LTC3544B

10

3544bfa

ΔΔ

V

I

ESR

C

OUT

L

OUT

≅+

⎛

⎝⎜

⎞

⎠⎟

1

8• •

ƒ

voltage, the output ripple is highest at maximum input

Using Ceramic Input and Output Capacitors

Higher value, lower cost, ceramic capacitors are now

widely available in smaller case sizes. Their high ripple

current, high voltage rating and low ESR make them

ideal for switching regulator applications. Because the

LTC3544B’s control loop does not depend on the output

capacitor’s ESR for stable operation, ceramic capacitors

can be used freely to achieve very low output ripple and

small circuit size.

However, care must be taken when ceramic capacitors

are used at the input and the output. When a ceramic

capacitor is used at the input and the power is supplied

by a wall adapter through long wires, a load step at the

ringing can couple to the output and be mistaken as loop

instability. At worst, a sudden inrush of current through

large enough to damage the part.

When choosing the input and output ceramic capacitors,

choose the X5R or X7R dielectric formulations. These

dielectrics have the best temperature and voltage charac-

teristics of all the ceramics for a given value and size.

Output Voltage Programming

according to the following formula:

VV

R

R

⎛

⎝⎜

⎞

⎠⎟

08

1

2

1

.

The external resistive divider is connected to the output

allowing remote voltage sensing as shown in Figure 2.

Keeping the current in the resistors small maximizes the

efficiency, but making them too small may allow stray

capacitance to cause noise problems or reduce the phase

margin of the control loop. It is recommended that the

total feedback resistor string be kept to under 100k.

To improve the frequency response of the control loop, a

be taken to route the feedback line away from noise sources

such as the inductor of the SW line.

Efficiency Considerations

The efficiency of a switching regulator is equal to the output

power divided by the input power times 100%. It is often

useful to analyze individual losses to determine what is

limiting the efficiency and which change would produce

the most improvement. Efficiency can be expressed as:

Efficiency = 100% – (L1 + L2 + L3 + ...) where L1, L2, etc.

are the individual losses as a percentage of input power.

Although all dissipative elements in the circuit produce

losses, two main sources usually account for most of the

the efficiency loss at medium to high load currents.

1. The quiescent current is due to two components: the

DC bias current as given in the electrical characteristics

and the internal main switch and synchronous switch

gate charge currents. The gate charge current results

from switching the gate capacitance of the internal power

MOSFET switches. Each time the gate is switched from

high to low to high again, a packet of charge, dQ, moves

APPLICATIONS INFORMATION

GND

LTC3544B

0.8V

R2

R1

3544B F02

Figure 2. Setting the LTC3544B Output Voltage