Linear Design Features: LTC3857_58 - Jason Leonard

Low IQ, High Efficiency Dual Output Controllers for Wide Ranging Input and Output Voltages Jason Leonard

The LTC3857 and LTC3858 are versatile low quiescent The LTC3857 and LTC3858 are pin-com- patible parts that differ only slightly,

current (IQ), 2-phase dual output synchronous buck

most notably in short-circuit operation

controllers that are ideal for applications demanding

and at light load operation. These dif-

high efficiency and minimal power loss over a broad

ferences are discussed below and sum-

range of load currents, from virtually no load to many

marized in Table 1. A “-1” version of each

amps. They feature a wide input supply range of 4V is available with slightly fewer features

to 38V (40V abs max) that covers a broad range of (Table 2). The LTC3857/LTC3858 are also battery chemistries and power sources. Each output mostly pin compatible with the popular LTC3827/LTC3826 family of low IQ control-

can be set from 0.8V to 24V (28V abs max).

lers. Figure 1 shows a typical applica- tion circuit in which the LTC3857 is used to convert a widely varying car battery voltage to two well regulated outputs.

215k INTVCC LOW IQ EXTENDS

LTC3857

100k BATTERY RUN TIME

CF1 SENSE1+ PGOOD2 15p F C1

In many applications, one or more sup-

SENSE1– PGOOD1

68.1k plies remain active at all times, often in

L1 VFB1 BG1 MB1 3.3µH VOUT1

a standby mode where little or no load

CITH1A 150p F SW1 3.3V

CB1 RSENSE1 5A current is drawn. In these “always-on”

BOOST1

0.1µF 5mΩ COUT1 COUT1A

RITH1 15k

TG1 MT1 150µF 100µF systems, the quiescent current of the

CITH1 820p F power supply circuit represents the vast

CSS1 0.1µF VIN

4.5V TO 38V majority of the current drawn from the

TRACK/SS1 CIN2 CIN1 47µF 47µF

ILIM INTVCC input supply (battery). Having a low

CINT ×2

PHASMD

IQ power supply is crucial to extending

CLKOUT PGND PLLIN/MODE

D2 battery run times. In Burst Mode® opera-

EXTVCC TG2 MT2 tion, the LTC3857 draws only 50µA when

CB2 one output is active and only 65µA when

RUN2 BOOST2

L2 RSENSE2 both outputs are enabled, while the

CSS2 0.1µF

7.2µH 8mΩ VOUT2

LTC3858 draws 170µA when one output is TRACK/SS2 SW2 8.5V

CITH2 680p F 3A

RITH2 27k active and only 300µA when both outputs

COUT2

ITH2 BG2 MB2 68µF

are enabled. Both devices consume only 8µA when both outputs are shut down.

CITH2A 100p F

44.2k SENSE2–

C2 CF2 1n F

39p F SENSE2+

Figure 1. High efficiency dual output RB2 3.3V/8.5V step-down converter. The 8.5V

output follows V when V is less than 8.5V.

COUT1: SANYO 4TPE220MF D1, D2: CENTRAL SEMI CMDSH-4E MT1, MT2, MB1, MB2: INFINEON BSZ097NO4LS

COUT1A: TAIYO YUDEN JMK325BJ107MM-T L1: SUMIDA CDEP105NP-3R2MC-88 COUT2: SANYO 10TPC68M L2: SUMIDA CDEP105NP-7R2MC-88

16 | October 2011 : LT Journal of Analog Innovation