LM5010SD datasheet(13/26 Pages) TI1

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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)

LM5010

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SNVS307E – SEPTEMBER 2004 – REVISED FEBRUARY 2005

This provides a minimum value for L1 - the next higher standard value (100 µH) will be used. 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 re-arranging Equation 9 using VIN(max), FS(min), and the minimum

inductor value, based on the manufacturer’s tolerance. Assume, for this exercise, the inductor’s tolerance is

±20%.

(11)

(12)

IPK+ = 1.0A + 0.234A / 2 = 1.117A

(13)

RCL: Since it is obvious that the lower peak of the inductor current waveform does not exceed 1.0A 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 1.0A, see the section below on

increasing the current limit threshold.

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, the minimum ripple current is determined.

(14)

The minimum ESR for C2 is then equal to:

(15)

If the capacitor used for C2 does not have sufficient ESR, R3 is added in series as shown in Typical Application

Circuit and Block Diagram. 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.

D1: The important parameters are reverse recovery time and forward voltage drop. The reverse recovery time

determines how long the current surge lasts each time the buck switch is turned on. The forward voltage drop is

significant in the event the output is short-circuited as it is mainly this diode’s voltage (plus the voltage across the

current limit sense resistor) which forces the inductor current to decrease during the off-time. For this reason, a

higher voltage is better, although that affects efficiency. A reverse recovery time of

≊30 ns, and a forward voltage

drop of

≊0.75V are preferred. The reverse leakage specification is important as that can significantly affect

efficiency. Other types of diodes may have a lower forward voltage drop, but may have longer recovery times, or

greater reverse leakage. D1 should be rated for the maximum VIN, and for the peak current when in current limit

(IPK in Figure 11) which is equal to:

IPK = 1.5A + IOR(max) = 1.734A

(16)

where 1.5A is the maximum guaranteed current limit threshold, and the maximum ripple current was previously

calculated as 234 mAp-p. Note that this calculation is valid only when RCL is not required.

C1: Assuming the voltage supply feeding VIN has a source impedance greater than zero, this capacitor limits the

ripple voltage at VIN while supplying most of the switch current during the on-time. At maximum load current,

when the buck switch turns on, the current into VIN increases to the lower peak of the output current waveform,

ramps up to the peak value, then drops to zero at turn-off. The average current into VIN during this on-time is the

load current. For a worst case calculation, C1 must supply this average load current during the maximum on-

time. The maximum on-time is calculated using Equation 5, with a 25% tolerance added:

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