LTC3218
DESCRIPTION
1
WARNING!
Do not look directly at operating LED.
This circuit produces light that can damage eyes.
Demonstration circuit 1030 is a 400mA Single Wire
Camera LED Charge Pump featuring the LTC3218. This
circuit will provide 50mA of continuous torch current or
a flash current at 300mA or 400mA. The LTC3218 has a
built-in 2-second flash timer to protect the LED in case of
faults during flash mode.
The LTC3218 is a low-noise, high current charge pump
capable of flashing a Camera LED up to 400mA. The
LTC3218 design requires only three capacitors and one
program resistor to provide a complete Camera Driver
circuit with one torch and two flash current settings. A
second programming resistor may be used to provide a
wider range of LED torch and flash current settings.
Pressing the ENT push button, SW1, enables torch
mode. Pressing the ENF push button, SW2, enables flash
mode. Pressing both ENT and ENF push buttons enables
the high intensity flash mode. Pulse generator(s) may be
connected to the ENF and/or ENT terminals to better con-
trol the pulse timing for evaluation of flash and current
performance.
Built-in soft-start circuit prevents excessive inrush cur-
rents during start up. The LED output current is regulated
using an internal high side resistor allowing the LED
cathode to be connected directly to ground. The Charge
Pump Output (CPO) voltage is monitored and automati-
cally switches to 2X mode only when a dropout has been
detected to optimize efficiency. Refer to the LTC3218
datasheet for more information on this device.
Design files for this circuit board are available. Call
the LTC factory.
L, LTC, LTM, LT, are registered trademarks of Linear Technology Corporation. Other prod-
uct names may be trademarks of the companies that manufacture the products.
PERFORMANCE SUMMARY
Specifications are at T
A
= 25°C
PARAMETER CONDITIONS MIN TYP MAX UNITS
V
IN
Input Supply Range
2.9 4.5
V
I
VIN
Shutdown Current ENF = ENT = LOW, V
CPO
= 0V 1.1 3 µ
A
I
VIN
Operating Current
ICPO
= 0mA, 1X Mode
ICPO
= 0mA, 2X Mode
980
6.7
µ
A
m
A
I
LED
Torch Current ENT = HIGH, ENF = LOW
45 m
A
I
LED
Flash Current ENT = LOW, ENF = HIGH
290 m
A
I
LED
High Flash Current ENT = HIGH, ENF = HIGH
390 m
A
I
LED
Dropout Voltage (V
ILED
) Mode Switching Threshold, V
CPO
– V
ILED
,
I
LED
= 100mA
7 m
V
Minimum LED Forward Voltage I
LED
= 50mA
2.2
V
Charge Pump Output Clamp Voltage
5.3
V
DEMO CIRCUIT 103
0
QUICK START GUID
E
LTC321
8
400mA Single Wire Camera
LED Charge Pump
LTC3218
QUICK START PROCEDURE
Demonstration circuit 1030 is easy to set up to evaluate
the performance of the LTC3218. Refer to Figure 1 for
proper measurement equipment setup and follow the
procedure below:
NOTE.
When measuring the input or output voltage ripple, care must be
taken to avoid a long ground lead on the oscilloscope probe. Measure the
input or output voltage ripple by touching the probe tip directly across the
Vin or Vout and GND terminals. See Figure 2 for proper scope probe
technique.
1.
With power off, connect a variable +5V, 1A input
power supply in series with an ammeter to VIN and
GND.
For best stability and noise performance, use
the shortest leads possible between the board and
the supply. Capacitor C4 has been placed on the
board to assist in offsetting the effects of long
power supply leads.
2.
Turn on the power at the input and set the supply to
4.5V.
NOTE.
Make sure that the input voltage does not exceed 6V.
3.
Press and hold the ENT push button, SW1, and ob-
serve that the LED illuminates.
4.
Press and hold the ENF push button, SW2, and ob-
serve the LED illuminates brightly for two seconds.
The two-second timer turns off the LED.
5.
Press and hold both the ENT and ENF push buttons
simultaneously. Observe that the LED illuminates even
brighter for two seconds. The two-second timer again
turns off the LED.
6.
Try steps 3 through 5 with VIN Voltages of 3.6V and
3.0V. Observe when the device drops out, the charge
pump automatically switches to 2X mode and the in-
put current doubles.
NOTE.
The ENT and ENF push buttons do not have any de-bounce
circuitry and may cause the part to switch to 2X mode. For better control
of the ENT and ENF pins, connect a pulse generator to the ENT and ENF
terminals. Make sure these terminals do not exceed VIN + 0.3V.
2
LTC3218
Figure 1.
Proper Measurement Equipment Setup
VIN
GND
Figure 2.
Measuring Input or Output Ripple
3
LTC3218
LTC3218
4
4
LTC3218
5
