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LTC3101

3101fb

APPLICATIONS INFORMATION

The basic LTC3101 application circuit is shown as the

Typical Application on the front page of this data sheet.

The external component selection is dependent upon the

required performance of the IC in each particular appli-

cation given considerations and tradeoffs such as PCB

area, output voltages, output currents, ripple voltages

and efﬁciency. This section of the data sheet provides

some basic guidelines and considerations to aid in the

selection of external components and the design of the

application circuit.

CRS Capacitor Selection

A capacitor from the CRS pin to ground is used to pro-

gram the duration of the microprocessor reset signal on

the RESET pin. A low leakage ceramic capacitor should

be utilized to ensure reliable temperature independent

operation. At the start of the active-low reset pulse, a 1μA

(typical) current begins charging the CRS capacitor. The

RESET pulse ends when the voltage at the CRS pin reaches

1.20V (typical). Therefore, the required CRScapacitorvalue,

CRS, is given by the following equation where tRESET is the

desired reset duration in milliseconds:

μF

RESET

()

1200

If the microprocessor reset function of the LTC3101 is

unused, the CRS pin can be left unconnected.

LDO Output Capacitance

The LDO has been speciﬁcally designed for stable opera-

tion with a wide range of output capacitors. For most ap-

plications, a low ESR ceramic capacitor of at least 4.7μF

should be utilized. Large valued supercapacitors can be

connected directly to the LDO output without requiring a

series isolation resistor for loop stability. However, if the

supercapacitor has signiﬁcant ESR, it may be necessary

to place a small 4.7μF ceramic in parallel with the super-

capacitor to maintain an adequate phase margin.

MAX Capacitor Selection

The MAX output serves as the input to the LDO. There-

fore, even if the MAX output is unused directly in the

application, it is recommended that it be bypassed with

a 1μF or larger ceramic capacitor. There is no limit to the

maximum capacitance on this pin. However, the soft-start

duration is formed by the current-limited output charg-

ing the capacitance attached to the pin so larger output

capacitors will result in proportionally longer soft-start

durations.

Buck Inductor Selection

The choice of buck inductor value inﬂuences both the ef-

ﬁciency and the magnitude of the output voltage ripple.

Larger inductance values will reduce inductor current

ripple and will therefore lead to lower output voltage

ripple. For a ﬁxed DC resistance, a larger value inductor

will yield higher efﬁciency by lowering the peak current to

be closer to the average output current. However, a larger

inductor within a given inductor family will generally have

a greater series resistance, thereby counteracting this

efﬁciency advantage.

Given a desired peak-to-peak current ripple, ΔIL, the

required inductance can be calculated via the following

expression, where f represents the switching frequency

in MHz:

L = VOUT

f• IL

1– VOUT

VIN

μH

()

A reasonable choice for ripple current is ΔIL = 140mA

which represents 40% of the maximum 350mA load

current. The DC current rating of the inductor should be

at least equal to the maximum load current plus half the

ripple current in order to prevent core saturation and loss

of efﬁciency during operation. To optimize efﬁciency the

inductor should have a low DC resistance (DCR).

In particularly space-restricted applications it may be

advantageous to use a much smaller value inductor at

the expense of larger ripple current. In such cases, the

converter will operate in discontinuous conduction for a

wider range of output loads and efﬁciency will be reduced.

In addition, there is a minimum inductor value required

to maintain stability of the current loop as determined by

the ﬁxed internal slope compensation. Speciﬁcally, if the

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