LM5010AMHSLASHNOPB datasheet(13/25 Pages) TI

13 / 25 page

ESR(min) =

34.5 mA

50 mV

= 1.45:

IOR(min) =

L1max x FS(max) x VIN(min)

VOUT x (VIN(min) - VOUT)

120 PH x 201 kHz x 6V

5V x (6V - 5V)

= 34.5 mAp-p

C1 =

IO x tON

= 13 PF

0.5V

1.0A x 6.5 Ps

tON(max) =

6V - 1.4V

1.18 x 10

-10

x (200k + 1.4k)

+ 67 ns x 1.25 = 6.5 Ps

IOR(max) =

80 PH x 154 kHz x 60V

5V x (60V - 5V)

= 372 mAp-p

IOR(max) =

L1min x FS(min) x VIN(max)

VOUT x (VIN(max) - VOUT)

LM5010A

LM5010A-Q1

www.ti.com

SNVS376E – OCTOBER 2005 – REVISED FEBRUARY 2013

This provides a minimum value for L1 - the next higher standard value (100 µH) will be used. To prevent

saturation, and possible destructive current levels, L1 must be rated for the peak current which occurs if the

current limit and maximum ripple current are reached simultaneously (IPK in Figure 10). The maximum ripple

amplitude is calculated by re-arranging Equation 12 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%.

(14)

(15)

IPK = ILIM + IOR(max) = 1.5A + 0.372A = 1.872A

(16)

where ILIM is the maximum current limit threshold. At the nominal maximum load current of 1.0A, the peak

inductor current is 1.186A.

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 11), 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 INCREASING THE

CURRENT LIMIT THRESHOLD.

C1: This capacitor limits the ripple voltage at VIN resulting from the source impedance of the supply feeding this

circuit, and the on/off nature of the switch current into VIN. At maximum load current, when the buck switch turns

on, the current into VIN steps up from zero to the lower peak of the inductor current waveform (IPK-in Figure 11),

ramps up to the peak value (IPK+), 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 current during the maximum on-

time. The maximum on-time is calculated at VIN = 6V using Equation 5, with a 25% tolerance added:

(17)

The voltage at VIN should not be allowed to drop below 5.5V in order to maintain VCC above its UVLO.

(18)

Normally a lower value can be used for C1 since the above calculation is a worst case calculation which

assumes the power source has a high source impedance. A quality ceramic capacitor with a low ESR should be

used for C1.

C2 and R3: Since the LM5010A requires a minimum of 25 mVp-p of ripple at the FB pin for proper operation, the

required ripple at VOUT is increased by R1 and R2, and is equal to:

VRIPPLE = 25 mVp-p x (R1 + R2)/R2 = 50 mVp-p

(19)

This necessary ripple voltage is created by the inductor ripple current acting on C2’s ESR + R3. First, the

minimum ripple current, which occurs at minimum VIN, maximum inductor value, and maximum frequency, is

determined.

(20)

The minimum ESR for C2 is then equal to:

(21)

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