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LTC3100
15
3100fb
For more information www.linear.com/LTC3100
Dropout Operation
As the input voltage decreases to a value approaching the
output regulation voltage, the duty cycle increases toward
the maximum on-time. Further reduction of the supply
voltage will force the main switch to remain on for more
than one cycle until 100% duty cycle operation is reached
where the main switch remains on continuously. In this
dropout state, the output voltage will be determined by
the input voltage less the resistive voltage drop across the
main switch and series resistance of the inductor.
Slope Compensation
Current mode control requires the use of slope compen-
sation to prevent subharmonic oscillations in the inductor
current waveform at high duty cycle operation. This is ac-
complished internally on the LTC3100 through the addition
of a compensating ramp to the current sense signal. In
some current mode ICs, current limiting is performed by
clamping the error amplifier voltage to a fixed maximum.
This leads to a reduced output current capability at low
step-down ratios. In contrast, the LTC3100 performs cur-
rent limiting prior to addition of the slope compensation
ramp and therefore achieves a peak inductor current limit
that is independent of duty cycle.
Short-Circuit Protection
When the buck output is shorted to ground, the error am-
plifier will saturate high and the P-channel MOSFET switch
will turn on at the start of each cycle and remain on until
the current limit trips. During this minimum on-time, the
inductor current will increase rapidly and will decrease very
slowly during the remainder of the period due to the very
small reverse voltage produced by a hard output short.
To eliminate the possibility of inductor current runaway
in this situation, the buck converter switching frequency
is reduced to approximately 375kHz when the voltage on
FBBK falls below 0.3V.
Soft-Start
The buck converter has an internal voltage mode soft-start
circuit with a nominal duration of 1.3ms. The converter
remains in regulation during soft-start and will therefore
respond to output load transients which occur during
this time. In addition, the output voltage rise time has
minimal dependency on the size of the output capacitor
or load current.
Error Amplifier and Compensation
The LTC3100 buck converter utilizes an internal transcon-
ductance error amplifier. Compensation of the feedback
loop is performed internally to reduce the size of the
application circuit and simplify the design process. The
compensation network has been designed to allow use of
a wide range of output capacitors while simultaneously
ensuring rapid response to load transients.
Undervoltage Lockout
If the V
INBK
supply voltage decreases below 1.6V (typical),
the buck converter will be disabled. The soft-start for the
buck converter will be reset during undervoltage lockout
to provide a smooth restart once the input voltage rises
above the undervoltage lockout threshold.
PGOOD Comparator
The PGBK pin is an open-drain output which indicates the
status of the buck converter output voltage. If the buck
output voltage falls 8% below the regulation voltage, the
PGBK open-drain output will pull low. The output voltage
must rise 3% above the falling threshold before the pull-
down will turn off. In addition, there is a 60祍 typical deg-
litching delay in order to prevent false trips due to voltage
transients on load steps. The PGBK output will also pull
low during overtemperature shutdown and undervoltage
lockout to indicate these fault conditions, or if the buck
converter is disabled. The typical PGBK pull-down switch
resistance is 13?when V
INBK
= 3.3V.
Schottky Diode
Although it is not required, adding a Schottky diode from
SWBK to the ground plane will improve efficiency by
about 2%.
OPERATION
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