AMS2115
Device Summary
The AMS2115 is a high speed, high current LDO
controller designed to drive an external N-Channel
MOSFET pass stage. An external resistor senses the
load current to provide current limit. The output
voltage is sensed through an external resistive divider
that feeds the negative input to an internal
transconductance error amplifier. The output of the
error amplifier is pinned out so that external
compensation can be adapted to the specific
application. A separate high voltage Vcc input
provides sufficient gate drive for low input voltage
applications. The error amplifier regulates the output
voltage by modulating the gate voltage of the external
N Channel MOSFET. External compensation allows
the converter to be stabilized with a low ESR ceramic
output capacitor.
For applications with large load transients an
additional capacitance may be required to keep the
output voltage within reasonable limits during large
load transients. In this case the required capacitance
can be examined for the load application and load
removal.
Input Capacitor
The bulk input capacitance required in the form of low
cost high ESR aluminum or tantalum electrolytic
capacitors will vary with the load transient
requirements for the specific applications. Small low
ESR ceramic input capacitors ranging from 10 to
22uF placed close to the input of the LDO are typically
a sufficient compliment to the bulk capacitance of the
converter feeding the input to this LDO.
Application
Enable
The LDO enable (S/D input pin) threshold is 1.2V with
about 150mV of hysteresis. A high voltage applied to
the S/D input enables the LDO controller.
Fault Protection
A current limit controller senses the pass transistor
current through an external resistor tied to pins IPOS
and INEG. The current limit threshold voltage is about
50mV. The initial current limit current limit point is
50mV divided by the external sense resistor.
Ilimit
=
Vos
Rsense
Feedback Resistor Selection
The LDO controller uses a 1.25V reference voltage at
the positive terminal of the internal error amplifier. To
set the output voltage a programming resistor form
the feedback node to ground must first be selected
(R2, R3 of figure 4). A 10k resistor is a good
selection for a programming resistor. A higher value
could result in an excessively sensitive feedback node
while a lower value will draw more current and
degrade the light load efficiency. The equation for
selecting the voltage specific resistor is:
R2
=
Vout
-1
Vref
·
R3
=
2.5V
1.25V
-1
·
10k =10k
Table 2. Feedback Resistor values
=
50mV
7m
=
7.14A
For an accurate current limit, the PCB traces for IPOS
and INEG should connect to the resistor directly at the
resistor in a Kelvin connection manner. This will
prevent the drop associated with the high current PCB
traces creating an error in the sense voltage.
For applications that require extremely low drop
out, and do not require the current limit protection
feature of the AMS2115, the current sense resistor
can be eliminated and Ipos and Ineg shorted together.
Output Capacitor
A low ESR X5R or X7R type ceramic capacitor is
sufficient for most applications. When selecting a
ceramic output capacitor always consider the voltage
rating and the capacitance voltage coefficient. For
most applications the output capacitor required
ranges from 10 to 22uF.
Vout (V)
2.5
5.0
3.3
R2 (k )
(R3=10k )
10.0
30.9
16.5
Compensation
External compensation is comprised of a lead-lag
network (R5, C5, and C8 of the schematic shown
figure 3) tied to the output of the internal error
amplifier.
9
Advanced Monolithic Systems
http://www.ams-semitech.com
AMS [ Advanced Monolithic Systems Ltd ]
