LTC3606B
7
3606bfb
OPERATION
The LTC3606B uses a constant-frequency, current mode
architecture. The operating frequency is set at 2.25MHz.
The output voltage is set by an external resistor divider
returned to the V
FB
pins. An error amplifi er compares the
divided output voltage with a reference voltage of 0.6V and
regulates the peak inductor current accordingly.
The LTC3606B continuously monitors the input current
via the voltage drop across the R
DS(ON)
of the internal
P-channel MOSFET. When the input current exceeds the
programmed input current limit set by an external resistor,
R
LIM
, the regulator’s input current is limited. The regulator
output voltage will drop to meet output current demand
and to maintain constant input current.
Main Control Loop
During normal operation, the top power switch (P-channel
MOSFET) is turned on at the beginning of a clock cycle
when the V
FB
voltage is below the reference voltage. The
current into the inductor and the load increases until the
peak inductor current (controlled by I
TH
) is reached. The
RS latch turns off the synchronous switch and energy
stored in the inductor is discharged through the bottom
switch (N-channel MOSFET) into the load until the next
clock cycle begins, or until the inductor current begins to
reverse (sensed by the I
RCMP
comparator).
The peak inductor current is controlled by the internally
compensated I
TH
voltage, which is the output of the error
amplifi er. This amplifi er regulates the V
FB
pin to the internal
0.6V reference by adjusting the peak inductor current
accordingly.
When the input current limit is engaged, the peak inductor
current will be lowered, which then reduces the switching
duty cycle and V
OUT
. This allows the input voltage to stay
regulated when its programmed current output capability
is met.
Light Load Operation
The LTC3606B will automatically transition from continuous
operation to the pulse-skipping operation when the load
current is low. The inductor current is not fi xed during the
pulse-skipping mode which allows the LTC3606B to switch
at constant-frequency down to very low currents, where it
will begin skipping pulses to maintain output regulation.
This mode of operation exhibits low output ripple as well
as low audio noise and reduced RF interference while
providing reasonable low current effi ciency.
Dropout Operation
When the input supply voltage decreases toward the
output voltage the duty cycle increases to 100%, which
is the dropout condition. In dropout, the PMOS switch is
turned on continuously with the output voltage being equal
to the input voltage minus the voltage drops across the
internal P-channel MOSFET and the inductor.
An important design consideration is that the R
DS(ON)
of the P-channel switch increases with decreasing input
supply voltage (see the Typical Performance Characteristics
section). Therefore, the user should calculate the worst-
case power dissipation when the LTC3606B is used at
100% duty cycle with low input voltage (see Thermal
Considerations in the Applications Information section).
Soft-Start
In order to minimize the inrush current on the input by-
pass capacitor, the LTC3606B slowly ramps up the output
voltage during start-up. Whenever the RUN pin is pulled
high, the corresponding output will ramp from zero to
full-scale over a time period of approximately 950µs. This
prevents the LTC3606B from having to quickly charge the
output capacitor and thus supplying an excessive amount
of instantaneous current.
When the output is loaded heavily, for example, with mil-
lifarad of capacitance, it may take longer than 950µs to
charge the output to regulation. If the output is still low
after the soft-start time, the LTC3606B will try to quickly
charge the output capacitor. In this case, the input current
limit (after it engages) can prevent excessive amount of
instantaneous current that is required to quickly charge
the output. See the Start-Up from Shutdown curve
(C
L
= 4.4mF)in the Typical Performance Characteristics
section. After input current limit is engaged, the output
slowly ramps up to regulation while limited by its 500mA
of input current.
