LM46000PWPR datasheet(30/56 Pages) TI2

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OUT

ESR

(

0.5)

u u '

OUT

(1 D )

(1 r)

/ I

)

LM46000

SNVSA45B – JUNE 2014 – REVISED MARCH 2018

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In general, choosing lower inductance is preferred in switching power supplies, because it usually corresponds to

faster transient response, smaller DCR, and reduced size for more compact designs. But too low of an

inductance can generate too large of an inductor current ripple such that over current protection at the full load

could be falsely triggered. It also generates more conduction loss, since the RMS current is slightly higher

relative that with lower current ripple at the same DC current. Larger inductor current ripple also implies larger

output voltage ripple with the same output capacitors. With peak current mode control, it is not recommended to

have too small of an inductor current ripple. Enough inductor current ripple improves signal-to-noise ratio on the

current comparator and makes the control loop more immune to noise.

Once the inductance is determined, the type of inductor must be selected. Ferrite designs have very low core

losses and are preferred at high switching frequencies, so design goals can concentrate on copper loss and

preventing saturation. Ferrite core material saturates hard, which means that inductance collapses abruptly when

the peak design current is exceeded. The ‘hard’ saturation results in an abrupt increase in inductor ripple current

and consequent output voltage ripple. Do not allow the core to saturate!

For the design example, a standard 27-μH inductor from Wurth, Coiltronics, or Vishay can be used for the 3.3-V

output with plenty of current rating margin.

8.2.2.6 Output Capacitor Selection

The device is designed to be used with a wide variety of LC filters. It is generally desired to use as little output

capacitance as possible to keep cost and size down. Choose the output capacitor (s), COUT, with care because it

directly affects the steady-state output voltage ripple, loop stability and the voltage over/undershoot during load

current transients.

The output voltage ripple is essentially composed of two parts. One is caused by the inductor current ripple going

through the ESR of the output capacitors:

ΔVOUT-ESR = ΔiL× ESR

(17)

The other is caused by the inductor current ripple charging and discharging the output capacitors:

ΔVOUT-C = ΔiL / ( 8 × FS × COUT )

(18)

The two components in the voltage ripple are not in phase, so the actual peak-to-peak ripple is smaller than the

sum of the two peaks.

Output capacitance is usually limited by transient performance specifications if the system requires tight voltage

regulation in the presence of large current steps and fast slew rates. When a fast large load transient happens,

output capacitors provide the required charge before the inductor current can slew to the appropriate level. The

initial output voltage step is equal to the load current step multiplied by the ESR. VOUT continues to droop until

the control loop response increases or decreases the inductor current to supply the load. To maintain a small

over- or under-shoot during a transient, small ESR and large capacitance are desired. But these also come with

higher cost and size. Thus, the motivation is to seek a fast control loop response to reduce the output voltage

deviation.

For a given input and output requirement, Equation 19 gives an approximation for an absolute minimum output

capacitor required:

(19)

Along with this for the same requirement, calculate the maximum ESR per Equation 20

where

•

r = Ripple ratio of the inductor ripple current (ΔIL / IOUT)

•

ΔVOUT = target output voltage undershoot

•

D’ = 1 – duty cycle

•

FS = switching frequency

•

IOUT = load current

(20)

LM46000PWPR Datasheet(PDF) 30 Page - Texas Instruments

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