ltc3528b-2 Linear Technology Corporation, ltc3528b-2 Datasheet - Page 8

• Manufacturer Part Number: ltc3528b-2
• Description: 1a, 2mhz Synchronous Step-up Dc/dc Converter In 2mm ? 3mm Dfn
• Manufacturer: Linear Technology Corporation

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 fi xed 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 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 LTC3528-2 achieves high effi ciency over a
wide range of load current. The LTC3528B-2 features con-
tinuous 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
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 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.
LTC3528B-2
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
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. 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.
V
OUT
= 1.20V • 1+
OUT
to ground programs the output voltage
⎛
⎝ ⎜
R2
R1
⎞
⎠ ⎟
IN
or V
OUT
, as long as it
3528b2f