Preliminary Datasheet

LP6343

LP6343 –00 Version 1.0 Datasheet

Feb.-2010

www.lowpowersemi.com

Page 8 of 12

Table 1 shows the resistor selection for different output

voltage settings.

Vout(V) R2=59KΩ

R1(KΩ)

R1=316KΩ

R2(KΩ)

0.8

19.6

105

0.9

29.4

158

1.0

39.2

210

1.1

49.9

261

1.2

59

316

1.3

68.1

365

1.4

78.7

422

1.5

88.7

475

1.8

118

634

1.85

124

655

2.0

137

732

2.5

187

1000

3.3

267

1430

Table 1: Resistor Selections for Different Output

Voltage Settings (Standard 1% Resistors

Substituted For Calculated Values).

Inductor Selection

For most designs, the LP6343 operates with inductor values

of 1μH to 4.7μH. Low inductance values are physically

smaller but require faster switching, which results in some

efficiency loss. The inductor value can be derived from

the following equation:

Where ΔIL is inductor ripple current. Large value inductors

lower ripple current and small value inductors result in high

ripple

currents.

Choose

inductor

ripple

current

approximately 30% of the maximum load current 2A, or

For output voltages above 2.0V, when light-load efficiency

is important, the minimum recommended inductor is 2.2μH.

Manufacturer’s specifications list both the inductor DC

current rating, which is a thermal limitation, and the peak

current rating, which is determined by the saturation

characteristics. The inductor should not show any

appreciable saturation under normal load conditions. Some

inductors may meet the peak and average current ratings yet

result in excessive losses due to a high DCR.

Always consider the losses associated with the DCR and its

effect on the total converter efficiency when selecting an

inductor. For optimum voltage-positioning load transients,

choose an inductor with DC series resistance in the 20mΩ

to 100mΩ range. For higher efficiency at heavy loads

(above 200mA), or minimal load regulation (but some

transient overshoot), the resistance should be kept below

100mΩ. The DC current rating of the inductor should be

at least equal to the maximum load current plus half the

ripple current to prevent core saturation (2A + 600mA).

Table 2 lists some typical surface mount inductors that meet

target applications for the LP6343. For example, the 2.2μH

CDRH5D16-2R2 inductor selected from Sumida has a

28.7mΩ DCR and a 3.0ADC cur-rent rating. At full load,

the inductor DC loss is 57mW which gives a 1.6% loss in

efficiency for a 1200mA, 1.8V output.

Slope Compensation

The LP6343 step-down converter uses peak current mode

control with slope compensation for stability when duty

cycles are greater than 50%. The slope compensation is set

to maintain stability with lower value inductors which

provide better overall efficiency. The output inductor value

must be selected so the inductor current down slope meets

the internal slope compensation requirements. As an

example, the value of the slope compensation is set to

1A/μs which is large enough to guarantee stability when

using a 2.2μH inductor for all output volt-age levels from

0.6V to 3.3V. The worst case external current slope (m)

using the 2.2μH inductor is when VOUT = 3.3V and is:

To keep the power supply stable when the duty cycle is

above 50%, the internal slope compensation (mA) should

be:

Therefore, to guarantee current loop stability, the slope of

the compensation ramp must be greater than one-half of the

down slope of the current waveform. So the internal slope

compensated value of 1A/μs will guarantee stability using a

2.2μH inductor value for all output volt-ages from 0.6V to

3.3V.

Input Capacitor Selection

The input capacitor reduces the surge current drawn from

the input and switching noise from the device. The input

capacitor impedance at the switching frequency should be

less than the input source impedance to pre-vent high

frequency switching current passing to the input. The

calculated value varies with input voltage and is a

maximum when VIN is double the output voltage.

A low ESR input capacitor sized for maximum RMS

cur-rent must be used. Ceramic capacitors with X5R or

X7R dielectrics are highly recommended because of their

low ESR and small temperature coefficients.

A 22μF

ceramic capacitor for most applications is sufficient. A

large value may be used for improved input voltage

filtering.The maximum input capacitor RMS current is: