Login
Register

ENGLISH简体中文
(Chinese)日本語
(Japanese)РУССКИЙ
(Russian)

Products

Solutions

Design

Buy

Support

About Us

My Maxim

Maxim > Design
support > App notes
> Amplifier
and Comparator Circuits > APP 4563
Maxim > Design support >
App notes
> Interface
Circuits > APP 4563
Maxim > Design support >
App notes
> Voltage
References > APP 4563

Jan 05, 2011

Keywords: USB ports, USB-port power,
current-sense amplifiers, voltage references, op amps,
n-channel MOSFETs
Related
parts

Download,
PDF format (85kB)

[?]

APPLICATION NOTE 4563

Dynamic siphon steals current from USB port

Abstract: This
circuit exploits all the power available from a USB port by
dynamically adjusting the amount of current delivered to the
load, thereby siphoning a relatively constant (and maximum)
current from the USB port. Included are a current-sense
amplifier (MAX4173), voltage reference (MAX6129), and
precision op amp (MAX4238).

A
similar version of this article appeared in the Decmeber 15,
2006 issue of EDN magazine.

USB ports can be a
handy source
of 5V power. Not only can a USB port power a microcontroller
and other essential circuitry, it often has enough current
headroom left over to charge an energy-storage element such as
a small battery or super-capacitor. The typical approach is to
estimate the maximum current drawn by the essential circuitry,
and then place an appropriate current-limiting device in the
path of the energy-storage device (Figure 1). Though
simple, this method doesn't utilize all of the available USB
current. It therefore takes longer to charge the
energy-storage device.

Figure
1. In this typical method for drawing power from a USB port,
the storage-element current is limited to a fixed value that
is less than optimal.

The circuit of Figure
2 exploits all of the available USB power by dynamically
adjusting the amount of current delivered to the
energy-storage device, thereby siphoning a relatively constant
(and maximum) current from the USB port. U1 (MAX4173), U2 (MAX6129), and the
load-switch configuration (P1, N1, R2, and C4) form a control
loop that limits the current flowing through P1. The circuit
maximizes current flowing to the energy-storage element
(Figure 3) by ensuring that the sum of battery and
essential-circuitry currents never exceeds the maximum allowed
for a USB high-power device (500mA).

Figure
2. This circuit continuously monitors the total current drawn
from the USB port, and dynamically adjusts the storage-element
current as required to avoid exceeding the port's maximum
output capability.

Figure
3. These waveforms from the Figure 2 circuit show that the sum
of the essential-circuitry current (middle trace) and
storage-element current (bottom trace) is always less than the
500mA maximum specified for the USB port (top
trace).

To reconfigure the circuit for USB
low-power operation (100mA max), you can replace U1 with a MAX4173HEUT (a
device with 100V/V gain) and R1 with a 250mΩ resistor.

Related parts

MAX4173

Low-Cost, SOT23,
Voltage-Output, High-Side Current-Sense
Amplifier

Free
samples

MAX4238

Ultra-Low Offset/Drift,
Low-Noise, Precision SOT23 Amplifiers

Free
samples

MAX6129

Ultra-Low-Power Series
Voltage Reference

Free
samples

Automatic updates
Would you like to be
automatically notified when new application notes are
published in your areas of interest? Sign
up for EE-Mail™.

Download,
PDF format (85kB)
APP 4563: Jan 05, 2011

AN4563, AN 4563, APP4563,
Appnote4563, Appnote 4563

|

|

|     Privacy Policy
|     Legal Notices

Copyright © 2011 by Maxim Integrated
Products