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Maxim > Design
support > App notes
> Amplifier
and Comparator Circuits > APP 4415
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Protection > APP 4415

Aug 27, 2009

Keywords: high-side current-sense amplifier,
open-drain output, npn transistors, p-channel MOSFETs

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APPLICATION NOTE 4415

High-Side Current Amplifier Forms 28V Circuit
Breaker

Abstract: Using
the MAX4373 high-side current-sense amplifier, a circuit can
be designed with a voltage range to 28V. All that is needed is
a general-purpose NPN transistor.

This
design idea appeared in the June 1, 2006 issue of Power
Electronics Technology magazine.

The
latching-comparator output (COUT1) on the MAX4373, a high-side
current-sense
amplifier with comparator and reference, makes possible a
circuit-breaker application for that device (see Figure 2 of
its data sheet). The data-sheet circuit, however, is suitable
only for supply voltages in the range +2.7V to +5.5V. You can
extend that range to 28V by adding a general-purpose NPN
transistor, as shown in Figure 1.

Figure
1. The NPN transistor, Q1, extends the output swing of the
comparator output, COUT1, in this circuit
breaker.

The open-drain
comparator output is driven low during normal operation,
allowing a nominal 430µA to flow from the transistor's emitter
through R2 to COUT1. The same (approximate) current flows
through the collector and R1, producing a VGS drive
just over -6.4V for the p-channel
MOSFET.

The
application needs a circuit-breaker threshold of 900mA. The
MAX4373FESA is chosen, because its gain from sense
resistor to output is 50V/V. Thus, 900mA through the 100mΩ
sense resistor, R6, drops 90mV, which is multiplied by the
gain of 50 to yield 4.5V at the OUT terminal. The comparator's
input threshold at CIN1 is 600mV, so a resistor-divider with
ratio 6.5:1 is the target value. Choosing R4 at 68kΩ and R5 at
10kΩ yields an actual load-current threshold of 936mA, which
is close enough for this application.

If the load
current exceeds this threshold, the comparator output is
latched into a high-impedance state. The transistor goes into
cutoff and causes the MOSFET to turn off. Actuating the reset
pushbutton restores the circuit to normal operation, provided
that the fault has been removed.

This circuit topology
works for applications in which the higher voltage
(+28V in this example) is at least double the VCC
voltage. Also, VCC is limited to +5.5V maximum to
stay within the limits of the comparator's output. The unequal
values for R1 and R2 in this circuit give a slight gain which,
in turn, provides a bit more gate drive for minimizing losses
due to the MOSFET's on-resistance.

The circuit responds
to an overcurrent condition in about 100µs (Figure 2).
Its response (unlike that of a thermo-mechanical circuit
breaker) is virtually unchanged for overcurrents only 10%
above the threshold.

Figure
2. The load current shown (110% of the nominal 900mA
threshold) trips the Figure 1 circuit breaker in about
100µs.

Related parts

MAX4373

Low-Cost, Micropower,
High-Side Current-Sense Amplifier + Comparator +
Reference ICs

Free
samples

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Download,
PDF format (43kB)
APP 4415: Aug 27, 2009

AN4415, AN 4415, APP4415,
Appnote4415, Appnote 4415

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