LTC3217_15 Datasheet, PDF (8/12 Pages) Linear Technology – 600mA Low Noise Multi-LED Camera Light Charge Pumps

LTC3217
OPERATION
ROL
+
+– _{1.5VBAT OR 2VBAT}
CPO
–
3217 F01
Figure 1. Equivalent Open-Loop Circuit
V_CPO in calculating Equations 2 and 3 is the minimum re-
quired voltage for the LED and not the regulated voltage.
Typical values of R_OL as a function of temperature are
shown in Figures 2 and 3.
3.2
V_BAT = 3V
V_CPO = 4.2V
3.0 C2 = C3 = C4 = 2.2μF
2.8
2.6
2.4
2.2
2.0
–40
–15 10
35
60
85
TEMPERATURE (°C)
3217 F02
Figure 2. 1.5x Mode Charge Pump Open-Loop Output
Resistance vs Temperature (1.5V_BAT – V_CPO)/I_CPO
3.8
V_BAT = 3V
V_CPO = 4.8V
3.6 C2 = C3 = C4 = 2.2μF
3.4
3.2
3.0
2.8
2.6
–40
–15 10
35
60
85
TEMPERATURE (°C)
3217 F03
Figure 3. 2x Mode Charge Pump Open-Loop Output
Resistance vs Temperature (2V_BAT – V_CPO)/I_CPO
Shutdown Current
In shutdown mode all the circuitry is turned off and the
LTC3217 draws a very low current from the V_BAT sup-
ply. Furthermore, CPO is weakly connected to V_BAT. The
LTC3217 enters shutdown mode when both the EN1 and
EN2 pins are brought low. EN1 and EN2 have 250k pull-
down resistors to ground.
Thermal Protection
The LTC3217 has built-in overtemperature protection.
At internal die temperatures of around 150°C thermal
shutdown will occur. This will disable all of the current
sources and charge pump until the die has cooled by
about 15°C. This thermal cycling will continue until the
fault has been corrected.
CPO Short-Circuit Protection
The LTC3217 has internal CPO short-circuit protection.
An internal comparator senses when CPO is below 0.8V
which forces the part into shutdown. A pull-up device
ensures start-up.
Mode Switching
The LTC3217 will automatically switch from 1x mode
to 1.5x mode and subsequently to 2x mode whenever a
dropout condition is detected at an LED pin. Dropout oc-
curs when a current source voltage becomes too low for
the programmed current to be supplied. The time from
dropout detection and mode switching is about 2.5ms. This
delay allows for the LED to warm up and reduce its forward
voltage which may remove the dropout condition.
If PWM is used on the EN2 pin, then the dropout time is
dependent on one to two PWM clock pulses.
The part is reset back to 1x mode when the part is shut
down (EN1 = EN2 = Low). The part may be set to the
desired output current level via EN1 and EN2. An internal
comparator will not allow the main switches to connect
V_BAT and CPO in 1x mode until the voltage at the CPO
pin has decayed to less than or equal to the voltage at
the V_BAT pin.
3217fa
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