LTC3218

8

3218fa

Charge Pump Strength

When the LTC3218 operates in 2x mode, the charge pump

can be modeled as a Thevenin-equivalent circuit to deter-

mine the amount of current available from the effective

input voltage and effective open-loop output resistance,

oscillator frequency, flying capacitor values and switch

resistances. From Figure 1, we can see that the output

current is proportional to:

2V

CPO

R

IN

OL

−

(3)

in 2x mode.

OPERATION

Mode Switching

The LTC3218 will automatically switch from 1x mode to

2x mode whenever the LED forward voltage approaches

the maximum CPO voltage for that mode. The part will

wait approximately 500µs before switching to the next

mode. This delay allows the LED to warm up and reduce

its forward voltage which may remove the dropout condi-

tion. The part may be reset to 1x mode by bringing the part

into shutdown by setting the ENF and ENT pins low. Once

these pins are low, either one or both may be immediately

brought high to re-enable the part.

The value and type of capacitors used with the LTC3218

determine several important parameters such as regulator

control loop stability, output ripple, charge pump strength

and minimum start-up time.

To reduce noise and ripple, it is recommended that low

equivalent series resistance (ESR) ceramic capacitors be

pacitors are not recommended because of their high ESR.

will reduce the output ripple at the expense of higher start-

up current. The peak-to-peak output ripple for 2x mode is

approximately given by the expression:

V

I

fC

RIPPLE

OUT

OSC

CPO

PP

()

•

−

=

2

Both the style and value of the output capacitor can sig-

nificantly affect the stability of the LTC3218. As shown in

the Block Diagram, the LTC3218 uses a control loop to

adjust the strength of the charge pump to match the cur-

rent required at the output. The error signal of this loop

is stored directly on the output charge storage capacitor.

The charge storage capacitor also serves as the dominant

pole for the control loop. To prevent ringing or instability,

it is important for the output capacitor to maintain at least

3µF of actual capacitance over all conditions.

Likewise, excessive ESR on the output capacitor will tend

to degrade the loop stability of the LTC3218. To prevent

poor load transient response and instability, the ESR of the

output capacitor should be kept below 80mΩ. Multilayer

ceramic chip capacitors typically have exceptional ESR

APPLICATIONS INFORMATION

–

+

+

–

CPO

3218 F01

Figure 1. Charge Pump Open-Loop Thevenin-Equivalent Circuit