LTC3217

6

3217f

BLOCK DIAGRA

EN2

EN1

CONTROL LOGIC

850kHz

OSCILLATOR

PWM TIMING

THERMAL

SHUTDOWN

LED CURRENT

SOURCES

CHARGE PUMP

ENABLE CP

4

0.8V

1.22V

MUX

GND2

LED4

LED3

LED2

LED1

CPO

SHORT-CIRCUIT

PROTECTION

CPO

GND1

C1P

C1M

C2P

C2M

3217 BD

+

–

–

+

1

15

9

10

3

11

14

16

12

13

2

4

5

6

7

8

250k

250k

–

+

+

–

OPERATIO

Power Management

The LTC3217 uses a switched capacitor charge pump to

boost CPO to as much as 2 times the input voltage up to

connected directly to CPO. This mode provides maximum

efficiency and minimum noise. The LTC3217 will remain

in 1x mode until an LED current source drops out. Dropout

occurs when a current source voltage becomes too low

for the programmed current to be supplied. When dropout

is detected, the LTC3217 will switch into 1.5x mode. The

CPO voltage will then start to increase and will attempt to

cause the part to enter the 2x mode. The CPO voltage will

reset to 1x mode whenever the part is shut down.

A two phase non-overlapping clock activates the charge

pump switches. In the 2x mode the flying capacitors are

input current ripple and CPO voltage ripple. In 1.5x mode

the flying capacitors are charged in series during the first

second phase. This sequence of charging and discharging

the flying capacitors continues at a constant frequency

of 850kHz.