LM5009EVAL National Semiconductor, LM5009EVAL Datasheet - Page 10

• Manufacturer Part Number: LM5009EVAL
• Description: BOARD EVALUATION LM5009
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM5009EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 10V
• Current - Output: 150mA
• Voltage - Input: 12 ~ 95V
• Regulator Topology: Buck
• Frequency - Switching: 240Hz
• Board Type: Fully Populated
• Utilized Ic / Part: LM5009
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -
• Lead Free Status / Rohs Status: Not Compliant

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At Vin = 90V, L1(min) calculates to 132 µH. The next larger
standard value (150 µH) is chosen and with this value I
calculates to 176 mA p-p at Vin = 90V, and 33 mA p-p at Vin
= 12V.
b) Maximum load current: At a load current of 150 mA, the
peak of the ripple waveform must not reach the minimum
guaranteed value of the LM5009’s current limit threshold (250
mA). Therefore the ripple amplitude must be less than 200
mA p-p, which is already satisfied in the above calculation.
With L1 = 150 µH, at maximum Vin and Io, the peak of the
ripple will be 238 mA. While L1 must carry this peak current
without saturating or exceeding its temperature rating, it also
must be capable of carrying the maximum guaranteed value
of the LM5009’s current limit threshold (370 mA) without sat-
urating, since the current limit is reached during startup.
C3: The capacitor on the V
filtering and stability, but also prevents false triggering of the
V
son, 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 capacitor.
However, this regulator requires a minimum amount of ripple
voltage at the feedback input for proper loop operation. For
the LM5009 the minimum ripple required at pin 5 is 25 mV p-
p, requiring a minimum ripple at V
minimum ripple current (at minimum Vin) is 33 mA p-p, the
minimum ESR required at V
itors for SMPS applications 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 Ω to 5.0 Ω.
b) Nature of the Load: The load can be connected to
V
ripple voltage which ranges from 100 mV (@ Vin = 12V) to
580 mV (@Vin = 90V). Alternatively, V
(3 mV to 13 mV) but lower regulation due to R3.
C2 should generally be no smaller than 3.3 µF. Typically, its
value is 10 µF to 20 µF, with the optimum value determined
by the load. If the load current is fairly constant, a small value
suffices for C2. If the load current includes significant tran-
sients, a larger value is necessary. For each application,
experimentation is needed to determine the optimum values
for R3 and C2.
C) Ripple Reduction: The ripple amplitude at V
reduced by reducing R3, and adding a capacitor across R1
so as to tranfer the ripple at V
out attenuation. The new value of R3 is calculated from:
where I
mAp-p in this example. The added capacitor's value is calcu-
lated from:
where T
selected capacitor should be larger than the value calculated
above.
CC
OUT1
UVLO at the buck switch on/off transitions. For this rea-
or V
OR(min)
ON(max)
OUT2
is the minimum ripple current amplitude - 33
is the maximum on-time (at minimum Vin). The
. V
OUT1
C = T
R3 = 25 mV/I
provides good regulation, but with a
ON(max)
CC
OUT1
OUT1
output provides not only noise
/(R1 // R2)
is 3 Ω. Since quality capac-
OR(min)
OUT1
directly to the FB pin, with-
OUT2
of 100 mV. Since the
provides low ripple
OUT1
can be
OR
10
R
off-time set by this resistor must be greater than the maximum
normal off-time which occurs at maximum Vin. Using equation
2, the minimum on-time is 0.329 µs, yielding a maximum off-
time of 2.63 µs. This is increased by 82 ns (to 2.72 µs) due to
a ±25% tolerance of the on-time. This value is then increased
to allow for:
(400ns),
ance,
Using equation 3, R
The closest standard value is 169 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, al-
though that affects efficiency. A good choice is an ultrafast
power or Schottky diode with a reverse recovery time of
ns, and a forward voltage drop of
may have a lower forward voltage drop, but may have longer
recovery times, or greater reverse leakage. D1’s reverse volt-
age rating must be at least as great as the maximum Vin, and
its current rating be greater than the maximum current limit
threshold (370 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 (150 mA). For a worst case calculation, C1 must
supply this average load current during the maximum on-time.
To keep the input voltage ripple to less than 2V (for this ex-
ercise), C1 calculates to:
Quality ceramic capacitors in this value have a low ESR which
adds only a few millivolts to the ripple. It is the capacitance
which is dominant in this case. To allow for the capacitor’s
tolerance, temperature effects, and voltage effects, a 1.0 µF,
100V, X7R capacitor will be used.
C4: The recommended value is 0.022 µF for C4, as this is
appropriate in the majority of applications. A high quality ce-
ramic capacitor, with low ESR is recommended as C4 sup-
plies the surge current to charge the buck switch gate at turn-
on. A low ESR also ensures a quick recharge during each off-
time. At minimum VIN, when the on-time is at maximum, it is
possible during start-up that C4 will not fully recharge during
each 300 ns off-time. The circuit will not be able to complete
the start-up, and achieve output regulation. This can occur
when the frequency is intended to be low (e.g., R
In this case C4 should be increased so it can maintain suffi-
cient voltage across the buck switch driver during each on-
time.
CL
The response time of the current limit detection loop
The off-time determined by equation 3 has a ±25% toler-
: When a current limit condition is detected, the minimum
t
OFFCL(MIN)
= (2.72 µs x 1.25) + 0.4 µs= 3.8 µs
CL
calculates to 167 kΩ (at V
≊
0.7V. Other types of diodes
ON
FB
= 500K).
= 2.5V).
≊
30