LM5008MM National Semiconductor, LM5008MM Datasheet - Page 11

• Manufacturer Part Number: LM5008MM
• Description: BUCK BIAS REG, 100V, 5008, MSOP8
• Manufacturer: National Semiconductor

Specifications of LM5008MM

• Primary Input Voltage: 95V
• No. Of Outputs: 1
• Output Voltage: 90V
• Output Current: 350mA
• Voltage Regulator Case Style: MSOP
• No. Of Pins: 8
• Operating Temperature Range: -40°C To +125°C
• Svhc: No SVHC
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Applications Information
false triggering of the V
transitions. For this reason, C3 should be no smaller than 0.1
µF.
C2, and R3: When selecting the output filter capacitor C2,
the items to consider are ripple voltage due to its ESR, ripple
voltage due to its capacitance, and the nature of the load.
a) ESR and R3: A low ESR for C2 is generally desirable so
as to minimize power losses and heating within the capaci-
tor. 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 mV p-p, requiring a minimum ripple at V
100 mV. Since the minimum ripple current (at minimum Vin)
is 34 mA p-p, the minimum ESR required at V
34mA = 2.94Ω. Since quality capacitors for SMPS applica-
Starting when the current reaches Io (300 mA in Figure 12)
half way through the on-time, the current continues to in-
crease 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:
the minimum value for C2 is 7.2 µF. 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 ca-
pacitor 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
off-time set by this resistor must be greater than the maxi-
mum normal off-time which occurs at maximum Vin. Using
equation 2, 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
is then increased to allow for:
(400ns),
CL
The response time of the current limit detection loop
: When a current limit condition is detected, the minimum
±
25% tolerance of the on-time. This value
C = I x ∆t / ∆V
CC
UVLO at the buck switch on/off
FIGURE 12. Inductor Current Waveform
(Continued)
OUT1
is 100mV/
OUT1
of
11
tions have an ESR considerably less than this, R3 is inserted
as shown in Figure 1. R3’s value, along with C2’s ESR, must
result in at least 25 mV p-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
or V
voltage which ranges from 100 mV (
(
lower regulation due to R3.
For a maximum allowed ripple voltage of 100 mVp-p at
V
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
12.
ance,
Using equation 3, R
The closest standard value is 267 kΩ.
D1: The important parameters are reverse recovery time and
forward voltage. The reverse recovery time determines how
long the reverse 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 only this diode’s
voltage which forces the inductor current to reduce during
the forced off-time. For this reason, a higher voltage is better,
although that affects efficiency. A good choice is an ultrafast
power diode, such as the MURA110T3 from ON Semicon-
ductor. Its reverse recovery time is 30ns, and its forward
voltage drop is approximately 0.72V at 300 mA at 25˚C.
Other types of diodes may have a lower forward voltage
drop, but may have longer recovery times, or greater reverse
leakage. D1’s reverse voltage rating must be at least as
great as the maximum Vin, and its current rating be greater
than the maximum current limit threshold (610 mA).
C1: This capacitor’s purpose is to supply most of the switch
current during the on-time, and limit the voltage ripple at Vin,
on the assumption that the voltage source feeding Vin has
an output impedance greater than zero. At maximum load
current, when the buck switch turns on, the current into pin 8
will suddenly increase to the lower peak of the output current
waveform, ramp up to the peak value, then drop to zero at
turn-off. The average input current during this on-time is the
load current (300 mA). For a worst case calculation, C1 must
@
OUT2
The off-time determined by equation 3 has a
Vin = 95V). Alternatively, V
OUT2
t
OFFCL(MIN)
(
@
. V
Vin = 95V), assume an ESR of 0.4Ω for C2. At
OUT1
= (4.11 µs + 0.40µs) x 1.25 = 5.64 µs
provides good regulation, but with a ripple
CL
calculates to 264kΩ (at V
OUT2
provides low ripple, but
@
Vin = 12V) to 500mV
20097926
±
www.national.com
FB
25% toler-
= 2.5V).
OUT1