LINEAR ThinSOT Micropower Buck Regulator Low Output Ripple Design Note 373

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POW ER LOSS (m

Thin SOT Micropower Buck Regulator Has Low Output Ripple Design Note 373 Keith Szolusha

Introduction If the load is light, the IC alternates between micropower and switching states to keep the output in regulation

High voltage monolithic step-down converters simplify

(Figure 3a). Hysteretic mode allows the IC to provide

circuit design and save space by integrating the high-side

single switch-cycle bursts for the lowest possible light-

power switch into the device. In most cases, the switch is

load output voltage ripple (<20m V peak-to-peak from 12V

an n-type transistor (NMOS or NPN) with a boot-strapped

to 3.3V at zero load.) During continuous switching mode

drive stage, requiring an external boost diode and capaci-

tor as well as the main catch diode, complicating the (Figure 3b) at higher current levels, the output voltage ripple is even smaller (<10m V peak-to-peak).

applications circuit.

, LTC, LT and Burst Mode are registered trademarks and Thin SOT is a

The LT®3470 is a 40V step-down converter with the power

trademark of Linear Technology Corporation. All other trademarks are the switch, catch diode and boost diode integrated in a tiny property of their respective owners.

Thin SOTTM package. The boosted NPN power stage pro- EF FI CI EN CY

vides high voltage capability, high power density and high

4V TO 40V 5.5V 0.22µF

switching speed without the cost and space of external

START-UP 6.3V

diodes. VIN BOOST 33µH VOUT

SHDN VSWOFF ON 3.3V BIAS 22µF 200m A

The LT3470 accepts an input voltage from 4V to 40V and 2.2µF 324k

LT3470 6.3V

CERAMIC

delivers up to 200m A to load. Micropower bias current CERAMIC FB

and Burst Mode® operation enable it to consume merely 1% 26µA with no load and a 12V input. Hysteretic current

L: TOKO A003AS-270M=P3

mode control and single-cycle bursts result in very low VIN 4V TO 23V 0.22µF

output ripple and stable operation with small ceramic 25V

VIN BOOST 15µH VOUT

capacitors. The combination of small circuit size, low

SHDN VSWOFF ON 1.8V BIAS 22µF 200m A

quiescent current and 40V input makes the LT3470 ideal

LT3470

25V 1%22p F CERAMIC

for automotive and industrial applications.

CERAMIC FB X5R

X5R GND 332k 1%

Current Mode Control

DN373 F01

L: MURATA LQH32CN150K53

The LT3470 uses a hysteretic current control scheme in conjunction with Burst Mode operation to provide low

Figure 1. Typical 3.3V and 1.8V Output Applications

output ripple and low quiescent current while using a tiny inductor and ceramic capacitors. The switch turns on until VIN = 12V

VOUT = 5V

the current ramps up to the level of the top current com- parator, then turns off and the inductor current ramps down through the catch diode until the bottom current compara- tor trips and the minimum off-time has been met. In continuous mode, the difference between the top and bottom current comparator levels is about 150m A. Since the switch only turns on when the catch diode current falls

below threshold, switching frequency decreases, keeping 0.1

LOAD CURRENT (m A) DN373 F02

switch current under control during start-up or short- circuit conditions.

Figure 2. Efficiency and Power Loss vs Load Current