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LT3800

High-Voltage Synchronous Current Mode Step-Down Controller

Linear Technology 

LT3800

APPLICATIO S I FOR ATIO

With the reverse-current inhibit feature enabled, an LT3800

converter will operate much like a nonsynchronous

converter during light loads. Reverse-current inhibit re-

duces resistive losses associated with inductor ripple

currents, which improves operating efficiencies during

light-load conditions.

An LT3800 DC/DC converter that is operating in reverse-

inhibit mode has a minimum load requirement of 1mA

(BURST_EN = VFB). Since most applications use output-

generated power for the LT3800, this requirement is met

by the bias currents of the IC, however, for applications

that do not derive power from the output, this require-

ment is easily accomplished by using a 1.2k resistor

connected from VFB to ground as one of the converter

output voltage programming resistors (R1). There are no

minimum load restrictions when in Burst Mode operation

(BURST_EN < 0.5V) or continuous conduction mode

(BURST_EN > 2.5V).

Soft-Start

The LT3800 incorporates a programmable soft-start func-

tion to control start-up surge currents, limit output over-

shoot and for use in supply sequencing. The soft-start

function directly monitors and controls output voltage

slew rate during converter start-up.

As the output voltage of the converter rises, the soft-start

circuit monitors δV/δt current through a coupling capaci-

tor and adjusts the voltage on the VC pin to maintain an

average value of 2µA. The soft-start function forces the

programmed slew rate while the converter output rises to

95% regulation, which corresponds to 1.185V on the VFB

pin. Once 95% regulation is achieved, the soft-start circuit

is disabled. The soft-start circuit will re-enable when the VFB

pin drops below 70% regulation, which corresponds to

300mV of control hysteresis on the VFB pin, which allows

for a controlled recovery from a ‘brown-out’ condition.

The desired soft-start rise time (tSS) is programmed via

a programming capacitor CSS1, using a value that

corresponds to 2µA average current during the soft-start

interval. This capacitor value follows the relation:

CSS1

=

2E–6 • tSS

VOUT

RSS is typically set to 200k for most applications.

VOUT

CSS1

A

RSS

LT3800

CSS

3800 AI06

Considerations for Low Voltage Output Applications

The LT3800 CSS pin biases to 220mV during the soft-start

cycle, and this voltage is increased at network node “A” by

the 2µA signal current through RSS, so the output has to

reach this value before the soft-start function is engaged.

The value of this output soft-start start-up voltage offset

(VOUT(SS)) follows the relation:

VOUT(SS) = 220mV + RSS • 2E–6

which is typically 0.64V for RSS = 200k.

In some low voltage output applications, it may be desir-

able to reduce the value of this soft-start start-up voltage

offset. This is possible by reducing the value of RSS. With

reduced values of RSS, the signal component caused by

voltage ripple on the output must be minimized for proper

soft-start operation.

Peak-to-peak output voltage ripple (∆VOUT) will be im-

posed on node “A” through the capacitor CSS1. The value

of RSS can be set using the following equation:

RSS

=

∆VOUT

1.3E–6

It is important to use low ESR output capacitors for

LT3800 voltage converter designs to minimize this ripple

voltage component. A design with an excessive ripple

component can be evidenced by observing the VC pin

during the start cycle.

3800fb

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