LM5085SDESLASHNOPB datasheet(19/33 Pages) TI

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R3 x C1 =

(7V - 4.81V) x 2.55 Ps

0.025V

= 2.23 x 10

-4

R3 x C1 =

(VIN(min) - VA) x tON

COUT =

1.19A

8 x 300 kHz x 0.005V

= 99.2 PF

COUT =

IOR(max)

8 x FS x VRIPPLE

ICL(min) =

0.01:

(2.1 k: x 32 PA) - 9 mV

= 5.82A

ICL(max) =

0.01:

(2.1 k: x 48 PA) + 9 mV

= 11A

ICL(nom) =

0.01:

(2.1 k: x 40 PA)

= 8.4A

LM5085, LM5085-Q1

www.ti.com

SNVS565I – NOVEMBER 2008 – REVISED AUGUST 2015

Typical Application (continued)

A standard value 2.1k

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

(21)

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

threshold is calculated to be:

(22)

The minimum current limit threshold is:

(23)

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

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

COUT: Since the maximum allowed output ripple voltage is very low in this example (5 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:

(24)

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

(25)

A 100µF 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 components are

determined using the following procedure:

Calculate VA = VOUT - (VSW x (1 – (VOUT/VIN(min))))

(26)

where VSW is the absolute value of the voltage at the SW node during the off-time, typically 0.5V to 1V

depending on the diode D1. Using a typical value of 0.65V, VA calculates to 4.81V. VA is the nominal DC voltage

at the R3/C1 junction, and is used in the next equation to calculate the R3-C3 product:

(27)

where tON is the maximum on-time (at minimum input voltage), and ΔV is the desired ripple amplitude at the R3-

C1 junction. For ripple voltage of 25 mVp-p:

(28)

R3 and C1 are then selected from standard value components to produce the product calculated above. Typical

values for C1 are 3000pF to 10,000pF, and R3 is typically from 10k

Ω to 300kΩ. C2 is then chosen large

compared to C1, typically 0.1µF. For this example, 3300pF is chosen for C1, requiring R3 to be 67.7k

Ω. A

standard value 66.5k

Ω resistor is selected.

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