3800EFE データシートの表示（PDF） - Linear Technology

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3800EFE

High-Voltage Synchronous Current Mode Step-Down Controller

Linear Technology 

LT3800

APPLICATIO S I FOR ATIO

Overview

The LT3800 is a high input voltage range step-down

synchronous DC/DC converter controller IC that uses a

200kHz constant frequency, current mode architecture

with external N-channel MOSFET switches.

The LT3800 has provisions for high efficiency, low load

operation for battery-powered applications. Burst Mode

operation reduces total average input quiescent currents

to 100µA during no load conditions. A low current shutdown

mode can also be activated, reducing quiescent current to

<10µA. Burst Mode operation can be disabled if desired.

The LT3800 also employs a reverse-current inhibit fea-

ture, allowing increased efficiencies during light loads

through nonsynchronous operation. This feature disables

the synchronous switch if inductor current approaches

zero. If full time synchronous operation is desired, this

feature can be disabled.

Much of the LT3800’s internal circuitry is biased from an

internal linear regulator. The output of this regulator is the

VCC pin, allowing bypassing of the internal regulator. The

associated internal circuitry can be powered from the

output of the converter, increasing overall converter effi-

ciency. Using externally derived power also eliminates the

IC’s power dissipation associated with the internal VIN to

VCC regulator.

Theory of Operation (See Block Diagram)

The LT3800 senses converter output voltage via the VFB

pin. The difference between the voltage on this pin and an

internal 1.231V reference is amplified to generate an error

voltage on the VC pin which is, in turn, used as a threshold

for the current sense comparator.

During normal operation, the LT3800 internal oscillator

runs at 200kHz. At the beginning of each oscillator cycle,

the switch drive is enabled. The switch drive stays enabled

until the sensed switch current exceeds the VC derived

threshold for the current sense comparator and, in turn,

disables the switch driver. If the current comparator

threshold is not obtained for the entire oscillator cycle, the

switch driver is disabled at the end of the cycle for 450ns.

This minimum off-time mode of operation assures regen-

eration of the BOOST bootstrapped supply.

Power Requirements

The LT3800 is biased using a local linear regulator to

generate internal operational voltages from the VIN pin.

Virtually all of the circuitry in the LT3800 is biased via an

internal linear regulator output (VCC). This pin is decoupled

with a low ESR 1µF capacitor to PGND.

The VCC regulator generates an 8V output provided there

is ample voltage on the VIN pin. The VCC regulator has

approximately 1V of dropout, and will follow the VIN pin

with voltages below the dropout threshold.

The LT3800 has a start-up requirement of VIN > 7.5V. This

assures that the onboard regulator has ample headroom

to bring the VCC pin above its UVLO threshold. The VCC

regulator can only source current, so forcing the VCC pin

above its 8V regulated voltage allows use of externally

derived power for the IC, minimizing power dissipation in

the IC. Using the onboard regulator for start-up, then

deriving power for VCC from the converter output maxi-

mizes conversion efficiencies and is common practice. If

VCC is maintained above 6.5V using an external source, the

LT3800 can continue to operate with VIN as low as 4V.

The LT3800 operates with 3mA quiescent current from the

VCC supply. This current is a fraction of the actual VCC

quiescent currents during normal operation. Additional

current is produced from the MOSFET switching currents

for both the boosted and synchronous switches and are

typically derived from the VCC supply.

Because the LT3800 uses a linear regulator to generate

VCC, power dissipation can become a concern with high

VIN voltages. Gate drive currents are typically in the range

of 5mA to 15mA per MOSFET, so gate drive currents can

create substantial power dissipation. It is advisable to

derive VCC and VBOOST power from an external source

whenever possible.

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