LTC3103EDD#PBF Datasheet LTC3103IDD#TRPBF, LTC3103IMSE#TRPBF, LTC3103EDD#TRPBF | P10-P12

LTC3103

3103fa

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OPERATION

Power Good Status Output

The PGOOD pin is an open-drain output which indicates

the output voltage status of the step-down converter. If the

output voltage falls 10% below the regulation voltage, the

PGOOD open-drain output will pull low. A built-in deglitch-

ing delay prevents false trips due to voltage transients on

load steps. The output voltage must rise 2% above the

falling threshold before the pull-down will turn off. The

PGOOD output will also pull low during overtemperature

shutdown and undervoltage lockout to indicate these fault

conditions. The PGOOD output is valid 1ms after the buck

converter is enabled. When the converter is disabled the

open-drain device is forced on into a low impedance state.

The PGOOD pull-up voltage must be below the 6V absolute

maximum voltage rating of the pin.

Current Limit

The peak inductor current limit comparator shuts off the

buck switch once the internal limit threshold is reached.

Peak switch current is no less than 400mA.

Slope Compensation

Current mode control requires the use of slope com-

pensation to prevent sub-harmonic oscillations in the

inductor current waveform at high duty cycle operation. In

some current mode ICs, current limiting is performed by

clamping the error amplifier voltage to a fixed maximum

which leads to a reduced output current capability at low

step-down ratios. Slope compensation is accomplished

on the LTC3103 internally through the addition of a com-

pensating ramp to the current sense signal. The current

limiting function is completed prior to the addition of the

compensation ramp and therefore achieves a peak inductor

current limit that is independent of duty cycle.

Short-Circuit Protection

When the output is shorted to ground, the error amplifier

will saturate high and the high side switch will turn on at

the start of each cycle and remain on until the current limit

trips. During this minimum on-time, the inductor current

will increase rapidly and will decrease very slowly during

the remainder of the period due to the very small reverse

voltage produced by a hard output short. To eliminate the

possibility of inductor current runaway in this situation, the

switching frequency is reduced to approximately 300kHz

when the voltage on FB falls below 0.3V.

BST Pin Function

The input switch driver operates from the voltage gener-

ated on the BST pin. An external capacitor between the SW

and BST pins and an internal synchronous PMOS boost

switch are used to generate a voltage that is higher than

the input voltage. When the synchronous rectifier is on

(SW is low) the internal boost switch connects one side

of the capacitor to V

replenishing its charge. When the

synchronous rectifier is turned off the input switch is turned

on forcing SW high and the BST pin is at a potential equal

to V

+ SW relative to ground.

A comparator ensures there is sufficient voltage across

the boost capacitor to guarantee start-up after long sleep

periods or if starting up into a pre-biased output.

Undervoltage Lockout

The LTC3103 has an internal UVLO which disables the

converter if the supply voltage decreases below 2.1V

(typical), the converter will be disabled. The soft-start for

the converter will be reset during undervoltage lockout to

provide a smooth restart once the input voltage increases

above the undervoltage lockout threshold. The RUN pin

can alternatively be configured as a precise undervoltage

lockout (UVLO) on the V

supply with a resistive divider

connected to the RUN pin.

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APPLICATIONS INFORMATION

The basic LTC3103 application circuit is shown as the

Typical Application on the front page of this data sheet.

The external component selection is determined by the

desired output voltage, output current, desired noise im-

munity and ripple voltage requirements for each particular

application. However, basic guidelines and considerations

for the design process are provided in this section.

Inductor Selection

The choice of inductor value influences both the efficiency

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. However, a larger value inductor within the

same family will generally have a greater series resistance,

thereby offsetting this efficiency advantage. Given a desired

peak-to-peak current ripple, ∆I

(A), the required inductance

can be calculated via the following expression:

L ≥

OUT

1.2•∆I

• 1–

OUT













µH

( )

A reasonable choice for ripple current is ∆I

= 120mA

which represents 40% of the maximum 300mA 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 series resistance. 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 (given the fixed internal slope compensation).

Specifically, if the buck converter is going to be utilized at

duty cycles greater than 40%, the inductance value must

be at least L

MIN

as given by the following equation:

MIN

≥ 2.5 • V

OUT

(µH)

Table 1 depicts the minimum required inductance for

several common output voltages using standard induc-

tor values.

Table 1. Minimum Inductance

OUTPUT VOLTAGE (V) MINIMUM INDUCTANCE (µH)

0.8 2.2

1.2 3.3

2.0 5.6

2.7 6.8

3.3 8.3

5.0 15

A large variety of low ESR, power inductors are available that

are well suited to the LTC3103 converter applications. The

trade-off generally involves PCB area, application height,

required output current and efficiency. Table 2 provides a

representative sampling of small surface mount inductors

that are well suited for use with the LTC3103. The induc-

tor specifications listed are for comparison purposes but

other values within these inductor families are generally

well suited to this application as well. Within each family

(i.e., at a fixed inductor size), the DC resistance generally

increases and the maximum current generally decreases

with increased inductance.

