LM25085AMYSLASHNOPB datasheet(16/31 Pages) TI1

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COUT =

0.85A

8 x 200 kHz x 0.01V

= 53.1 PF

COUT =

IOR(max)

8 x FS x VRIPPLE

ICL(min) =

0.01:

(2.05 k: x 32 PA) - 9 mV

= 5.66A

ICL(max) =

0.01:

(2.05 k: x 48 PA) + 9 mV

= 10.7A

ICL(nom) =

0.01:

(2.05 k: x 40 PA)

= 8.2A

RADJ =

32 PA

6.33A x 0.01:

= 1.98 k:

IPK

IOUT

IOR

1/FS

LM25085A

SNVS601B – JANUARY 2009 – REVISED MARCH 2013

www.ti.com

Figure 27. Inductor Current Waveform

•

RSEN, RADJ: To achieve good current limit accuracy and avoid over designing the power stage components,

the sense resistor method is used for current limiting in this example. A standard value 10 m

Ω resistor is

selected for RSEN, resulting in a 50 mV drop at maximum load current, and a maximum 0.25W power

dissipation in the resistor. Since the LM25085A uses peak current detection, the minimum value for the

current limit threshold must be equal to the maximum load current (5A) plus half the maximum ripple

amplitude calculated above:

ICL(min) = 5A + 0.85A/2 = 5.43A

At this current level the voltage across RSEN is 54.3 mV. Adding the current limit comparator offset of 9 mV

(max) increases the required current limit threshold to 6.33A. Using Equation 7 with the minimum value for

the ADJ pin current (32 µA), the required RADJ resistor calculates to:

A standard value 2.05 k

Ω resistor is selected. The nominal current limit threshold calculates to:

Using the tolerances for the ADJ pin current and the current limit comparator offset, the maximum current limit

threshold calculates to:

The minimum current limit thresholds calculate to:

The load current in each case is equal to the current limit threshold minus half the current ripple amplitude.

The recommended value of 1000 pF for CADJ is used in this example.

•

COUT: Since the maximum allowed output ripple voltage is very low in this example (10 mVp-p), the minimum

ripple configuration (R3, C1, and C2 in the Block Diagram) must be used. The resulting ripple at VOUT is then

due to the inductor’s ripple current passing through COUT. This capacitor’s value can be selected based on the

maximum allowable ripple voltage at VOUT, or based on transient response requirements. The following

calculation, based on ripple voltage, provides a first order result for the value of COUT:

where IOR(max) is the maximum ripple current calculated above, and VRIPPLE is the allowable ripple at VOUT.

A 68 µF capacitor is selected. Typically the ripple amplitude will be higher than the calculations indicate due

to the capacitor’s ESR.

•

R3, C1, C2: The minimum ripple configuration uses these three components to generate the ripple voltage

required at the FB pin since there is insufficient ripple at VOUT. A minimum of 25 mVp-p must be applied to

the FB pin to obtain stable constant frequency operation. R3 and C1 are selected to generate a sawtooth

waveform at their junction, and that waveform is AC coupled to the FB pin via C2. The values of the three

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