LTC3548

9

3548fc

APPLICATIONS INFORMATION

tantalum capacitors are all available in surface mount

packages. The OS-CON semiconductor dielectric capacitor

available from Sanyo has the lowest ESR (size) product

of any aluminum electrolytic at a somewhat higher price.

Special polymer capacitors, such as Sanyo POSCAP,

Panasonic Special Polymer (SP), and Kemet A700, of-

fer very low ESR, but have a lower capacitance density

than other types. Tantalum capacitors have the highest

capacitance density, but they have a larger ESR and it

is critical that the capacitors are surge tested for use in

switching power supplies. An excellent choice is the AVX

TPS series of surface mount tantalums, available in case

heights ranging from 2mm to 4mm. Aluminum electrolytic

capacitors have a significantly larger ESR, and are often

used in extremely cost-sensitive applications provided that

consideration is given to ripple current ratings and long

term reliability. Ceramic capacitors have the lowest ESR

and cost, but also have the lowest capacitance density,

a high voltage and temperature coefficient, and exhibit

audible piezoelectric effects. In addition, the high Q of

ceramic capacitors along with trace inductance can lead

to significant ringing.

In most cases, 0.1μF to 1μF of ceramic capacitors should

also be placed close to the LTC3548 in parallel with the

main capacitors for high frequency decoupling.

Ceramic Input and Output Capacitors

Higher value, lower cost ceramic capacitors are now be-

coming available in smaller case sizes. These are tempting

for switching regulator use because of their very low ESR.

Unfortunately, the ESR is so low that it can cause loop

stability problems. Solid tantalum capacitor ESR generates

a loop “zero” at 5kHz to 50kHz that is instrumental in giving

acceptable loop phase margin. Ceramic capacitors remain

capacitive to beyond 300kHz and usually resonate with their

Figure 2. LTC3548 General Schematic

RUN2

TO 5.5V

RUN1

POR

SW1

GND

SW2

MODE/SYNC

LTC3548

R5

POWER-ON

RESET

C4

C5

L1

L2

R4

R2

R1

R3

3548 F02

PS*

BM*

*MODE/SYNC = 0V: PULSE SKIP

ESL before ESR becomes effective. Also, ceramic caps are

prone to temperature effects which requires the designer

to check loop stability over the operating temperature

range. To minimize their large temperature and voltage

coefficients, only X5R or X7R ceramic capacitors should

be used. A good selection of ceramic capacitors is available

from Taiyo Yuden, AVX, Kemet, TDK and Murata.

Great care must be taken when using only ceramic input

and output capacitors. When a ceramic capacitor is used

at the input and the power is being supplied through long

wires, such as from a wall adapter, a load step at the output

couple to the output and be mistaken as loop instability.

At worst, the ringing at the input can be large enough to

damage the part.

Since the ESR of a ceramic capacitor is so low, the input

and output capacitor must instead fulfill a charge storage

requirement. During a load step, the output capacitor must

instantaneously supply the current to support the load

until the feedback loop raises the switch current enough

to support the load. The time required for the feedback

loop to respond is dependent on the compensation and the

output capacitor size. Typically, 3 to 4 cycles are required to

respond to a load step, but only in the first cycle does the

about 2 to 3 times the linear drop of the first cycle. Thus,

a good place to start is with the output capacitor size of

approximately: More capacitance may be required depend-

ing on the duty cycle and load step requirements.

In most applications, the input capacitor is merely required

to supply high frequency bypassing, since the impedance

to the supply is very low. A 10μF ceramic capacitor is

usually enough for these conditions.

Setting the Output Voltage

The LTC3548 develops a 0.6V reference voltage between

Figure 2. The output voltage is set by a resistive divider

according to the following formula:

V

1

+

R2

R1

⎛

⎝⎜

⎞

⎠⎟

Keeping the current small (<5μA) in these resistors maxi-

mizes efficiency, but making them too small may allow