LINEAR LTC3631 User Guide and Manual

L DESIGN IDEAS

Robust DC/DC Step-Down Converter in 3mm × 3mm DFN Resists 60V

Input Surges by Chuen Ming Tan

Introduction

VOUT Industrial and test equipment must 1V/DIV

often run on relatively unregulated 9V-to-24V rails that also support high current and inductive load switching of

VIN electromechanical devices. When such 10V/DIV

devices switch on and off, momentary power surges disrupt power flow, causing voltage fluctuations and large

50µs/DIV

overvoltage spikes on the rail.

Figure 1. The LTC3642 continues to regulate Figure 2. The solution size of LTC3642-3.3/5

The LTC3631, LTC3632 and

the output despite a >45V spike on the input. in a 3mm × 3mm DFN package

LTC3642 are robust, monolithic DC/

150µH VOUT

DC step-down solutions that produce

VIN SWVIN 5V

5V TO 45VCIN

a well-regulated supply even in volatile COUT 50m A

LTC3642-5

voltage environments. All can operate RUN VOUT

HYST SS

from a wide input voltage ranges and

sustain repetitive 60V surges (see CIN: TDK C3225X7R1H105KT

COUT: MURATA GRM32DR71C106KA01

Table 1). The output voltage is immune

L1: COILCRAFT LPS6225-154ML

to large voltage swings in the input

Figure 3. With the LTC3642EDD-3.3/5 only two capacitors

(see Figure 1).

and an inductor are required for operation

EF FI CI EN CY

Compact and Easy to Use increase its undervoltage lockout regulator which does not suffer sig- The LTC3642 comes in compact 3mm (UVLO). Until the input voltage ex- nificant power loss as a result of IR × 3mm DFN and MS8E packages with ceeds the UVLO, the input remains drop between the input and output. integrated MOSFETs, as shown in disconnected from the load. The RUN High efficiency is also achieved with Figure 2. It is extremely easy to use, pin can be tied directly to the input Burst Mode® operation, which reduces requiring no loop compensation. The voltage and can be used together with switching activity at light loads to 3.3V and 5V fixed output versions only the hysteresis pin to prevent unwanted minimize switching losses. Figure need two capacitors and an inductor UVLO triggering due to noisy input 4 shows a fairly constant efficiency for operation (see Figure 3). supplies and high voltage coupling curve from light load all the way to The constant peak switch current in harsh environments. When above full load. During shutdown, this device thresholds of these devices inherently the UVLO, the LTC3642 soft starts only draws 3µA even at a maximum protect them from output short cir- its output with an internal 0.75ms input voltage of 45V. With such high cuits. Moreover, each of these devices timer. The duration of the soft-start efficiency, the LTC3642 is a good fit

can reduce its peak switch current PO ER OS (m timer can be increased by adding an in battery-operated motorized vehicles,

threshold such that smaller input and external capacitor in the SS pin. output capacitors can be used.

VIN = 10V

When operating with a high input High Efficiency EFFICIENCY

voltage source, the LTC3642’s RUN Unlike a linear regulator, the LTC3642

VIN = 24V

pin can be optionally configured to is a monolithic synchronous buck

VIN = 10V

Table 1. Comparison of monolithic wide input range buck regulators

LTC3631 LTC3632 LTC3642

POWER LOSS

Maximum Output Current 100m A 20m A 50m A

VOUT = 5V

Input Voltage Operating Range 4.5V–45V 4.5V–50V 4.5V–45V

LOAD CURRENT (m A)

Input Voltage Abs Max 60V 60V 60V

Figure 4. Efficiency for circuit in Figure 3

Linear Technology Magazine • September 2009