Design Note 493: LINEAR Bootstrap Biasing of High Input Voltage Step-Down Controller Increases Converter Efficiency - Goran Perica and Victor Khasiev

Bootstrap Biasing of High Input Voltage Step-Down Controller Increases Converter Efficiency Design Note 493 Goran Perica and Victor Khasiev Introduction Figure 1 shows a block diagram for this scheme. The output

High voltage buck DC/DC controllers such as the LTC3890 can be directly connected to the EXTVCC pin of the chip as (dual output) and LTC3891 (single output) are popular in long as the output voltage is above 4.7V. However, extra automotive applications due to their extremely wide 4V to circuitry (described in the following section) is required 60V input voltage range, eliminating the need for a snub- for outputs below 4.7V.

ber and voltage suppression circuitry. These controllers are also well suited for 48V telecom applications where

VIN (4.5V to 60V)INTVCC

no galvanic isolation is required. In a typical application for these controllers, the IC’s

LTC3890

VOUT (< 4.7V)

supply voltage (INTVCC) is provided by the on-chip LDO. LTC3891

This LDO produces 5V from input voltages up to 60V to EXTVCC

bias control circuitry and provide power FET gate drive. Although simple, this built-in biasing scheme can be inef- ficient. Power losses can be significant in applications where the input voltage is consistently high, such as in

48V telecom applications. Reducing the power losses VOLTAGE DOUBLER OR BOOST (FIGURE 2 OR FIGURE 3) DN493 F01

in the bias conversion can increase efficiency and also reduce the controller case operating temperature.

Figure 1. Block Diagram Showing External Bias

Employing EXTVCC to Improve Efficiency

One of the attractive features of the LTC3890 and LTC3891 Voltage Doubler for Output Voltages Below 4.7V

controllers is the external power input (EXTVCC). This is When the controller’s output is below 4.7V, it must be a second on-chip LDO, which can be used to bias the stepped up to allow the built-in LDO to work. A simple chip. When the input voltage is consistently high, it is voltage doubler solves this problem as long as the output more efficient to produce the biasing voltage by step- is higher than 2.5V. Below 2.5V output, a multivibrator- ping down the converter’s output voltage, which is fed based circuit can be used. into EXTVCC, rather than generating 5V INTVCC from the

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