LTC3535_15 Datasheet, PDF (9/16 Pages) Linear Technology – Dual Channel 550mA 1MHz Synchronous Step-Up DC/DC Converter

LTC3535
OPERATION ^{(Refer to Block Diagram)}
The LTC3535 is a dual channel 1MHz synchronous boost
converter housed in a 12-lead 3mm × 3mm DFN package.
Each channel is identical and fully independent. They can
operate from the same source, or from different voltage
sources.
In addition, their output voltages can be tied together
to allow operation of a single output from two different
input sources. However, note that the two channels are
not designed to current share, so if both input voltages
are present either one may be supplying the load.
The following description of operation applies to each
channel. Note that references to V_IN or V_OUT apply to the
corresponding channel.
With a guaranteed ability to start up and operate from
inputs less than 0.8V, each channel features fixed fre-
quency, current mode PWM control for exceptional line
and load regulation. The current mode architecture with
adaptive slope compensation provides excellent transient
load response, requiring minimal output filtering. Internal
soft-start and internal loop compensation simplifies the
design process while minimizing the number of external
components.
With its low R_DS(ON) and low gate charge internal N-channel
MOSFET switch and P-channel MOSFET synchronous
rectifier, the LTC3535 achieves high efficiency over a wide
range of load currents. Burst Mode operation maintains
high efficiency at very light loads, reducing the quiescent
current to just 9µA per channel. Operation can be best
understood by referring to the Block Diagram.
LOW VOLTAGE START-UP
The LTC3535 includes an independent start-up oscillator
designed to start up at an input voltage of 0.68V (typical).
Soft-start and inrush current limiting are provided during
start-up, as well as normal mode.
When either V_IN or V_OUT for a given channel exceeds 1.3V
typical, the channel enters normal operating mode. When
the output voltage exceeds the input by 0.24V, the channel
powers itself from V_OUT instead of V_IN. At this point the
internal circuitry has no dependency on the V_IN input volt-
age, eliminating the requirement for a large input capacitor.
The input voltage can drop as low as 0.5V. The limiting
factor for the application becomes the availability of the
power source to supply sufficient energy to the output at
low voltages, and maximum duty cycle, which is clamped at
90% typical. Note that at low input voltages, small voltage
drops due to series resistance become critical, and greatly
limit the power delivery capability of the converter.
LOW NOISE FIXED FREQUENCY OPERATION
Soft-Start
The LTC3535 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 750mA
(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.
Oscillator
An internal oscillator (independent for each channel) sets
the switching frequency to 1MHz.
Shutdown
Shutdown is accomplished by pulling the SHDN pin below
0.3V and enabled by pulling the SHDN pin above 0.8V.
Although SHDN can be driven above V_IN or V_OUT (up to the
absolute maximum rating) without damage, the LTC3535
has a proprietary test mode that may be engaged if SHDN
is held in the range of 0.5V to 1V higher than the greater
of V_IN or V_OUT. If the test mode is engaged, normal PWM
switching action is interrupted, which can cause undesir-
able operation in some applications. Therefore, in appli-
cations where SHDN may be driven above V_IN, a resistor
divider or other means must be employed to keep the SHDN
voltage below (V_IN + 0.4V) to prevent the possibility of
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