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APPLICATIONS INFORMATION

Output Capacitor Selection

A low ESR output capacitor should be utilized at the buck

output in order to minimize voltage ripple. Multilayer ce-

ramic capacitors are an excellent choice as they have low

ESR and are available in small footprints. In addition to

controlling the output ripple magnitude, the value of the

output capacitor also sets the loop crossover frequency

and therefore can impact loop stability. There is both a

minimum and maximum capacitance value required to

ensure stability of the loop. If the output capacitance is

too small, the loop crossover frequency will increase to

the point where switching delay and the high frequency

parasitic poles of the error amplifier will degrade the

phase margin. In addition, the wider bandwidth produced

by a small output capacitor will make the loop more sus-

ceptible to switching noise. At the other extreme, if the

output capacitor is too large, the crossover frequency

can decrease too far below the compensation zero and

also lead to degraded phase margin. Table 3 provides a

guideline for the range of allowable values of low ESR

output capacitors assuming a feedforward capacitor is

used. See the Output Voltage Programming section for

details on selecting a feedforward capacitor. Larger value

output capacitors can be accommodated provided they

have sufficient ESR to stabilize the loop, or by increasing

the value of the feedforward capacitor in parallel with the

upper resistor divider resistor.

In Burst Mode operation, the output capacitor stores energy

to satisfy the load current when the LTC3103 is in a low

current sleep state between the burst pulses. It can take

several cycles to respond to a large load step during a sleep

period. If large transient load currents are required then

a larger capacitor can be used at the output to minimize

output voltage droop until the part transitions from Burst

Mode operation to continuous mode operation.

Note that even X5R and X7R type ceramic capacitors have

a DC bias effect which reduces their capacitance when a

DC voltage is applied. It is not uncommon for capacitors

offered in the smallest case sizes to lose more than 50%

of their capacitance when operated near their rated volt-

age. As a result it is sometimes necessary to use a larger

capacitance value or use a higher voltage rating in order to

realize the intended capacitance value. Consult the manu-

facturer’s data for the capacitor you select to be assured

of having the necessary capacitance in your application.

Table 3. Recommended Output Capacitor Limits

OUTPUT VOLTAGE (V) C

MIN

(µF) C

MAX

(µF)

0.8 22.0 220

1.2 15.0 220

2.0 12.0 100

2.7 6.8 68

3.3 4.7 47

5.0 4.7 47

Table 2. Representative Inductor Selection

PART NUMBER

VALUE

(µH)

DCR

(Ω)

MAX DC

CURRENT

(A)

SIZE (MM)

W × L × H

Coilcraft

EPL3015 6.8 0.19 1.00 3.0 × 3.0 × 1.5

LPS3314 10 0.33 0.70 3.3 × 3.3 × 1.3

LPS4018 15 0.26 1.12 4.0 × 4.0 × 1.8

Cooper-Bussman

SD3114 6.8 0.30 0.98 3.1 × 3.1 × 1.4

SD3118 10 0.3 0.75 3.2 × 3.2 × 1.8

Murata

LQH3NPN 6.8 0.20 1.25 3.0 × 3.0 × 1.4

LQH44PN 10 0.16 1.10 4.0 × 4.0 × 1.7

Sumida

CDRH3D16 6.8 0.17 0.73 3.8 × 3.8 × 1.8

CDRH3D16 10 0.21 0.55 3.8 × 3.8 × 1.8

Taiyo-Yuden

CBC3225 6.8 0.16 0.93 3.2 × 2.5 × 2.5

NR3015 10 0.23 0.70 3.0 × 3.0 × 1.5

NR4018 15

0.30 0.65 4.0 × 4.0 × 1.8

Würth

744029006 6.8 0.25 0.95 2.8 × 2.8 × 1.4

744031006 6.8 0.16 0.85 3.8 × 3.8 × 1.7

744031100 10 0.19 0.74 3.8 × 3.8 × 1.7

744031100 15 0.26 0.62 3.8 × 3.8 × 1.7

Panasonic

ELLVGG6R8N 6.8 0.23 1.00 3.0 × 3.0 × 1.5

ELL4LG100MA 10 0.20 0.80 3.8 × 3.8 × 1.8

TDK

VLF3012 6.8 0.18 0.78 3.0 × 2.8 × 1.2

VLC4018 10 0.16 0.85 4.0 × 4.0 × 1.8

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Switching Voltage Regulators 300mA, 15V Step-Down DC/DC Converter with Ultralow Quiescent Current

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