LTC3544B datasheet(11/18 Pages) LINER

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LTC3544B

3544bfb

generally far exceeds the IRIPPLE(P-P) requirement. The

output ripple

ΔVOUT is determined by:

I ESR

OUT

≅









8 • •

where f = operating frequency, COUT = output capacitance

and

ΔIL = ripple current in the inductor. For a fixed output

voltage, the output ripple is highest at maximum input

voltage since

ΔIL increases with input voltage.

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 output can

induce ringing at the input, VIN. At best, this ringing can

couple to the output and be mistaken as loop instability. At

worst, a sudden inrush of current through the long wires

can potentially cause a voltage spike at VIN, 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

The output voltage is set by tying VFB to a resistive divider

according to the following formula:

OUT =









0 8 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

feed forward capacitor, CF, may be used. Great care should

betakentoroutethefeedbacklineawayfromnoisesources

such as the inductor of the SW line.

Efficiency Considerations

Theefficiencyofaswitchingregulatorisequaltotheoutput

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

losses in LTC3544B circuits: VIN quiescent current and I2R

losses.VINquiescentcurrentlossdominatestheefficiency

loss at low load currents, whereas the I2R loss dominates

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

from PVIN to ground. The resulting dQ/dt is the current out

of PVIN that is typically larger than the DC bias current and

applicaTions inForMaTion

VFB

GND

LTC3544B

0.8V ≤ VOUT ≤ 5.5V

3544B F02

Figure 2. Setting the LTC3544B Output Voltage

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