Datasheet

ESR
(min)
=
R2 x I
OR(min)
25 mV x (R1 + R2)
= 2.8:
I
OR(min)
=
L1
MAX
x F
S(max)
x V
IN(min)
V
OUT1
x (V
IN(min)
- V
OUT1
)
120 PH x 772 kHz x 15V
10V x (15V - 10V)
= 36 mA
=
I
OR(max)
=
80 PH x 463 kHz x 75V
10V x (75V - 10V)
= 234 mAp-p
I
OR(max)
=
L1
MIN
x F
S(min)
x V
IN(max)
V
OUT1
x (V
IN(max)
- V
OUT1
)
LM5010
www.ti.com SNVS307F –SEPTEMBER 2004–REVISED FEBRUARY 2013
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 (I
PK+
) 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 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%.
(11)
(12)
I
PK+
= 1.0A + 0.234A / 2 = 1.117A (13)
R
CL
: 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 R
CL
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 V
OUT1
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 V
OUT1
. 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 V
IN
, and for the peak current when in current limit
(I
PK
in Figure 11) which is equal to:
I
PK
= 1.5A + I
OR(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 R
CL
is not required.
C1: Assuming the voltage supply feeding V
IN
has a source impedance greater than zero, this capacitor limits the
ripple voltage at V
IN
while supplying most of the switch current during the on-time. At maximum load current,
when the buck switch turns on, the current into V
IN
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 V
IN
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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