Detailed Description
The MAX40200 mimics a near-ideal diode. The device
blocks reverse-voltages and passes current when forward-
biased just as a normal diode. The improvements are that
instead of a cut-in voltage around 500mV and a logarithmic
voltage-current transfer curve, the MAX40200 has a near
constant voltage drop independent of the magnitude of
the forward current flowing through it. This voltage drop is
around 45mV at 500mA of forward or load current.
The constant forward voltage drop significantly helps with
supply regulation; a normal diode typically drops an additional
60mV for every 10 times change in current through it.
Similar to a normal diode, the MAX40200 also becomes
resistive as the forward current exceeds the specified
limit (see Figure 1). Unlike a normal diode, should the
MAX40200 exceed the specified temperature, it will turn
off in order to protect itself and the circuitry connected to
it. Like a normal diode MAX40200 will turn-off when it is
reverse biased. The turn-on and turn-off times for enable
and disable responses are similar to those of forward and
reverse bias conditions.
MAX40200 has an enable function feature. Unlike a normal
diode the device can be turned off when not required.
When turned off, it blocks voltages on either side to
a maximum of 6V above ground. This feature allows
MAX40200 to be used, to switch supply sources, or to
control which sub-systems are to be powered up.
It should be noted, however, that, unlike normal diodes,
this “ideal diode” is not suited to rectifying AC. In applications
where the supply is inductively coupled, conventional
diodes should be used for the rectification part of the
circuitry. MAX40200 is designed to be used in applications
to switch between different DC sources.
Principle of Operation
The MAX40200 features an internal pMOSFET to pass
the current from the V
DD
input to the OUT output. The
internal MOSFET is controlled by circuitry that:
1) Creates the 18mV constant forward drop when the
MAX40200 is forward-biased.
2) Turns the MOSFET off when the part is reverse-biased.
3) Turns the MOSFET off if the enable input is pulled low.
4) Turns the device off when the device temperature
exceeds the thermal protection threshold.
This control circuitry consumes 7µA typical current and
this limits the rate at which the internal MOSFET can be
turned on/off.
To ensure the control loop remains stable for all output
current levels, there should always be a minimum of
0.33µF connected to the OUT output and likewise, a minimum
of 0.33µF on the V
DD
input.
These capacitors also improve the surge capability of
power supply. In general for higher Output Capacitive
Loads [e.g., C
OUT
= 10µF], then C
IN
should be kept to
C
OUT
/10 (µF) for optimum transient response.
Applications Information
The simplest application would be as shown in Figure 2,
where the battery has to be disconnected from the load
when the wall-supply is connected. Often, the wall-supply
can handle the additional losses of a normal diode, so it
would use a regular diode to prevent battery power from
flowing back into it.
The battery, on the other hand, benefits significantly by
only losing 30mV when powering the load, thus increasing
the battery life between charging cycles.
For systems that require more than the 500mA that the
MAX40200 is specified for, it may be convenient to split
the load up into various sections that could also benefit
from the individual power enabling that the MAX40200’s
Enable pins offer.
This also suggests that any integrated circuit without built-
in power-down capability can have it added by powering it
through a MAX40200.
This allows many parts to be used in portable and other
power-sensitive products.
Figure 1. Forward Voltage vs. Forward Current (WLP)
0
25
50
75
100
125
150
1 10 100 1000
FORWARD VOLTAGE (mV)
FORWARD CURRENT (mA)
V
DD
= 3.3V
REFER TO FIGURE 2 FOR TEST SETUP
CONDITIONS
V
DD
= 3.3V
T
A
= -40°C
T
A
= 25°C
T
A
= 85°C
T
A
= 125°C
MAX40200 Ultra-Tiny Micropower, 1A Ideal Diode
with Ultra-Low Voltage Drop
www.maximintegrated.com
Maxim Integrated
│
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