Datasheet

C1 =
'V
I
O
x t
ON
= 3.8 PF
0.5V
0.6A x 3.22 Ps
=
t
ON(max)
=
8V - 1.5V
1.14 x 10
-10
x (140k + 1.4k)
+ 95 ns
x 1.25 = 3.22 Ps
I
OR(max)
=
120 PH x 189 kHz x 30V
5V x (30V - 5V)
= 184 mAp-p
I
OR(max)
=
L1
MIN
x F
S(min)
x V
IN(max)
V
OUT
x (V
IN(max)
- V
OUT
)
L1 =
0.2A x 189 kHz x 30V
5V x (30V - 5V)
= 110 PH
L1 =
I
OR
x F
S(min)
x V
IN(max)
V
OUT
x (V
IN(max)
- V
OUT
)
L1 Current
0 mA
I
OR
1/Fs
I
PK+
I
O
I
PK-
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
• F
S(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 V
IN(max)
, F
S(min)
, and the minimum inductor value, based on the
manufacturer’s tolerance. Assume, for this exercise, the inductor’s tolerance is ±20%.
(13)
(14)
I
PK
= I
LIM
+ I
OR(max)
= 0.74A + 0.18A = 0.92A
where
• I
LIM
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 (I
PK-
in Figure 13),
ramps up to the peak value (I
PK+
), 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 V
IN
= 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 V
CC
above its UVLO.
(17)
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