LT8471 View Datasheet(PDF) - Linear Technology

Part Name

Description

View to exact match

LT8471

Dual Multitopology DC/DC Converters with 2A Switches and Synchronization

Linear Technology 

LT8471

Operation

The LT8471 consists of two primary channels, each with

a 2A power switch. One Skyhook channel is also avail-

able with a 500mA power switch to support the primary

channels when performing step-down conversions. The

maximum voltage between VIN1 and E1 (or VIN2 and E2) is

40V when E1 (or E2) is grounded. This is the case for boost,

SEPIC, flyback and dual-inductor inverting topologies. The

maximum allowed voltage between VIN1 and E1 (or VIN2

and E2) is 60V when E1 (or E2) is allowed to toggle, such

as in buck, ZETA and single-inductor inverting topologies.

Primary Channels

The two primary channels, 1 and 2, can be independently

configured as boost, buck, SEPIC, ZETA, flyback or invert-

ing DC/DC converters to adapt into various applications.

Both channels use a constant-frequency, current mode

control scheme to provide line and load regulation (refer

to the Block Diagram). The channel 1 clock is in phase

with the internal oscillator or the SYNC pin if it is toggling.

In order to reduce transient switching spikes, the clock

for channel 2 is approximately 180° out-of-phase with

the channel 1 clock.

At the start of each clock phase, an SR latch (SR11/SR12

in the Block Diagram) is set, turning on the internal power

switch (Q1/Q2 in the Block Diagram) for the respective

channel. An amplifier (A41/A42 in the Block Diagram) and

a comparator (A31/A32 in the Block Diagram) monitor the

current flowing through the internal power switch, turning

the switch off when the current reaches a level determined

by the voltage at VC1/ VC2. An error amplifier measures

the output voltage through an external resistor divider tied

to the FB1/FB2 pin and servos the VC1/ VC2 voltage. If

the error amplifier’s output (VC1/ VC2) increases, more

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

current is delivered. An internal clamp on the VC1/ VC2

voltage provides current limit.

Both of the primary channels contain a power good com-

parator which trips when the corresponding FB pin voltage

is at 92.5% of its regulated value. The PG1 and PG2 outputs

are driven by open-drain N-channel MOSFET devices that

are off when the respective output is in regulation, allow-

ing external resistors to pull the PG1/PG2 pins high. The

PG1 and PG2 pin states are only valid when the respective

channel is enabled and VIN1 is above 2.6V.

Skyhook Channel

When either channel is configured as a buck or single-

inductor inverting converter, the respective VIN pin(s) must

be boosted above the input voltage, VCC. The boosted

supply provides base current to the appropriate Q1 and/

or Q2 NPN power switch. The Skyhook channel provides

this boosted voltage to the SHOUT pin which must also

be connected to VIN2 and/or VIN1 as needed.

The Skyhook is a constant-frequency, voltage mode

boost converter including a Schottky diode integrated on

chip. The Skyhook output, SHOUT, is regulated to a fixed

voltage (4.25V typical) above the C2 pin which must be

connected to a DC voltage (typically VCC). If the Skyhook is

not needed it can be disabled by tying the C3 pin to GND.

This also reduces the current draw from VIN1. Refer to

the Applications Information section for more information

about the proper use of the Skyhook channel.

The Skyhook operates as follows: An error amplifier mea-

sures the output voltage (SHOUT) through the SHOUT-C2

voltage comparator and servos an internal control volt-

age. The control voltage determines the on-time of the

Q3 power switch for each cycle, and thus, the amount of

current being delivered to SHOUT. Loop compensation

is integrated in the chip. Comparator A43 monitors the

current in the power switch Q3 in order to detect over

current conditions. If current in excess of 500mA (typ) is

detected, switch Q3 is immediately turned off.

Start-Up Operation

Several functions are provided to enable a clean start-up

for the LT8471.

• First, the OV/UV pin voltage is monitored by an inter-

nal voltage reference to give a precise turn-on voltage

range. An external resistor (or resistor divider) can be

connected from the input power supply to the OV/UV

pin to provide a user-programmable undervoltage and

overvoltage lockout function.

• Second, the soft-start circuitry provides for a gradual

ramp-up of the switch current for the primary channels

For more information www.linear.com/8471

8471fc

9

Share Link: