Datasheet

LTC3528B-2
8
3528b2fa
OPERATION
The LTC3528B-2 is a 2MHz synchronous boost converter
housed in an 8-lead 3mm × 2mm DFN package. With the
ability to start-up and operate from inputs less than 0.88V,
the device features fixed frequency, current mode PWM
control for exceptional line and load regulation. The cur-
rent 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 LTC3528-2 achieves high efficiency over a
wide range of load current. The LTC3528B-2 features
continuous PWM operation at 2MHz. At very light loads,
the LTC3528B-2 will exhibit pulse-skip operation. Operation
can be best understood by referring to the Block Diagram.
LOW VOLTAGE START-UP
The LTC3528B-2 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 92% 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 LTC3528B-2 contains internal circuitry to provide
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
2MHz.
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 LTC3528B-2 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 undesirable opera-
tion 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
LTC3528B-2
R
1M
V
CNTRL
V
CNTRL
4M
±30%
SHDN
LTC3528B-2
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