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LTC3541EDD

High Efficiency Buck + VLDO Regulator

Linear Technology 

LTC3541

APPLICATIONS INFORMATION

be obtained by choosing an output capacitor value of 10µF

to 22μF. Typically, once the ESR requirement for COUT has

been met, the RMS current rating generally far exceeds

the IRIPPLE(P-P) requirement. The output ripple ΔVOUT is

determined by:

ΔVOUT

≅

ΔIL



ESR

+

1

8fCOUT



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.

Aluminum electrolytic and dry tantalum capacitors are both

available in surface mount configurations. In the case of

tantalum, 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 tantalum. These are

specially constructed and tested for low ESR so they give

the lowest ESR for a given volume. Other capacitor types

include Sanyo POSCAP, Kemet T510 and T495 series, and

Sprague 593D and 595D series. Consult the manufacturer

for other specific recommendations.

Using Ceramic Input and Output Capacitors

High value, low cost ceramic capacitors are now becoming

available in smaller case sizes. Their high ripple current,

high voltage rating, and low ESR make them ideal for

switching regulator applications. Since the LTC3541’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 char-

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

Output Voltage Programming

The output voltage is set by tying BUCKFB to a resistive

divider according to the following formula:

VOUT = 0.8V  1+ RR21

Since the impedance at the BUCKFB pin is dependent upon

the resistor divider network used, and phase shift due to

this impedance directly impacts the transient response of

the buck, R1 should be chosen <125k. In addition, stray

capacitance at this pin should be minimized (<5pF) to

prevent excessive phase shift. Finally, special attention

should be given to any stray capacitances that can couple

external signals onto the BUCKFB pin producing unde-

sirable output ripple. For optimum performance connect

the BUCKFB pin to R1 and R2 with a short PCB trace and

minimize all other stray capacitance to the BUCKFB pin.

The external resistive divider is connected to the output,

allowing remote voltage sensing as shown in Figure 6.

Checking Transient Response

The regulator loop response can be checked by looking

at the load transient response. Switching regulators take

several cycles to respond to a step in load current. When

a load step occurs, VOUT immediately shifts by an amount

equal to (ΔILOAD • ESR), where ESR is the effective series

0.8V VOUT 5V

R2

BUCKFB

LTC3541

R1

GND

3541 F06

Figure 6. Setting the LTC3541 Output Voltage

For more information www.linear.com/LTC3541

3541fb

13

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