LINEAR High Input Voltage Monolithic Switcher Steps Up and Down Using a Single Inductor – Design Note 330A

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High Input Voltage Monolithic Switcher Steps Up and Down Using a Single Inductor – Design Note 330A Jay Celani Introduction step-down functionality using a single inductor, so volt-

Ultrawide input voltage requirements are a common age conversion can continue when VIN approaches or is design problem for DC/DC converter applications, but less than VOUT.

when that range includes voltages both above and below

4V-60V Input to 5V Output

the output voltage, the converter must perform both

DC/DC Automotive Converter

step-up and step-down functions. The LT®3433 is a high

A 4V-60V to 5V DC/DC converter is shown in Figure 2. This

voltage monolithic DC/DC converter that incorporates

converter is well suited for 12V automotive battery appli-

two switch elements, allowing for a unique topology that

cations, maintaining output voltage regulation with bat-

accommodates both step-up and step-down conversion

tery line voltages from 4V cold crank through 60V load

using a single inductor.

dump. The threshold for bridged mode operation is about

The LT3433 uses a 200k Hz constant frequency, current 8V, so the converter will operate primarily in buck mode mode architecture and operates with input voltages from except during a cold crank condition. During buck opera- 4V to 60V. An internal 1% accurate voltage reference tion, this converter can provide load currents up to 350m A allows programming of precision output voltages up to 20V with input voltages up to 60V. Operating with a nominal using an external resistor divider. Burst Mode® operation 13.8V input, this LT3433 converter accommodates loads improves efficiencies during light-load conditions, reduc- of 400m A and produces efficiencies up to 82%. ing the device’s quiescent current to 100µA during no-load

When the input voltage drops below 8V, the converter

conditions. A soft-start feature reduces output overshoot

switches into bridged operation to maintain output volt-

and inrush currents during start-up, and both current limit

age regulation. Because the LT3433 switch current limit is

foldback and frequency foldback are employed to control inductor current runaway during start-up and short-circuit , LTC, LT and Burst Mode are registered trademarks of Linear Technology

Corporation.

conditions. The LT3433 is available in a 16-pin fused TSSOP exposed pad package which provides a small footprint and

VIN SW VOUT

excellent thermal characteristics.

CIN COUT

When the converter input voltage is significantly higher than the output voltage, the LT3433 operates as a modi-

(1a) Step-Down (VIN > VOUT)

fied buck converter using a boosted-drive high side switch. If the converter input voltage becomes close enough to

VIN VOUT

the output voltage to require a duty cycle greater than 75%

CIN SW COUT

in buck mode, the LT3433 automatically enables a second switch. This second switch pulls the output side of the

switched inductor to ground during the “switch on” time, (1b) Step-Up (VIN < VOUT)

creating a bridged switching configuration. SW

VIN VOUT

During bridged switching, the LT3433 merges the elements

CIN SW COUT

of buck and boost DC/DC converters as shown in Figure␣ 1.

DN330A F01

In the simplest terms, a buck DC/DC converter switches

(1c) Step-Up/Step-Down (VIN > VOUT or VIN < VOUT)

the VIN side of the inductor, while a boost converter switches the VOUT side of the inductor. Combining the Figure 1. The LT3433 Merges the Elements of

elements of both topologies achieves both step-up and Step-Up and Step-Down DC/DC Converters