8

LTC3202

3202fa

tances and the nonoverlap period of the switching circuit.

will be directly proportional to the

advantage voltage

Consider the example of driving white LEDs from a

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

1.5V – 3.8V – 0.6V or only 250mV. However if the input

voltage is raised to 3.2V the advantage voltage jumps to

400mV—a 60% improvement in available strength! Note

that a similar improvement in advantage voltage can be

achieved by operating the LTC3202 at a lower voltage

setting such as the 0.4V setting.

The style and value of capacitors used with the LTC3202

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

capacitors are not recommended because of their high ESR.

will reduce the output ripple at the expense of higher

minimum turn-on time and higher start-up current. The

peak-to-peak output ripple is approximately given by the

expression:

V

I

fC

RIPPLEP P

OUT

OSC

OUT

3•

capacitor.

Both the style and value of the output capacitor can

significantly affect the stability of the LTC3202. As shown

in the block diagram, the LTC3202 uses a control loop to

adjust the strength of the charge pump to match the

current 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 to form the

dominant pole for the control loop. To prevent ringing or

instability, it is important for the output capacitor to

maintain at least 0.6

µF of capacitance over all conditions.

Likewise, excessive ESR on the output capacitor will tend

to degrade the loop stability of the LTC3202. The closed-

loop output resistance of the LTC3202 is designed to be

0.35

Ω. For a 100mA load current change, the feedback

voltage will change by about 35mV. If the output capacitor

has 0.35

Ω or more of ESR the closed-loop frequency

response will cease to roll-off in a simple one-pole fashion

and poor load transient response or instability could

result. Multilayer ceramic chip capacitors typically have

exceptional ESR performance and combined with a tight

board layout should yield very good stability and load

transient performance.

relatively constant while the charge pump is on either the

input charging phase or the output charging phase but will

drop to zero during the clock nonoverlap times. Since the

nonoverlap time is small (~25ns) these missing “notches”

will result in only a small perturbation on the input power

supply line. Note that a higher ESR capacitor such as

tantalum will have higher input noise due to the input

current change times the ESR. Therefore ceramic capaci-

tors are again recommended for their exceptional ESR

performance.

Further input noise reduction can be achieved by powering

the LTC3202 through a very small series inductor as

shown in Figure 5. A 10nH inductor will reject the fast

current notches, thereby presenting a nearly constant

current load to the input power supply. For economy the

10nH inductor can be fabricated on the PC board with

about 1cm (0.4") of PC board trace.

OPERATIO