6

LTC3202

3202fa

Figure 1. Current Control Mode

3202 F01

LTC3202

FB

GND

1

µF

• • •

5, 11

3

2

OPERATIO

The LTC3202 uses a fractional conversion switched ca-

the input voltage. A two-phase nonoverlapping clock acti-

vates the charge pump switches. On the first phase of the

On the second phase of the clock they are connected in

charging and discharging the flying capacitors continues

at a free running frequency of 1.5MHz (typ).

Regulation is achieved by sensing the voltage at the FB pin

and modulating the charge pump strength based on the

error signal. The control pins, D0 and D1, program the set

point of the internal digital-to-analog converter. The regu-

the set-point voltage. Table 1 shows the regulation voltage

as a function of D0 and D1.

Table 1. Feedback Control Voltage Settings

D1

D0

Feedback Set Point Voltage

0

0

Shutdown

0

1

0.2V

1

0

0.4V

1

1

0.6V

In shutdown mode all circuitry is turned off and the

D1 pins are CMOS inputs with a threshold voltage of

approximately 0.8V. The LTC3202 is in shutdown when a

logic low is applied to both D0 and D1. Since the D0 and D1

pins are high impedance CMOS inputs they should never

be allowed to float. To ensure that their states are defined

they must always be driven with valid logic levels.

Shutdown Current

Output voltage detection circuitry will draw a current of

5

µA when the LTC3202 is in shutdown. This current will be

Short-Circuit/Thermal Protection

The LTC3202 has built-in short-circuit current limiting as

well as over temperature protection. During short-circuit

conditions it will automatically limit its output current to

approximately 250mA. At higher temperatures, or if the

input voltage is high enough to cause excessive self

heating on-chip, thermal shutdown circuitry will

shut down the charge pump when the junction tempera-

ture exceeds approximately 160

°C. It will reenable the

charge pump once the junction temperature drops back to

approximately 155

°C. The LTC3202 will cycle in and out of

thermal shutdown indefinitely without latchup or damage

Soft-Start

the LTC3202 has built-in soft-start circuitry. Soft-start is

achieved by increasing the amount of current available to

the output charge storage capacitor linearly over a period

of approximately 500

µs.

The soft-start feature activates any time an input, D0 or D1,

changes state. This will prevent large inrush current

during initial start-up as well as when the feedback setting

is changed from one value to the next. Note that the set

point voltage will drop to zero during the soft-start period.

Under heavy load conditions there may be observable

Programming the LTC3202 for Voltage or Current

The LTC3202 can be configured to control either a voltage

or a current. In white LED applications the LED current is

programmed by the ratio of the feedback set point voltage

and a sense resistor as shown in Figure 1. The current of

the remaining LEDs is controlled by virtue of their similar-

ity to the reference LED and the ballast voltage across the

sense resistor.

(Refer to Simplified Block Diagram)