Datasheet

391 mA
300 mA
209 mA
0 mA
1/Freq.
= Ts
Ts/2
L1 Current
LM5008
SNVS280G –APRIL 2004–REVISED MARCH 2013 www.ti.com
a) ESR and R3: A low ESR for C2 is generally desirable so as to minimize power losses and heating within the
capacitor. However, a hysteretic regulator requires a minimum amount of ripple voltage at the feedback input for
proper loop operation. For the LM5008 the minimum ripple required at pin 5 is 25 mVp-p, requiring a minimum
ripple at V
OUT1
of 100 mV. Since the minimum ripple current (at minimum Vin) is 34 mA p-p, the minimum ESR
required at V
OUT1
is 100mV/34mA = 2.94Ω. Since quality capacitors for SMPS applications have an ESR
considerably less than this, R3 is inserted as shown in Figure 2. R3’s value, along with C2’s ESR, must result in
at least 25 mVp-p ripple at pin 5. Generally, R3 will be 0.5 to 3.0Ω.
b) Nature of the Load: The load can be connected to V
OUT1
or V
OUT2
. V
OUT1
provides good regulation, but with a
ripple voltage which ranges from 100 mV (at Vin = 12V) to 500mV (at Vin = 95V). Alternatively, V
OUT2
provides
low ripple, but lower regulation due to R3.
For a maximum allowed ripple voltage of 100 mVp-p at V
OUT2
(at Vin = 95V), assume an ESR of 0.4Ω for C2. At
maximum Vin, the ripple current is 181 mAp-p, creating a ripple voltage of 72 mVp-p. This leaves 28 mVp-p of
ripple due to the capacitance. The average current into C2 due to the ripple current is calculated using the
waveform in Figure 13.
Figure 13. Inductor Current Waveform
Starting when the current reaches Io (300 mA in Figure 13) half way through the on-time, the current continues to
increase to the peak (391 mA), and then decreases to 300 mA half way through the off-time. The average value
of this portion of the waveform is 45.5mA, and will cause half of the voltage ripple, or 14 mV. The interval is one
half of the frequency cycle time, or 2.23 µs. Using the capacitor’s basic equation (see Equation 8), the minimum
value for C2 is 7.2 µF.
C = I x Δt / ΔV (8)
The ripple due to C2’s capacitance is 90° out of phase from the ESR ripple, and the two numbers do not add
directly. However, this calculation provides a practical minimum value for C2 based on its ESR, and the target
spec. To allow for the capacitor’s tolerance, temperature effects, and voltage effects, a 15 µF, X7R capacitor will
be used.
c) In summary: The above calculations provide a minimum value for C2, and a calculation for R3. The ESR is
just as important as the capacitance. The calculated values are guidelines, and should be treated as starting
points. For each application, experimentation is needed to determine the optimum values for R3 and C2.
R
CL
: When a current limit condition is detected, the minimum off-time set by this resistor must be greater than the
maximum normal off-time which occurs at maximum Vin. Using Equation 4, the minimum on-time is 0.470 µs,
yielding a maximum off-time of 3.99 µs. This is increased by 117 ns (to 4.11 µs) due to a ±25% tolerance of the
on-time. This value is then increased to allow for:
The response time of the current limit detection loop (400ns).
The off-time determined by Equation 5 has a ±25% tolerance.
t
OFFCL(MIN)
= (4.11 µs + 0.40 µs) × 1.25 = 5.64 µs (9)
Using Equation 5, R
CL
calculates to 264kΩ (at V
FB
= 2.5V). The closest standard value is 267 kΩ.
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