LT3504IUFD データシートの表示（PDF） - Linear Technology

部品番号

コンポーネント説明

メーカー

LT3504IUFD

Quad 40V/1A Step-Down Switching Regulator with 100% Duty Cycle Operation

Linear Technology 

LT3504

Operation

A comparator starts the reference when the EN/UVLO pin

rises above the 1.44V rising threshold. Other comparators

prevent switching when the input voltage is below 2.9V or

the die temperature is above 175°C. When the EN/UVLO

is above 1.44V, the input voltage is above 3.2V, and the

temperature is below 175°C, the boost regulator begins

switching and charges the SKY capacitor to 4.85V above

VIN. When the SKY voltage is less than 4.5V above VIN,

the RUN/SS pins and VC nodes are actively pulled low to

prevent the buck regulators from switching.

The boost regulator (Channel 5) consists of an internal

0.4A power switch (Q5), an internal power Schottky diode

(D5), and the necessary logic and other control circuitry

to drive the switch. The switch current is monitored to

enforce cycle-by-cycle current limit. The diode current

is monitored to prevent inductor current runaway during

transient conditions. An error amplifier servos the SKY

voltage to 4.85V above VIN. A comparator detects when

the SKY voltage is 4.5V above VIN and allows the buck

regulators to begin switching.

The oscillator produces two antiphase clock signals running

at 50% duty cycle. Channels 1, 3 and 5 run antiphase to

Channels 2 and 4. The oscillator can be programmed by

connecting a single resistor from RT/SYNC to ground, or

by applying an external clock signal to RT/SYNC. A sync

detect circuit distinguishes between the type of input.

Tying a resistor to GND directly sets the bias current of

the oscillator. The sync signal is converted to a current to

set the bias current of the oscillator.

The oscillator enables an RS flip-flop, turning on the

internal 1.7A power switch Q1. An amplifier and compara-

tor monitor the current flowing between the VIN and SW

pins, turning the switch off when this current reaches a

level determined by the voltage at the VC node. A second

comparator enforces a catch diode current limit to prevent

inductor current runaway during transient conditions. An

error amplifier measures the output voltage through an

external resistor tied to the FB pin and servos the VC node.

If the error amplifier’s output increases, more current is

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

delivered. A clamp on the VC pin provides switch current

limit. Each buck regulator switch driver operates by drawing

current from the SKY pin. Regulating the SKY pin to 4.85V

above the VIN pin voltage is necessary to fully saturate the

bipolar power switch for efficient operation.

Soft-start is implemented by generating a voltage ramp at

the RUN/SS pin. An internal 1.3µA current source pulls the

RUN/SS pin up to 2.1V. Connecting a capacitor from the

RUN/SS pin to ground programs the rate of the voltage

ramp on the RUN/SS pin. A voltage follower circuit with a

0.1V offset connected from the RUN/SS pin to the RAMP

node prevents switching until the voltage at the RUN/SS

pin increases above 0.1V. When the voltage at the RAMP

node is less than 0.9V, the error amplifier servos the FB

voltage to the RAMP node voltage. When the RAMP node

voltage increases above 0.9V, then the error amplifier ser-

vos the FB voltage to 0.8V. Additionally, a current amplifier

reduces the catch diode current limit when the FB voltage

is below 0.8V to limit the inductor current during startup.

Each individual buck regulator can be placed in shutdown

by pulling the respective RUN/SS pin below 0.1V. The EN/

UVLO pin can be pulled low (below a VBE) to place the

entire part in shutdown, disconnecting the outputs and

reducing the input current to less than 2µA.

The LT3504 is pin compatible with the 28-lead QFN package

LT3514. The LT3514 is a three channel step-down converter

and has one channel (CH3) that outputs 2A instead of 1A.

For more information www.linear.com/LT3504

3504fa

9

Share Link: