
decays very slowly during a single switching cycle. To
prevent current runaway from occurring, a secondary
current limit is imposed on the inductor current. If the
inductor valley current increases greater than 5A, the top
power MOSFET will be held off and switching cycles will be
skipped until the inductor current is reduced.