LTC3535 Datasheet, PDF (11/16 Pages) Linear Technology – Dual Channel 550mA 1MHz Synchronous Step-Up DC/DC Converter

LTC3535
OPERATION ^{(Refer to Block Diagram)}
Output Disconnect
The LTC3535 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 allows for inrush current limiting at turn-
on, minimizing surge currents seen by the input supply.
Note that to obtain the advantages of output disconnect,
there must not be an external Schottky diode connected
between SW and V_OUT. The output disconnect feature also
allows V_OUT to be pulled high, without any reverse current
into a battery connected to V_IN.
Thermal Shutdown
If the die temperature exceeds 160°C, the LTC3535 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
about 15°C.
Burst Mode OPERATION
Each channel of the LTC3535 will enter Burst Mode
operation at light load current and return to fixed frequency
PWM mode when the load increases. Refer to the Typical
Performance Characteristics to see the output load Burst
Mode threshold 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 LTC3535 still switches at a
fixed frequency of 1MHz, 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 operation,
energy is delivered to the output until it reaches the
nominal regulation value, then the LTC3535 transitions
to sleep mode where the outputs are off and the LTC3535
consumes only 9μA of quiescent current from V_OUT for
each channel. When the output voltage droops slightly,
switching resumes. This maximizes efficiency at very light
loads by minimizing switching and quiescent losses. Burst
Mode output voltage ripple, which is typically 1% peak-to-
peak, can be reduced by using more output capacitance
(10μF or greater), or with a small capacitor (10pF to 50pF)
connected between V_OUT and FB.
As the load current increases, the LTC3535 will automati-
cally leave Burst Mode operation. Note that larger output
capacitor values may cause this transition to occur at
lighter loads. Once the LTC3535 has left Burst Mode op-
eration and returned to normal operation, it will remain
there until the output load is reduced below the burst
threshold current.
Burst Mode operation is inhibited during start-up and
soft-start and until V_OUT is at least 0.24V greater than V_IN.
Note that each channel can enter or leave Burst Mode
operation independent of the other channel.
3535f
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