LINEAR 42V, 2.5A Synchronous Step-Down Regulator with 2.5μA Quiescent Current Design Note 504

42V, 2.5A Synchronous Step-Down Regulator with 2.5µA Quiescent Current Design Note 504 Hua (Walker) Bai Introduction Short-Circuit Robustness Using Small Inductors The LT®8610 and LT8611 are 42V, 2.5A synchronous The LT8610 and LT8611 are specifi cally designed to step-down regulators that meet the stringent high minimize solution size by allowing inductor size to be input voltage and low output voltage requirements of selected based on the output load requirements of the automotive, industrial and communications applica- application, rather than the maximum current limits of tions. To minimize external components and solution the IC. During overload or short-circuit conditions, the size, the top and bottom power switches are integrated LT8610 and LT8611 safely tolerate operation with satu- in a synchronous regulator topology, including internal rated inductors through the use of a high speed peak-

compensation. The regulator consumes only 2.5μA FF IC IE current mode architecture and a robust switch design.

quiescent current from the input source even while For example, an application that requires a maximum regulating the output. of 1.5A should use an inductor that has an RMS rating

of >1.5A and a saturation current rating of >1.9A. This

High Effi ciency Synchonous Operation

fl exibility allows the user to avoid oversize inductors for

Replacing an external Schottky diode with an internal

applications requiring less than maximum output current.

synchronous power switch not only minimizes the solu-

tion size, but also increases effi ciency and reduces power Current Sense and Monitoring with the LT8611 dissipation. The effi ciency improvement is signifi cant The LT8611 includes a fl exible current control and in low output voltage applications where the voltage monitor loop using the ISN, ISP, IMON and ICTRL pins. drop of the Schottky diode represents a relatively large The ISP and ISN pins connect to an external sense portion of the output voltage. Figure 1 shows a 12V resistor that may be in series with the input or output to 3.3V circuit. Figure 2 shows the effi ciency of this of the LT8611 or in series with other system currents. circuit reaching 94%, which is 5% to 10% higher than The current limit loop functions by limiting the LT8611 a comparable nonsynchronous circuit. output current such that the voltage between the ISP L, LT, LTC, LTM, Linear Technology, the Linear logo and μModule are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.

BSTVIN

3.8V TO 42V

EN/UV VO = 5V

L1 4.7μH VOUT

PG SW 3.3V

LT8610 C1 2.5A VO = 3.3V

SYNC BIAS 47μF

TR/SS FB 1μF

INTVCC

RT GNDPGND

L1: HCM0703-4R7, COILTRONICS

C1: GRM32ER71A476K, MURATA

LOAD CURRENT (A)

f SW = 700k Hz dn504 F01

dn504 F02

Figure 1. LT8610 12V to 3.3V Application Figure 2. Effi ciency of the 12V to 3.3V Application Achieves High Effi ciency (Circuit Shown in Figure 1)