LTC3642
(Refer to Block Diagram)
OPERATION
greaterthantheaverageloadcurrent.Forthisarchitecture,
the maximum average output current is equal to half of
the peak current.
TheLTC3642isastep-downDC/DCconverterwithinternal
power switches that uses Burst Mode control, combin-
ing low quiescent current with high switching frequency,
which results in high efficiency across a wide range of
load currents. Burst Mode operation functions by using
short “burst” cycles to ramp the inductor current through
the internal power switches, followed by a sleep cycle
where the power switches are off and the load current is
supplied by the output capacitor. During the sleep cycle,
the LTC3642 draws only 12ꢀA of supply current. At light
loads, the burst cycles are a small percentage of the total
cycle time which minimizes the average supply current,
greatly improving efficiency.
The hysteretic nature of this control architecture results
in a switching frequency that is a function of the input
voltage, output voltage and inductor value. This behavior
provides inherent short-circuit protection. If the output
is shorted to ground, the inductor current will decay very
slowly during a single switching cycle. Since the high side
switch turns on only when the inductor current is near
zero,theLTC3642inherentlyswitchesatalowerfrequency
during start-up or short-circuit conditions.
Start-Up and Shutdown
IfthevoltageontheRUNpinislessthan0.7V, theLTC3642
enters a shutdown mode in which all internal circuitry is
disabled,reducingtheDCsupplycurrentto3ꢀA.Whenthe
voltageontheRUNpinexceeds1.21V,normaloperationof
the main control loop is enabled. The RUN pin comparator
has 110mV of internal hysteresis, and therefore must fall
below 1.1V to disable the main control loop.
Main Control Loop
The feedback comparator monitors the voltage on the V
FB
pin and compares it to an internal 800mV reference. If
this voltage is greater than the reference, the comparator
activates a sleep mode in which the power switches and
current comparators are disabled, reducing the V pin
IN
supplycurrenttoonly12ꢀA.Astheloadcurrentdischarges
The HYST pin provides an added degree of flexibility for
the RUN pin operation. This open-drain output is pulled
to ground whenever the RUN comparator is not tripped,
signaling that the LTC3642 is not in normal operation. In
applications where the RUN pin is used to monitor the
the output capacitor, the voltage on the V pin decreases.
FB
When this voltage falls 5mV below the 800mV reference,
the feedback comparator trips and enables burst cycles.
At the beginning of the burst cycle, the internal high side
power switch (P-channel MOSFET) is turned on and the
inductor current begins to ramp up. The inductor current
increases until either the current exceeds the peak current
V voltage through an external resistive divider, the HYST
IN
pin can be used to increase the effective RUN comparator
hysteresis.
comparatorthresholdorthevoltageontheV pinexceeds
FB
An internal 1ms soft-start function limits the ramp rate of
the output voltage on start-up to prevent excessive input
supply droop. If a longer ramp time and consequently less
supplydroopisdesired,acapacitorcanbeplacedfromthe
SS pin to ground. The 5ꢀA current that is sourced out of
thispinwillcreateasmoothvoltageramponthecapacitor.
If this ramp rate is slower than the internal 1ms soft-start,
then the output voltage will be limited by the ramp rate
800mV, at which time the high side power switch is turned
off. The low side power switch (N-channel MOSFET) is
then turned on. The inductor current ramps down until
the reverse current comparator trips, signaling that the
current is close to zero. If the voltage on the V pin is
FB
still less than the 800mV reference, the high side power
switch is turned on again and another cycle commences.
The average current during a burst cycle will normally be
3642f
8
Linear [ Linear ]
