LM5117PMH/NOPB National Semiconductor, LM5117PMH/NOPB Datasheet - Page 23

• Manufacturer Part Number: LM5117PMH/NOPB
• Description: IC BUCK CONTROLLER 20-TSSOP
• Manufacturer: National Semiconductor
• Series: -r

Specifications of LM5117PMH/NOPB

• Pwm Type: Current Mode
• Number Of Outputs: 1
• Frequency - Max: 530kHz
• Duty Cycle: 95%
• Voltage - Supply: 5.5 V ~ 65 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 20-TSSOP (0.173", 4.40mm Width) Exposed Pad
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

Design Example
OPERATING CONDITIONS
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•
•
•
•
TIMING RESISTOR R
Generally, higher frequency applications are smaller but have
higher losses. Operation at 230 kHz was selected for this ex-
ample as a reasonable compromise between small size and
high efficiency. The value of R
quency can be calculated from the equation (3) as follows:
A standard value of 22.1kΩ was chosen for R
OUTPUT INDUCTOR L
The maximum inductor ripple current occurs at the maximum
input voltage. Typically, 20% to 40% of the full load current is
a good compromise between core loss and copper loss of the
inductor. Higher ripple current allows for a smaller inductor
size, but places more of a burden on the output capacitor to
smooth the ripple voltage on the output. For this example, a
ripple current of 40% of 9A was chosen. Knowing the switch-
ing frequency, maximum ripple current, maximum input volt-
age and the nominal output voltage, the inductor value can
be calculated as follows:
The closest standard value of 10μH was chosen for L
the value of 10μH for L
essary if the chosen value of L
calculated value.
From the equation (11),
At the minimum input voltage, this value is 1.04A
DIODE EMULATION FUNCTION
The DEMB pin is left floating since this example uses diode
emulation to reduce the power loss under no load or light load
conditions.
Output Voltage
Full Load Current
Minimum Input Voltage
Maximum Input Voltage
Switching Frequency
Diode Emulation
External VCC Supply
T
O
, calculate I
O
O
T
differs significantly from the
for 230 kHz switching fre-
PP
again. This step is nec-
T
V
V
.
f
SW
IN(MAX)
IN(MIN)
V
OUT
= 230kHz
I
OUT
O
. Using
= 12V
= 15V
= 55V
= 9A
Yes
Yes
23
CURRENT SENSE RESISTOR R
The performance of the converter will vary depending on the
K value. For this example, K=1 was chosen to control sub-
harmonic oscillation and achieve one-cycle damping. The
maximum output current capability (I
20~50% higher than the required output current, to account
for tolerances and ripple current. For this example, 130% of
9A was chosen. The current sense resistor value can be cal-
culated from the equation (9), (10) as follows:
A value of 7.41mΩ was realized for R
tional 0.1Ω sense resistor in parallel with 8mΩ. The sense
resistor must be rated to handle the power dissipation at max-
imum input voltage when current flows through the low-side
NMOS for the majority of the PWM cycle. The maximum pow-
er dissipation of R
The worst case peak inductor current under the output short
condition can be calculated from the equation (12) as follows:
Where t
CURRENT SENSE FILTER R
The LM5117 itself is not affected by the large leading edge
spike because it samples valley current just prior to the onset
of the high-side switch. A current sense filter is used to mini-
mize a noise injection from any external noise sources. In
general, a current sense filter is not necessary. In this exam-
ple, a current sense filter is not used
RAMP RESISTOR R
The positive slope of the inductor current ramp signal is em-
ulated by R
C
the inductor, sense resistor and the K factor selected, the val-
ue of R
follows:
RAMP
was set at the standard capacitor value of 820pF. With
ON(MIN)
RAMP
RAMP
can be calculated from the equation (4) as
is normally 100ns.
and C
S
can be calculated as:
RAMP
RAMP
and RAMP CAPACITOR C
. For this example, the value of
CS
and C
S
S
CS
OUT(MAX)
by placing an addi-
) should be
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RAMP