LINEAR LTC3851 User's Guide
Summary
This advanced synchronous buck controller is a high-precision step-down switching regulator designed for critical automotive and industrial power applications. Exceptional performance in a compact 3mm x 3mm package, it regulates power from a wide input range of 4V to 38V, supporting outputs up to 20A at frequencies as high as 750kHz. The manual details its sophisticated features, including DCR current sensing—which eliminates external resistors—and multiple modes (Burst/Continuous) for maximum efficiency and robust load regulation in demanding electronics environments.
Page 1 Text Content
L DESIGN FEATURES
Synchronous Buck Controller in 3mm × 3mm QFN Fits Automotive and Industrial Applications with 4V–38V Input Capability by Mark Mercer Introduction The LTC3851 is a versatile synchro- The LTC3851 has ±1% output voltage nous step-down switching regulator tolerance over temperature. The part’s controller that is available in a space low minimum on-time (90ns, typical) saving 16-lead 3mm × 3mm QFN or allows for low duty cycle operation convenient narrow SSOP packages. Its even with switching frequencies as wide input range of 4V to 38V makes it high as 750k Hz. well-suited for regulating power from a
variety of sources, including automo- Two Current Sensing Options
tive batteries, 24V industrial supplies The LTC3851 features a high input and unregulated wall transformers. impedance current sense comparator. The strong onboard drivers allow the This allows the use of the inductor’s
RFREQ (k)
use of high power external MOSFETs DC resistance (DCR) as the current to produce output currents up to 20A sense element in conjunction with an
Figure 1. Relationship between oscillator
with output voltages ranging from RC filter. DCR current sensing allows frequency and resistor connected between
0.8V to 5.5V. the designer to eliminate the need for a FREQ/PLLFLTR and GND
The constant frequency peak cur- discrete sense resistor, thereby maxi- rent mode control architecture provides mizing efficiency and lowering solution 50m V and 75m V) to accommodate a
OS CI LL AT OR RE QU EN CY (k Hz
excellent line and load regulation along cost. Alternately, higher current sense wide range of DCR values and output with load current sharing capability accuracy may be achieved by connect- current levels. and dependable cycle-by-cycle current ing the SENSE+ and SENSE– pins to a As with all constant frequency, limiting. OPTI-LOOP® compensation precision sense resistor in series with peak current mode control switching simplifies loop stability design and the inductor. The LTC3851 offers the regulators, the LTC3851 utilizes slope provides well-behaved regulation over choice of three pin-selectable maxi- compensation to prevent sub-harmon- a broad range of operating conditions. mum current sense thresholds (30m V, ic oscillations at high duty cycles. This
VIN 4.5V TO 32V
MODE/PLLIN VIN
RFREQ CIN 82.5k 22µF
FREQ/PLLFLTR TG HAT2170H
0.1µF RUN BOOST LTC38510.1µF L1
0.68µH VOUT
TK/SS SW
330p F2200p F CMDSH05-4 15A
ITH INTVCC
VFB BG HAT2170H 48.7k
SENSE– GND
SENSE+ COUT: SANYO 6TPE330MIL
CIN: SANYO 63HVH22M
30.1k L1: VISHAY IHLP5050-EZERR68M01
Figure 2. High efficiency 3.3V/15A power supply with DCR sensing
Linear Technology Magazine • September 2008
Page Summary Contents For LINEAR LTC3851 User's Guide
Manual Details
| Brand | Linear |
|---|---|
| Pages | 3 |
| File Size | 277.00 KB |
| Published | June 21, 2026 |
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Frequently Asked Questions
What is the voltage input range for the LTC3851 regulator?
It supports a wide input range of 4V to 38V.
How does this device sense current without an external resistor?
It utilizes DCR (DC resistance) current sensing, eliminating the need for a discrete sense element.
Which operating mode offers maximum efficiency at light loads?
Burst Mode operation is available when configured via the RUN pin to minimize power consumption.
Is this regulator suitable for controlling LED lights?
Yes, it includes protection and supports a broad PWM frequency range over 60Hz for flicker-free operation.