LINEAR LTC3619 User's Manual

Choose a Regulator with an Accurate Input Current Limit to Safely Extract Maximum Power from USB Albert Lee

Most of us would not use

3.4V TO 5.5V

CIN RPGD2 RPGD1

a paintbrush to sign our 10µF 499k VINRUN2 RUN1 499k PGOOD1 PGOOD1PGOOD2PGOOD2

names on our checks. It’s L2 L1

VOUT2 1.5µH LTC3619B 3.3µH

VOUT1

simply the wrong tool for the 3.4V AT SW2 SW1 1.8V AT 800m A CF1, 22p F 400m A

job—wasting paint where a R4

COUT1

thin line of ink offers better VFB2 VFB1

COUT2

2.2m F RLIM GND

results. Likewise, using 255k 255k 511k

×2 SUPERCAP

a power supply IC with a broad-brush ±40% output CLIM RLIM ILIM = 475m A

current limit is wasteful,

requiring a designer to leave CIN, COUT1: AVX 08056D106KAT2A L1: COILCRAFT LPS4012-332ML COUT2: VISHAY 592D228X96R3X2T20H L2: COILCRAFT LPS4012-152ML

80% of the input current

on the table, or worse, Figure 1. GSM receiver power supply operates from the USB. Although GSM receivers require bursts of power beyond the USB current limit, this design complies with USB current limits by incorporating a supercap to

ignore the tolerance and risk provide short bursts of current at VOUT2. Only VOUT2 is actively input current limited. The current at VOUT1

must operate within the power constraints of the USB input, but it is not actively input current limited, so the

collapse of the input supply.

voltage at VOUT1 remains stable for important circuits.

To design an input-current- limited supply that maximizes

An increasing number of portable elec- GSM APPLICATION

input power usage requires Users expect a high level of mobile func-

tronics are powered from the USB, which is

a detailed approach, ideally current limited to 500m A. Surges in current tionality in their electronic devices—they

incorporating a regulator on the USB commonly occur from plugging want their GSM modems to detect a strong a device into a port. If the surge current is signal at all corners of the city. As more

with tight tolerances.

high, non-current-limited load dumps at current is required for better transmission The LTC3619 and LTC3619B are 400m A and the USB port can glitch the source power and reception, the importance of an accu- 800m A dual monolithic synchronous supply, which can affect other systems rate input current limit cannot be ignored.

buck regulators with tight ±5% program- that depend on the supply. Plugging in an

Programming the LTC3619B’s accurate

mable average input current limits. The improperly current-limited USB device into

input current limit to maximize avail-

LTC3619 uses Burst Mode operation to a laptop can glitch the laptop CPU, causing

able current usage is simple with an

improve efficiency at light loads, while it to lock up or reboot. High peak current

external resistor RLIM and capacitor CLIM,

the LTC3619B uses pulse-skipping to pulses during GSM wireless data trans-

sized using the following expression:

improve efficiency and also reduce noise. fers can also cause supply glitches. The The accurate input current limit allows accurate current limits of the LTC3619 and

LTC3619B protect the supply while maxi- ILIM DC

utilization of 90% of the maximum input

current for fast supercap charging, strong mizing usage of available input current.

Determine RLIM for the average input

signal and lag-free operation without

current being limited (IDC), and choose

risk of collapsing the input supply.

22 | July 2010 : LT Journal of Analog Innovation