LTC3526-2/LTC3526B-2
OPERATION
(Refer to Block Diagram)
The LTC3526-2/LTC3526B-2 are 2MHz synchronous boost
converters housed in a 6-lead 2mm
×
2mm DFN package.
With the ability to start up and operate from inputs less
than 1V, these devices feature fixed frequency, current
mode PWM control for exceptional line and load regula-
tion. 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-chan-
nel MOSFET switch and P-channel MOSFET synchronous
rectifier, the LTC3526-2 achieves high efficiency over a wide
range of load currents. Automatic Burst Mode operation
maintains high efficiency at very light loads, reducing
the quiescent current to just 9μA. Operation can be best
understood by referring to the Block Diagram.
LOW VOLTAGE START-UP
The LTC3526-2/LTC3526B-2 include an independent start-
up oscillator designed to start up at an input voltage of
0.85V (typical). Soft-start and inrush current limiting are
provided during start-up, as well as normal mode.
When either V
IN
or V
OUT
exceeds 1.4V typical, the IC
enters normal operating mode. When the output voltage
exceeds the input by 0.24V, the IC powers itself from
V
OUT
instead of V
IN
. At this point the internal circuitry has
no dependency on the V
IN
input voltage, eliminating the
requirement for a large input capacitor. The input voltage
can drop as low as 0.5V. The limiting factor for the ap-
plication 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 LTC3526-2/LTC3526B-2 contain internal circuitry to
provide soft-start operation. The soft-start circuitry slowly
ramps the peak inductor current from zero to its peak value
of 700mA (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 sets the switching frequency to
2MHz.
Shutdown
Shutdown is accomplished by pulling the SHDN pin
below 0.3V and enabled by pulling the SHDN pin above
0.8V typical. 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.
Error Amplifier
The positive input of the transconductance error amplifier
is internally connected to the 1.195V reference and the
negative input is connected to FB. Clamps limit the mini-
mum and maximum error amp output voltage for improved
large-signal transient response. Power converter control
loop compensation is provided internally. An external
resistive voltage divider from V
OUT
to ground programs
the output voltage via FB from 1.6V to 5.25V.
V
OUT
=
1.195V • 1+
Current Sensing
Lossless current sensing converts the peak current signal of
the N-channel MOSFET switch into a voltage that is summed
with the internal slope compensation. The summed signal
is compared to the error amplifier output to provide a peak
current control command for the PWM.
Current Limit
The current limit comparator shuts off the N-channel
MOSFET switch once its threshold is reached. The cur-
rent limit comparator delay to output is typically 60ns.
Peak switch current is limited to approximately 700mA,
independent of input or output voltage, unless V
OUT
falls
below 0.7V, in which case the current limit is cut in half.
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LINER [ LINEAR TECHNOLOGY ]
