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LTC3528-2

1A, 2MHz Synchronous Step-Up DC/DC Converter in 2mm × 3mm DFN

Linear Technology 

LTC3528-2

OPERATION (Refer to Block Diagram)

The LTC3528-2 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 RDS(ON) and low gate charge internal N-channel

MOSFET switch and P-channel MOSFET synchronous recti-

fier, the LTC3528-2 achieves high efficiency over a wide

range of load current. The LTC3528-2 features continuous

2MHz PWM operation over a wide range of load current.

At very light loads, the LTC3528‑2 will enter Burst Mode

operation to maintain high efficiency. Operation can be

best understood by referring to the Block Diagram.

LOW VOLTAGE START-UP

The LTC3528-2 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 VIN or VOUT 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 VOUT instead

of VIN. At this point the internal circuitry has no dependency

on the VIN 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 LTC3528-2 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. Al-

though SHDN can be driven above VIN or VOUT (up to the

absolute maximum rating) without damage, the LTC3528-

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 VIN or VOUT. 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 VIN,

a resistor divider or other means must be employed to

keep the SHDN voltage below (VIN + 0.4V) to prevent the

possibility of the test mode being engaged. Please refer

to Figure 1 for two possible implementations

LTC3528-2

4M

±30% SHDN

R

VCNTRL

1M

R > (VCNTRL/(VIN + 0.4) – 1) MΩ

LTC3528-2

VIN

4M

±30% SHDN

3528 F01

ZETEX ZC2811E

1M

VCNTRL

Figure 1. Recommended Shutdown Circuits

when Driving SHDN Above VIN

35282fb

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