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LTC3214EDD

500mA Camera LED Charge Pump

Linear Technology 

LTC3214

APPLICATIONS INFORMATION

VIN, CPO Capacitor Selection

The value and type of capacitors used with the LTC3214

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

used for both CVIN and CCPO. Tantalum and aluminum ca-

pacitors are not recommended because of their high ESR.

The value of CCPO directly controls the amount of output

ripple for a given load current. Increasing the size of CCPO

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

up current. The peak-to-peak output ripple for 1.5x mode

is approximately given by the expression:

VRIPPLE(P-P) = IOUT/(3fOSC • CCPO)

where fOSC is the LTC3214’s oscillator frequency (typically

900kHz) and CCPO is the output storage capacitor.

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

nificantly affect the stability of the LTC3214. As shown in

the Block Diagram, the LTC3214 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 LTC3214. To prevent

poor load transient response and instability, the ESR of the

output capacitor should be kept below 50mΩ. Multilayer

ceramic chip capacitors typically have exceptional ESR

performance. MLCCs combined with a tight board layout

will yield very good stability. As the value of CCPO controls

the amount of output ripple, the value of CVIN controls the

amount of ripple present at the input pin (VIN). The input

current to the LTC3214 will be 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

(~15ns), these missing “notches” will result in only a small

8

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 capacitors are again recommended for

their exceptional ESR performance. Input noise can be

further reduced by powering the LTC3214 through a very

small series inductor as shown in Figure 3. A 10nH induc-

tor 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.

10nH

VIN

0.1μF

2.2μF

LTC3214

GND

3214 F03

Figure 3. 10nH Inductor Used for Input Noise Reduction

(Approximately 1cm of Wire)

Flying Capacitor Selection

Warning: Polarized capacitors such as tantalum or

aluminum should never be used for the flying capaci-

tors since their voltage can reverse upon start-up of the

LTC3214. Ceramic capacitors should always be used for

the flying capacitors.

The flying capacitors control the strength of the charge

pump. In order to achieve the rated output current it is

necessary to have at least 1.6μF of actual capacitance

for each of the flying capacitors. Capacitors of different

materials lose their capacitance with higher temperature

and voltage at different rates. For example, a ceramic

capacitor made of X7R material will retain most of its

capacitance from –40°C to 85°C whereas a Z5U or Y5V

style capacitor will lose considerable capacitance over

that range. Z5U and Y5V capacitors may also have a very

poor voltage coefficient causing them to lose 60% or more

of their capacitance when the rated voltage is applied.

Therefore, when comparing different capacitors, it is often

more appropriate to compare the amount of achievable

capacitance for a given case size rather than comparing

the specified capacitance value. For example, over rated

voltage and temperature conditions, a 1μF, 10V, Y5V

3214fb

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