LTC3217

7

3217f

OPERATIO

Table 1. Charge Pump Output Regulation Voltages

CHARGE PUMP MODE

1.5x

4.5V

2x

5.05V

The LED currents are delivered by the four programmable

current sources. Three discrete current settings (Low, High,

Low + High) are available and may be selected via the EN1 and

EN2 pins. The values of these currents may be selected by

choosing the appropriate programming resistors. Each re-

The resistor values required to attain the desired current

levels can be determined by Equation 1.

R

I

SET

LEDx

12

488

(1)

cause the LTC3217 to enter overcurrent shutdown mode.

This mode prevents damage to the part and external LEDs by

shutting down the high power sections of the part.

Each LED output can be disabled by connecting the pin

directly to CPO. Do not leave pins open as this will cause

dropout and subsequently mode changing.

Pulse Width Modulation Option

EN2 can be pulse width modulated to control the

LED brightness. The minimum allowable pulse width is 50

µs

and the maximum low time is 1ms. Pulse width modulating

the EN2 input can be performed with EN1 high or low. If EN1

is high then there is no limitation on the EN2 low time. When

EN1 is low the part would normally go into shutdown

whenever EN2 goes low. Prevention of shutdown in this case

is achieved by an internal timer which delays shutdown until

EN2 has remained low for at least 1ms.

Soft-Start

Initially, when the part is in shutdown, a weak switch con-

output capacitor and prevent large charging currents to

occur.

The LTC3217 also employs a soft-start feature on its charge

pump to prevent excessive inrush current and supply droop

whenswitchingintothestep-upmodes.Thecurrentavailable

to the CPO pin is increased linearly over a typical period of

125

µs. Soft-start occurs at the start of both 1.5x and 2x

mode changes.

Charge Pump Strength and Regulation

Regulation is achieved by sensing the voltage at the CPO pin

and modulating the charge pump strength based on the error

signal. The CPO regulation voltages are set internally, and are

dependent on the charge pump modes as shown in Table 1.

When the LTC3217 operates in either 1.5x mode or 2x mode,

the charge pump can be modeled as a Thevenin-equivalent

circuit to determine the amount of current available from the

effective input voltage and effective open-loop output resis-

tances and the non-overlap period of the switching circuit.

will be directly proportional to the advantage voltage of

mode. Consider the example of driving white LEDs from

a 3.1V supply. If the LED forward voltage is 3.8V and the

current sources require 100mV, the advantage voltage for

1.5x mode is 3.1V

• 1.5 - 3.8V – 0.1V or 750mV. Notice

that if the input voltage is raised to 3.2V, the advantage

voltage jumps to 900mV—a 20% improvement in avail-

able strength.

From Figure 1, for 1.5x mode the available current is given by:

I

VV

R

OUT

BAT

CPO

OL

=

15

.–

(2)

For 2x mode, the available current is given by:

I

VV

R

OUT

BAT

CPO

OL

=

2–

(3)

Notice that the advantage voltage in this case is

3.1V

but a significant overall increase in available current

is achieved.