LT8471 View Datasheet(PDF) - Linear Technology

Part Name

Description

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LT8471

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

Linear Technology 

LT8471

Operation

and a gradual ramp-up of duty cycle for the Skyhook

channel. When the part is brought out of shutdown, the

external SS capacitors are first discharged (providing

protection against OV/UV pin glitches and slow ramp-

ing). Next, internal 250k resistors pull the SS pins up

to ~2.15V. By connecting an external capacitor to each

of the SS pins, the voltage ramp rates on the pins can

be set. Typical values for the soft-start capacitor range

from 100nF to 1μF.

• Finally, the primary channels’ switching frequency is

folded back by 2, 4, or 8 times when the correspond-

ing FB pin voltage is below certain thresholds (see

the Typical Performance Characteristics section). This

feature reduces the minimum duty cycle that the part

can achieve, thus allowing better control of the switch

current during start-up. The slope compensation func-

tion is disabled during foldback to increase the available

current that can be delivered to the output.

Thermal Shutdown Operation

Not shown in the Block Diagram is the thermal shutdown

circuit. If the temperature of the part exceeds approximately

164°C, the SR21 and SR22 latches are set. A full soft-start

cycle will then be initiated after the temperature drops

below approximately 162.5°C. The thermal shutdown

circuit protects the power switches as well as the external

components connected to the LT8471.

Applications Information

Input Supply Requirements

VIN1 is the main power supply of the LT8471. It powers

channel 1, the Skyhook channel and most of the internal

control and bias circuits for both channels. It must be

powered up to enable any channel of the LT8471. VIN2

is the power supply for channel 2, and only needs to be

powered up when channel 2 is used. When VIN2 is not

powered up, channel 2 will shut down.

Switch Configurations and the Skyhook Regulator

The primary channel NPN power switches can be connected

in low side or high side configurations. A low side connec-

tion is when the power switch is on the lower voltage side

of the inductor while the switch is on. The boost, SEPIC,

flyback and dual-inductor inverting configurations use low

side power switches. Conversely, a high side connection is

when the power switch is on the higher voltage side of the

inductor when it is on. The buck, ZETA and single-inductor

inverting configurations use high side power switches.

Channels 1 and 2 can be configured for high side or low

side switching and do not need to be configured in the

same way as the other.

In the low side configurations, the E pin is typically tied

to ground and the respective C pin toggles. VIN for the

respective channel must operate in a range of 2.6V to 40V.

High side configurations require that the C pin be tied to a

positive DC voltage supply and the respective E pin toggles.

The channel’s VIN pin should be at least 2.2V (typical) higher

than the respective C pin to provide adequate drive to the

base of the NPN power switch. When configured with a

high side switch, the channel’s VIN can operate up to 50V

above ground, 60V above the respective E pin voltage, and

40V above the respective C pin voltage.

The Skyhook boost regulator is available to provide ad-

ditional VIN voltage when it is needed to support high side

switch topologies. When enabled, the Skyhook output

voltage (SHOUT) is regulated to 4.25V (typical) above

the C2 pin voltage. Figure 1 shows an example where the

application input voltage is 6V to 32V. The Skyhook boost

converter regulates SHOUT, VIN1 and VIN2 to 10.25V to

36.25V insuring that the VIN1 and VIN2 pins are typically

4.25V above the C1 and C2 pins. More information about

the Skyhook regulator is available in later sections.

8471fc

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For more information www.linear.com/8471

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