LTC3528B Linear Technology Corporation, LTC3528B Datasheet - Page 8

• Manufacturer Part Number: LTC3528B
• Description: 1MHz Synchronous Step-Up DC/DC Converters
• Manufacturer: Linear Technology Corporation

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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 fi xed 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 fi ltering. Internal soft-start and
internal loop compensation simplifi es the design process
while minimizing the number of external components.
With its low R
nel MOSFET switch and P-channel MOSFET synchronous
rectifi er, the LTC3528 achieves high effi ciency over a wide
range of load current. Burst Mode operation maintains
high effi ciency 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
normal operating mode. Once the output voltage exceeds
the input by 0.24V, the IC powers itself from V
V
on the V
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 suffi cient
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
LTC3528/LTC3528B
8
IN
. At this point the internal circuitry has no dependency
IN
input voltage, eliminating the requirement for
DS(ON)
IN
or V
OUT
and low gate charge internal N-chan-
(Refer to Block Diagram)
exceeds 1.6V typical, the IC enters
OUT
instead of
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. Note
that SHDN can be driven above V
is limited to less than the absolute maximum rating.
Error Amplifi er
The error amplifi er is a transconductance type. The nonin-
verting input is internally connected to the 1.20V reference
and the inverting input is connected to FB. Clamps limit
the minimum and maximum error amp output voltage for
improved large-signal transient response. Power converter
control loop compensation is provided internally. A voltage
divider from V
via FB from 1.6V to 5.25V.
Current Sensing
Lossless current sensing converts the peak current signal
of the N-channel MOSFET switch into a voltage which
is summed with the internal slope compensation. The
summed signal is compared to the error amplifi er 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 current
limit comparator delay to output is typically 60ns. Peak
switch current is limited to approximately 1.5A, indepen-
dent of input or output voltage, unless V
0.7V, in which case the current limit is cut in half.
V
OUT
= 1.20V • 1+
OUT
to ground programs the output voltage
R2
R1
IN
or V
OUT
OUT
, as long as it
falls below
3528fa