AAT3171
High Current, High Efficiency
Charge Pump with Auto-Timer
Real-Time Control of the CT Pin
Application Information
To achieve flash and torch mode control, the auto-
timer can be enabled or disabled in real-time. This
can be done by connecting a microcontroller or
microprocessor GPIO port to the CT pin.
Selecting a Timer Capacitor for CT
The CT pin must be configured according to the
desired flash function. There are two options: con-
nect a small valued capacitor or connect the pin to
GND.
To set up flash mode in real-time, enable the auto-
timer by tri-stating the GPIO port. Do not drive the
port high. It can only be set to high-impedance so
that the CT capacitor can be charged and dis-
charged by the AAT3171.
Configure the timer feature for the AAT3171 by
connecting a small valued capacitor between the
CT pin and GND. The value of the capacitor will
determine the timeout duration according to the fol-
lowing formula:
To set up torch mode in real-time, disable the auto-
timer by driving the GPIO port low so that the CT
pin will be pulled to GND through the GPIO port.
s
T = CT · 10
µF
AAT3171
EN/SET
As an example, the result of connecting a 0.1µF
capacitor between the CT pin and GND will be a 1
second timeout duration. Operationally, when the
AAT3171 is enabled, it will automatically disable
itself after 1 second. When using the auto-timer
feature, select a CT capacitor value according to
the formula above.
Tri-state = TMR ON
CT
Low = TMR OFF
0.1µF = 1 sec
To disable the auto-timer feature, connect the CT
pin to ground. The result will be a small bias current
of approximately 3µA. A large valued pull-down
resistor should not be used for connecting the CT
pin to GND. The CT pin can be connected directly
to GND. If a pull-down resistor is preferred, it
should be of low value (i.e., less than 10K).
Figure 2: Enable or Disable the Auto-Timer in
Real-Time for Flash/Torch Control.
Flash/Torch Control Using the RSET Pin
The primary considerations for selecting the capac-
itor type are leakage current and tolerance of the
capacitor value. The auto-timer duration is deter-
mined by charging and discharging the timing
capacitor with a small charging/discharging current
of approximately 3µA. Avoid leaky capacitor types
as they will distort the time-out duration. Select the
capacitor value's tolerance according to the
desired time-out tolerance.
An alternative method can be used for flash/torch
control that eliminates the need to use the S2Cwire
single-wire interface. By using any typical digital
I/O port, an additional enable can be created (see
Figure 3).
The I/O port output configuration can be any one of
open-drain NMOS, open-drain PMOS, or push-pull
type. The control will always act as an active-low
flash enable or, equivalently, an active-high torch
enable (see Table 2).
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3171.2007.03.1.1
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
