LM2694MTSLASHNOPB datasheet(12/26 Pages) TI

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C1 =

IO x tON

= 3.8 PF

0.5V

0.6A x 3.22 Ps

tON(max) =

8V - 1.5V

1.14 x 10

-10 x (140k + 1.4k)

+ 95 ns x 1.25 = 3.22 Ps

IOR(max) =

120 PH x 189 kHz x 30V

5V x (30V - 5V)

= 184 mAp-p

IOR(max) =

L1MIN x FS(min) x VIN(max)

VOUT x (VIN(max) - VOUT)

L1 =

0.2A x 189 kHz x 30V

5V x (30V - 5V)

= 110 PH

L1 =

IOR x FS(min) x VIN(max)

VOUT x (VIN(max) - VOUT)

0 mA

IOR

1/Fs

IPK+

IPK-

LM2694

SNVS444A – MAY 2006 – REVISED APRIL 2013

www.ti.com

Figure 13. Inductor Current

To keep the circuit in continuous conduction mode, the maximum allowed ripple current is twice the minimum

load current, or 200 mAp-p. Using this value of ripple current, the inductor (L1) is calculated using the following:

where

•

FS(min) is the minimum frequency of 189 kHz (252 kHz - 25%)

(11)

(12)

This provides a minimum value for L1 - the next higher standard value (150 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. The maximum ripple amplitude is

calculated by re-arranging Equation 11 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%.

(13)

(14)

IPK = ILIM + IOR(max) = 0.74A + 0.18A = 0.92A

where

•

ILIM is the maximum specified current limit threshold

(15)

At the nominal maximum load current of 0.6A, the peak inductor current is 692 mA.

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 13),

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 = 8V using Equation 5, with a 25% tolerance added:

(16)

The voltage at VIN should not be allowed to drop below 7.5V in order to maintain VCC above its UVLO.

(17)

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