AAT3171 Просмотр технического описания (PDF) - Analog Technology Inc

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AAT3171

High Current, High Efficiency Charge Pump with Auto-Timer

Analog Technology Inc 

Application Information

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.

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:

T

=

CT

·

10

s

µF

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.

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).

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.

AAT3171

High Current, High Efficiency

Charge Pump with Auto-Timer

Real-Time Control of the CT Pin

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.

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.

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.

Tri-state = TMR ON

Low = TMR OFF

0.1µF = 1 sec

AAT3171

EN/SET

CT

Figure 2: Enable or Disable the Auto-Timer in

Real-Time for Flash/Torch Control.

Flash/Torch Control Using the RSET Pin

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).

10

3171.2007.03.1.1

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