LM5005MH National Semiconductor, LM5005MH Datasheet - Page 16

• Manufacturer Part Number: LM5005MH
• Description: IC, BUCK REGULATOR, TSSOP-20
• Manufacturer: National Semiconductor

Specifications of LM5005MH

• Primary Input Voltage: 75V
• No. Of Outputs: 1
• Output Voltage: 1.225V
• Output Current: 2.5A
• No. Of Pins: 20
• Operating Temperature Range: -40°C To +125°C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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Application Information
When the buck switch turns on, the current into the VIN pin
steps to the lower peak of the inductor current waveform,
ramps up to the peak value, then drops to zero at turn-off.
The average current into VIN during the on-time is the load
current. The input capacitance should be selected for RMS
current rating and minimum ripple voltage. A good approxi-
mation for the required ripple current rating necessary is
I
Quality ceramic capacitors with a low ESR should be se-
lected for the input filter. To allow for capacitor tolerances
and voltage effects, two 2.2 µF, 100V ceramic capacitors will
be used. If step input voltage transients are expected near
the maximum rating of the LM5005, a careful evaluation of
ringing and possible spikes at the device VIN pin should be
completed. An additional damping network or input voltage
clamp may be required in these cases.
C8
The capacitor at the VCC pin provides noise filtering and
stability for the V
should be no smaller than 0.1 µF, and should be a good
quality, low ESR, ceramic capacitor. A value of 0.47 µF was
selected for this design.
C7
The bootstrap capacitor between the BST and the SW pins
supplies the gate current to charge the buck switch gate at
turn-on. The recommended value of C7 is 0.022 µF, and
should be a good quality, low ESR, ceramic capacitor.
C4
The capacitor at the SS pin determines the soft-start time,
i.e. the time for the reference voltage and the output voltage,
to reach the final regulated value. The time is determined
from:
For this application, a C4 value of 0.01 µF was chosen which
corresponds to a soft-start time of 1 ms.
R5, R6
R5 and R6 set the output voltage level, the ratio of these
resistors is calculated from:
For a 5V output, the R5/R6 ratio calculates to 3.082. The
resistors should be chosen from standard value resistors, a
good starting point is selection in the range of 1.0 kΩ - 10 kΩ.
Values of 5.11 kΩ for R5, and 1.65 kΩ for R6 were selected.
R1, R2, C12
A voltage divider can be connected to the SD pin to set a
minimum operating voltage Vin
feature is required, the easiest approach to select the divider
resistor values is to select a value for R1 (between 10 kΩ
and 100 kΩ recommended) then calculate R2 from:
RMS
>
I
OUT
/ 2.
R5/R6 = (V
CC
regulator. The recommended value of C8
OUT
/ 1.225V) - 1
(min)
for the regulator. If this
(Continued)
16
Capacitor C12 provides filtering for the divider. The voltage
at the SD pin should never exceed 8V, when using an
external set-point divider it may be necessary to clamp the
SD pin at high input voltage conditions. The reference de-
sign utilizes the full range of the LM5005 (7V to 75V); there-
fore these components can be omitted. With the SD pin open
circuit the LM5005 responds once the Vcc UVLO threshold
is satisfied.
R7, C11
A snubber network across the power diode reduces ringing
and spikes at the switching node. Excessive ringing and
spikes can cause erratic operation and couple spikes and
noise to the output. In the limit, spikes beyond the rating of
the LM5005 or the re-circulating diode can damage these
devices. Selecting the values for the snubber is best accom-
plished through empirical methods. First, make sure the lead
lengths for the snubber connections are very short. For the
current levels typical for the LM5005 a resistor value be-
tween 5 and 20 Ohms is adequate. Increasing the value of
the snubber capacitor results in more damping but higher
losses. Select a minimum value of C11 that provides ad-
equate damping of the SW pin waveform at high load.
R4, C5, C6
These components configure the error amplifier gain char-
acteristics to accomplish a stable overall loop gain. One
advantage of current mode control is the ability to close the
loop with only two feedback components, R4 and C5. The
overall loop gain is the product of the modulator gain and the
error amplifier gain. The DC modulator gain of the LM5005 is
as follows:
The dominant low frequency pole of the modulator is deter-
mined by the load resistance (R
tance (C
For R
DC Gain
For the design example of Figure 1 the following modulator
gain vs. frequency characteristic was measured as shown in
Figure 8.
LOAD
DC Gain
OUT
(MOD)
= 5 Ω and C
). The corner frequency of this pole is:
f
(MOD)
= 2 x 5 = 10 = 20 dB
p(MOD)
= G
= 1 / (2π R
OUT
m(MOD)
= 177 µF then f
x R
LOAD
LOAD
LOAD
,) and output capaci-
C
OUT
= 2 x R
p(MOD)
)
LOAD
= 180Hz