LTC3528/LTC3528B
8
3528fd
OPERATION
The LTC3528/LTC3528B are 1MHz synchronous boost
converters housed in an 8-lead 3mm × 2mm DFN package.
With the ability to start-up and operate from inputs less
than 0.88V, the 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
and requires 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 LTC3528 achieves high efficiency over a wide
range of load current. Burst Mode operation maintains
high efficiency at very light loads, reducing the quiescent
current to 12µA. Operation can be best understood by
referring to the Block Diagram.
LOW VOLTAGE START-UP
The LTC3528/LTC3528B includes an independent start-up
oscillator designed to operate at an input voltage of 0.70V
(typical). Soft-start and inrush current limiting are provided
during start-up, as well as normal operating mode.
When either V
IN
or V
OUT
exceeds 1.6V typical, the IC enters
normal operating mode. Once 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 application becomes
the availability of the power source to supply sufficient
power to the output at the low voltages, and the maximum
duty cycle, which is clamped at 93% 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 LTC3528/LTC3528B contains internal circuitry to pro-
vide soft-start operation. The internal soft-start circuitry
slowly ramps the peak inductor current from zero to its
peak value of 1.5A (typical), allowing start-up into heavy
loads. The soft-start time is approximately 0.5ms. The
soft-start circuitry is reset in the event of a commanded
shutdown or a thermal shutdown.
Oscillator
An internal oscillator sets the frequency of operation to
1MHz.
Shutdown
The converter is shut down by pulling the SHDN pin below
0.25V, and activated by pulling SHDN above 0.88V. Although
SHDN can be driven above V
IN
or V
OUT
(up to the absolute
maximum rating) without damage, the LTC3528/LTC3528B
have 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
undesirable operation in some applications. Therefore,
in applications 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 the test mode being engaged. Please refer
to Figure 1 for two possible implementations
(Refer to Block Diagram)
4M
±30%
SHDN
LTC3528/LTC3528B
R
1M
V
CNTRL
V
CNTRL
4M
±30%
SHDN
LTC3528/LTC3528B
ZETEX ZC2811E
R > (V
CNTRL
/(V
IN
+ 0.4) – 1) MΩ
V
IN
1M
3528 F01
Figure 1. Recommended Shutdown Circuits
when Driving SHDN Above V
IN
