LTC3201

7

3201f

appropriate to compare the achievable capacitance for a

given case size rather than discussing the specified ca-

pacitance value. For example, over the rated voltage and

temperature, a 1

µF,10V,Y5Vceramiccapacitorinan0603

case may not provide any more capacitance than a 0.22

µF

10V X7R available in the same 0603 case. The capacitor

manufacturer’s data sheet should be consulted to deter-

mine what value of capacitor is needed to ensure 0.22

µF

at all temperatures and voltages.

Below is a list of ceramic capacitor manufacturers and

how to contact them:

AVX

(843) 448-9411

www.avxcorp.com

Kemet

(864) 963-6300

www.kemet.com

Murata

(770) 436-1300

www.murata.com

Taiyo Yuden

(800) 348-2496

www.t-yuden.com

Vishay

(610) 644-1300

www.vishay.com

Open-Loop Output Impedance

The theoretical minimum open-loop output impedance of

a voltage doubling charge pump is given by:

R

VV

IFC

OUT MIN

IN

OUT

OUT

()

–

==

21

where F if the switching frequency (1.8MHz typ) and C is

the value of the flying capacitor. (Using units of MHz and

µF is convenient since they cancel each other). Note that

the charge pump will typically be weaker than the theoreti-

cal limit due to additional switch resistance. Under normal

value of ~0.5

Ω.

Output Ripple

the output ripple at the expense of higher minimum turn-

on time and higher start-up current. The peak-to-peak

output ripple is approximated by the expression:

V

I

FC

RIPPLE P P

OUT

OUT

()

•

−

≅

2

F is the switching frequency (1.8MHz typ).

Loop Stability

of the LTC3201. The device uses a closed loop to adjust

the strength of the charge pump to match the required

output current. The error signal of this loop is directly

stored on the output capacitor. The output capacitor also

serves to form the dominant pole of the loop. To prevent

ringing or instability, it is important for the output capaci-

tor to maintain at least 0.47

µF over all ambient and

operating conditions.

Excessive ESR on the output capacitor will degrade the

loop stability of the LTC3201. The closed loop DC imped-

ance is nominally 0.5

Ω. The output will thus change by

50mV with a 100mA load. Output capacitors with ESR of

0.3

Ω or greater could cause instability or poor transient

response. To avoid these problems, ceramic capacitors

should be used. A tight board layout with good ground

plane is also recommended.

Soft-Start

The LTC3201 has built-in soft-start circuitry to prevent

soft-start time is programmed at approximately 30

µs.

Layout Considerations

Due to the high switching frequency and large transient

currents produced by the LTC3201, careful board layout is

necessary. A true ground plane is a must. To minimize

high frequency input noise ripple, it is especially important

that the filter capacitor be placed with the shortest dis-

tance to the LTC3201 (1/8 inch or less). The filter capacitor

should have the highest possible resonant frequency.

Conversely, the input capacitor does not need to be placed

close to the pin. The input capacitor serves to cancel out

the lower frequency input noise ripple. Extra inductance

it starts to look like an antenna and worsen the radiated

noise. It is recommended that the filter capacitor be placed

on the left hand side next to Pin 3. The flying capacitor can

then be placed on the top of the device. It is also important

APPLICATIO S I FOR ATIO

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.