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LT3740

3740fc

OPERATION

The LT3740 is a constant-frequency, valley current mode

controller for DC/DC step-down converters. At the start

of each oscillator cycle, the switch logic is set, which

turns on the bottom MOSFET. After a 500ns blanking

time, the bottom MOSFET current is sensed and added

to a stabilizing ramp, and the resulting sum is fed into the

PWM comparator A1. When this voltage goes below the

voltage at VC pin, the switch logic is reset, which turns off

the bottom MOSFET, and turns on the top MOSFET. The

top MOSFET remains on until the next oscillator cycle. The

bottom MOSFET current can be determined by sensing

the voltage between the drain and source of the MOSFET

using the bottom MOSFET on-resistance, or by sensing

the voltage drop across a resistor between the source of

the bottom MOSFET and ground. The two current sensing

pins are SN+ and SN–. The gm error ampliﬁer adjusts the

voltage on the VC pin by comparing the feedback signal

VFB with the reference, which is determined by the lower

of the internal 0.8V reference and the voltage at the XREF

pin. If the error ampliﬁer’s output increases, more current

is delivered to the output; if it decreases, less current is

delivered.

The LT3740 features an open collector PGOOD signal. When

the voltage at FB pin is less than 720mV, the PGOOD output

is pulled low by a NPN transistor. The 720mV threshold is

independent of the voltage on XREF pin.

The small internal step-up converter provides a BIAS

voltage about 7V higher than the input voltage VIN for the

drive of the top MOSFET. This enables the LT3740 to work

from an input voltage as low as 2.2V. The controller starts

operation when the BIAS pin is about 7V higher than VIN

pin. The voltage supply for the bottom MOSFET drive is

provided through the BGDP pin. For VIN lower than 7V,

BGDP should be connected to BIAS to get enough drive

bias. For VIN higher than 7V, BGDP can be connected

directly to VIN to reduce power loss.

Grounding the SHDN pin turns both the internal step-up

converter and the controller off. The SHDN pin can also

be used to implement an optional soft-start function.

Start-Up and Shutdown

During normal operation, when the feedback voltage is

above 720mV, the LT3740 operates in forced continuous

mode. When the feedback voltage is below 720mV, either

during the start-up or because an external reference is

applied, a zero current detect comparator is enabled to

monitor the on-state bottom MOSFET current. When the

current reaches zero, both the top and bottom MOSFETs

are turned off, resulting in discontinuous operation. During

the time that both top and bottom MOSFETs are off, no

current signal is fed into the LT3740. Only the stabilizing

ramp is fed into the PWM comparator to decide the next

turn on of the top MOSFET.

The LT3740 uses the SHDN pin to implement one of the

two different startup schemes. As shown in the block

diagram, the VC pin is clamped to SHDN pin through a

PNP transistor. If the SHDN pin is slowly ramped up, the

VC pin will track it up proportionally. As the VC pin voltage

is compared to the current signal at comparator A1, this

will, in turn, slowly ramp up the switching current.

The tracking capability built into XREF can be used to

implement another startup scheme. If less than 0.8V is

applied to XREF, the LT3740 will use this voltage as the

reference for regulation. Slowly ramping up the voltage

at XREF forces the output to increase slowly, which limits

the start-up current, as shown in Typical Performance

Characteristics.

A sharp SHDN signal is recommended to shut down the

LT3740. If SHDN slowly ramps down, the VC signal will

be dragged low for a considerable period of time before

SHDN reaches its turn-off threshold. During this period

of time, the output voltage could still be in regulation and

the circuit operates in forced continuous mode. A low

VC voltage will result in large bottom MOSFET on-time,

which may cause a reverse inductor current that pumps

the energy from the output to the input. If there is another

supply at the output or the output has a big capacitor, the

input voltage could overshoot, and may cause overvoltage

damage to certain devices.

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