LTC3412A
9
3412afe
operAtion
The operating frequency is externally set by an external
resistor connected between the R
T
pin and ground. The
practical switching frequency can range from 300kHz to
4MHz.
Overvoltage and undervoltage comparators will pull the
PGOOD output low if the output voltage comes out of
regulation by ±7.5%. 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.
Forced Continuous Mode
Connecting the SYNC/MODE pin to SV
IN
will disable Burst
Mode operation and force continuous current operation.
At light loads, forced continuous mode operation is less
efficient than Burst Mode operation, but may be desirable in
some applications where it is necessary to keep switching
harmonics out of a signal band. The output voltage ripple
is minimized in this mode.
Burst Mode Operation
Connecting the SYNC/MODE pin to a voltage in the range
of 0V to 1V enables Burst Mode operation. In Burst Mode
operation, the internal power MOSFETs operate intermit-
tently at light loads. This increases efficiency by minimiz-
ing switching losses. During Burst Mode operation, the
minimum peak inductor current is externally set by the
voltage on the SYNC/MODE pin and the voltage on the I
TH
pin is monitored by the burst comparator to determine
when sleep mode is enabled and disabled. When the
average inductor current is greater than the load current,
the voltage on the I
TH
pin drops. As the I
TH
voltage falls
below 150mV, the burst comparator trips and enables
sleep mode. During sleep mode, the top power MOSFET
is held off and the I
TH
pin is disconnected from the output
of the error amplifier. The majority of the internal circuitry
is also turned off to reduce the quiescent current to 64µA
while the load current is solely supplied by the output
capacitor. When the output voltage drops, the I
TH
pin is
reconnected to the output of the error amplifier and the
top power MOSFET along with all the internal circuitry is
switched back on. This process repeats at a rate that is
dependent on the load demand.
Pulse-skipping operation is implemented by connecting
the SYNC/MODE pin to ground. This forces the burst
clamp level to be at 0V. As the load current decreases, the
peak inductor current will be determined by the voltage
on the I
TH
pin until the I
TH
voltage drops below 400mV. At
this point, the peak inductor current is determined by the
minimum on-time of the current comparator. If the load
demand is less than the average of the minimum on-time
inductor current, switching cycles will be skipped to keep
the output voltage in regulation.
Frequency Synchronization
The internal oscillator of the LTC3412A can be synchro-
nized to an external clock connected to the SYNC/MODE
pin. The frequency of the external clock can be in the
range of 300kHz to 4MHz. For this application, the oscil-
lator timing resistor should be chosen to correspond to
a frequency that is 25% lower than the synchronization
frequency. During synchronization, the burst clamp is set
to 0V, and each switching cycle begins at the falling edge
of the clock signal.
Dropout Operation
When the input supply voltage decreases toward the output
voltage, the duty cycle increases toward the maximum
on-time. Further reduction of the supply voltage forces
the main switch to remain on for more than one cycle
eventually reaching 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 LTC3412A is designed to operate down to an 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 LTC3412A is used
at 100% duty cycle with low input voltages to ensure that
thermal limits are not exceeded.
