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LTC3541EDD-3

High Efficiency Buck + VLDO Regulator

Linear Technology 

LTC3541-3

APPLICATIO S I FOR ATIO

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 LTC3541-3’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.

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

resistance of COUT. ΔILOAD also begins to charge or dis-

charge COUT, which generates a feedback error signal. The

regulator loop then acts to return VOUT to its steady-state

value. During this recovery time VOUT can be monitored

for overshoot or ringing that would indicate a stability

problem. For a detailed explanation of switching control

loop theory see Application Note 76.

A second, more severe transient is caused by switching

in loads with large (>1µF) supply bypass capacitors. The

discharged bypass capacitors are effectively put in paral-

lel with COUT, causing a rapid drop in VOUT. No regulator

can deliver enough current to prevent this problem if the

load switch resistance is low and it is driven quickly. The

only solution is to limit the rise time of the switch drive

so that the load rise time is limited to approximately

(25 • CLOAD). Thus, a 10µF capacitor charging to 3.3V

would require a 250µs rise time, limiting the charging

current to about 130mA.

VLDO/LINEAR REGULATOR

Output Capacitance and Transient Response

The LTC3541-3 is designed to be stable with a wide range

of ceramic output capacitors. The ESR of the output capaci-

tor affects stability, most notably with small capacitors. A

minimum output capacitor of 2.2µF with an ESR of 0.05Ω

35413fc

13

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