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LTC3537

2.2 MHz, 600mA Synchronous Step-Up DC/DC Converter and 100mA LDO

Linear Technology 

LTC3537

OPERATION

The LTC3537 is a 2.2MHz synchronous step-up (boost)

converter and LDO regulator housed in a 16-lead 3mm

× 3mm QFN package. Included with the ability to start

up and operate from inputs less than 0.7V, the LTC3537

features fixed frequency, current mode PWM control for

exceptional line and load regulation.

The current mode architecture with adaptive slope com-

pensation provides excellent transient load response,

requiring minimal output filtering. Internal soft-start and

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 rectifier, the

LTC3537 achieves high efficiency over a wide range of

load currents. Automatic Burst Mode operation maintains

high efficiency at very light loads, reducing the quiescent

current to just 30μA. Operation can be best understood

by referring to the Block Diagram.

LOW VOLTAGE START-UP

The LTC3537 step-up converter includes an independent

start-up oscillator designed to operate at an input voltage

of 0.68V (typical). Soft-start and inrush current limiting

are provided during start-up, as well as normal mode.

When either VINB or VOUTB exceeds 1.4V typical, the IC

enters normal operating mode. When the output voltage

exceeds the input by 0.24V, the IC powers itself from VOUTB

instead of VINB. At this point the internal circuitry has no

dependency on the VINB input voltage, eliminating the

requirement for a large input capacitor. The input voltage

can drop as low as 0.5V after start-up is achieved. The

limiting factor for the application becomes the availability

of the power source to supply sufficient energy to the

output at low voltages, and 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 LTC3537 contains internal circuitry to provide soft-

start operation. The soft-start circuitry slowly ramps the

peak inductor current from zero to its peak value of 750mA

(typical) in approximately 0.5ms, allowing start-up into

heavy loads. The soft-start circuitry is reset in the event

of a shutdown command or a thermal shutdown.

Oscillator

An internal oscillator sets the switching frequency to

2.2MHz.

Shutdown

Shutdown of the boost converter is accomplished by

pulling ENBST below 0.3V and enabled by pulling ENBST

above 0.8V. Note that ENBST can be driven above VINB or

VOUTB, as long as it is limited to less than the absolute

maximum rating.

Boost Error Amplifier

The non-inverting input of the transconductance error

amplifier is internally connected to the 1.2V reference

and the inverting input is connected to FBB. Clamps limit

the minimum and maximum error amp output voltage for

improved large-signal transient response. Power converter

control loop compensation is provided internally. An exter-

nal resistive voltage divider from VOUTB to ground programs

the output voltage via FBB from 1.5V to 5.25V.

VOUTB

=

1.21V

⎛⎝⎜1+

R2

R1

⎞

⎠⎟

Boost Current Sensing

Lossless current sensing converts the peak current signal of

the N-channel MOSFET switch into a voltage that is summed

with the internal slope compensation. The summed signal

is compared to the error amplifier output to provide a peak

current control command for the PWM.

3537fd

11

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