LTC3125_15 Datasheet, PDF (9/18 Pages) Linear Technology – 1.2A Synchronous Step-Up DC/DC Converter with Input Current Limit

LTC3125
OPERATION
prevents the inductor current from reversing in polarity,
improving efficiency at light loads.
OSCILLATOR
An internal oscillator sets the switching frequency to
1.6MHz.
SHUTDOWN
Shutdown of the boost converter is accomplished by
pulling SHDN below 0.35V and enabled by pulling SHDN
above 1V. Note that SHDN can be driven above V_IN or
V_OUT, as long as it is limited to less than the absolute
maximum rating.
OUTPUT DISCONNECT
The LTC3125 is designed to allow true output disconnect
by eliminating body diode conduction of the internal
P-channel MOSFET rectifier. This allows for V_OUT to go
to zero volts during shutdown, drawing no current from
the input source. It also limits inrush current at turn-on,
minimizing surge currents seen by the input supply. Note
that to obtain the advantages of output disconnect, there
cannot be an external Schottky diode connected between
the SW pin and V_OUT. The output disconnect feature also
allows V_OUT to be pulled high, without any reverse current
into the power source connected to V_IN.
THERMAL SHUTDOWN
If the die temperature exceeds 160°C typical, the LTC3125
will go into thermal shutdown. All switches will be off and
the soft-start capacitor will be discharged. The device
will be enabled again when the die temperature drops by
approximately 15°C.
SYNCHRONOUS RECTIFIER
To control inrush current and to prevent the inductor
current from running away when V_OUT is close to V_IN, the
P-channel MOSFET synchronous rectifier is only enabled
when V_OUT > (V_IN + 0.38V).
ANTI-RINGING CONTROL
The anti-ringing control connects a resistor across the
inductor to prevent high frequency ringing on the SW pin
during discontinuous current mode operation. Although
the ringing of the resonant circuit formed by L and C_SW
(capacitance on SW pin) is low energy, it can cause EMI
radiation.
SOFT-START
The LTC3125 contains internal circuitry to provide soft-
start operation. The soft-start circuitry slowly ramps the
peak inductor current from zero to its peak value of 1.8A
(typical) in approximately 0.5ms, allowing start-up into
heavy loads. The soft-start circuitry is reset in the event
of a shutdown command or a thermal shutdown.
Burst Mode OPERATION
The LTC3125 will automatically enter Burst Mode opera-
tion at light load and return to fixed frequency PWM mode
when the load increases. Refer to the Typical Performance
Characteristics to see the output load Burst Mode thresh-
old current vs V_IN. The load current at which Burst Mode
operation is entered can be changed by adjusting the
inductor value. Raising the inductor value will lower the
load current at which Burst Mode operation is entered.
In Burst Mode operation, the LTC3125 still switches at a
fixed frequency of 1.6MHz, using the same error amplifier
and loop compensation for peak current mode control.
This control method eliminates any output transient
when switching between modes. In Burst Mode opera-
tion, energy is delivered to the output until it reaches the
nominal regulation value, then the LTC3125 transitions to
sleep mode where the outputs are off and the LTC3125
consumes only 15μA of quiescent current from V_OUT. When
the output voltage droops slightly, switching resumes.
This maximizes efficiency at very light loads by minimizing
switching and quiescent losses.
As the load current increases, the LTC3125 will automati-
cally leave Burst Mode operation. Once the LTC3125 has left
Burst Mode operation and returned to normal operation,
it will remain there until the output load is reduced below
the burst threshold.
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