LTC3536_15 Datasheet, PDF (12/28 Pages) Linear Technology – 1A Low Noise, Buck-Boost DC/DC Converter

LTC3536
Operation
soft-start period is reset by thermal shutdown and UVLO
events on V_IN and the mode of operation is always PWM.
In case the output voltage at start -up is already precharged
above 90% (typically) of the target value, the internal soft-
start is skipped and the LTC3536 immediately enters the
mode of operation that has been set on the MODE pin.
If the MODE pin is tied high and Burst Mode operation is
selected, the output voltage is regulated smoothly to the
target voltage value. Instead if the MODE pin is tied low and
PWM mode is selected, the error amplifier needs to charge
up the VC pin and the output voltage might be pulled to
lower voltage values for a short period of time, proportional
to the value of the main compensation capacitor.
Undervoltage Lockout
To ensure proper operation, the LTC3536 incorporates
internal undervoltage lockout (UVLO) circuitry. The con-
verter is disabled if V_IN falls below its respective UVLO
threshold (typical 1.67V). If the input voltage falls below
this level all switching is disabled until the input voltage
rises above 1.75V (nominal).
Output Disconnect
The LTC3536 is designed to allow true output disconnect
by opening both P-channel MOSFET rectifiers. This allows
V_OUT to go to zero volts during shutdown, drawing no
current from the input source.
Thermal Considerations
The power switches in the LTC3536 are designed to operate
continuously with currents up to the internal current limit
thresholds. However, when operating at high current levels
there may be significant heat generated within the IC. As a
result, careful consideration must be given to the thermal
environment of the IC in order to optimize efficiency and
ensure that the LTC3536 is able to provide its full-rated
output current. Specifically, the exposed pad of both the
DD and MSOP packages shall be soldered to the PC board
and the PC board should be designed to maximize the
conduction of heat out of the IC package.
If the die temperature exceeds approximately 165°C, the
IC will enter overtemperature shutdown and all switching
will be inhibited. The part will remain disabled until the
die cools by approximately 10°C. The soft-start circuit
is reinitialized in overtemperature shutdown to provide
a smooth recovery when the fault condition is removed.
If the SHDN pin is driven externally to a level higher than
V_IN, a current limiting resistor should be used. 1M for 6V
on the SHDN pin limits the current to 6µA. Also, a Schottky
diode from the SHDN pin to V_IN can be used in addition
to current limiting resistor.
Applications Information
The standard LTC3536 application circuit is shown as the
Typical Application on the front page of this data sheet. The
appropriate selection of external components is dependent
upon the required performance of the IC in each particular
application given considerations and trade-offs such as
PCB area, cost, output and input voltage, allowable ripple
voltage, efficiency and thermal considerations. This section
of the data sheet provides some basic guidelines and con-
siderations to aid in the selection of external components
and the design of the application circuit.
Inductor Selection
The choice of inductor used in LTC3536 application circuits
influences the maximum deliverable output current, the
magnitude of the inductor current ripple, and the power
conversion efficiency. The inductor must have low DC
series resistance or output current capability and efficiency
will be compromised. Larger inductance values reduce
inductor current ripple and will therefore generally yield
greater output current capability. For a fixed DC resistance,
a larger value of inductance will yield higher efficiency by
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