ESR(min) =

R2 x IOR(min)

25 mV x (R1 + R2)

= 2.8:

IOR(min) =

L1MAX x FS(max) x VIN(min)

VOUT1 x (VIN(min) - VOUT1)

120 PH x 772 kHz x 15V

10V x (15V - 10V)

= 36 mA

=

IOR(max) =

80 PH x 463 kHz x 75V

10V x (75V - 10V)

= 234 mAp-p

IOR(max) =

L1MIN x FS(min) x VIN(max)

VOUT1 x (VIN(max) - VOUT1)

L1 =

0.30A x 463 kHz x 75V

10V x (75V - 10V)

= 63 PH

L1 =

IOR x FS(min) x VIN(max)

VOUT1 x (VIN(max) - VOUT1)

17

LM5010

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where

•

FS(min) is the minimum frequency (FS - 25%)

(9)

(10)

Equation 10 provides the minimum value for inductor L1. When selecting an inductor, use a higher standard

value (100 uH). L1 must be rated for the peak current (IPK+) to prevent saturation. The peak current occurs at

maximum load current with maximum ripple. The maximum ripple is calculated by rearranging Equation 9 using

VIN(max), FS(min), and the minimum inductor value, based on the manufacturer’s tolerance. Assume for

Equation 11, Equation 12, and Equation 13 that the inductor’s tolerance is ±20%.

(11)

(12)

IPK+ = 1 A + 0.234 A / 2 = 1.117 A

(13)

8.2.2.1.4

RCL

Since it is obvious that the lower peak of the inductor current waveform does not exceed 1 A at maximum load

current (see Figure 13), it is not necessary to increase the current limit threshold. Therefore RCL is not needed for

this exercise. For applications where the lower peak exceeds 1 A, see Increasing The Current Limit Threshold.

8.2.2.1.5

C2 and R3

Since the LM5010 requires a minimum of 25 mVP-P of ripple at the FB pin for proper operation, the required ripple

at VOUT1 is increased by R1 and R2. This necessary ripple is created by the inductor ripple current acting on C2’s

ESR + R3. First, determine the minimum ripple current with Equation 14.

(14)

The minimum ESR for C2 is then equal to Equation 15.

(15)

If the capacitor used for C2 does not have sufficient ESR, R3 is added in series as shown in Figure 12. C2

should generally be no smaller than 3.3 µF, although that is dependent on the frequency and the allowable ripple

amplitude at VOUT1. Experimentation is usually necessary to determine the minimum value for C2, as the nature

of the load may require a larger value. A load which creates significant transients requires a larger value for C2

than a non-varying load.