lm25117psqx National Semiconductor Corporation, lm25117psqx Datasheet - Page 23

• Manufacturer Part Number: lm25117psqx
• Description: Wide Input Range Synchronous Buck Controller With Analog Current Monitor
• Manufacturer: National Semiconductor Corporation

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 20% 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 6.8μH was chosen for L
the value of 6.8μH for L
necessary if the chosen value of L
the calculated value.
From the equation (11),
At the minimum input voltage, this value is 0.95A.
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
O
, calculate I
T
for 230 kHz switching fre-
O
differs significantly from
PP
again. This step is
T
V
.
f
SW
IN(MAX)
V
V
IN(MIN)
OUT
= 230kHz
I
OUT
O
. Using
= 3.3V
= 36V
= 9A
= 6V
Yes
No
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, 150% of
9A was chosen. The current sense resistor value can be cal-
culated from the equation (9), (10) as follows:
A value of 8mΩ was chosen for R
be placed in parallel with R
current capability. The sense resistor must be rated to handle
the power dissipation at maximum input voltage when current
flows through the low-side NMOS for the majority of the PWM
cycle. The maximum power dissipation of R
lated as:
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 LM25117 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
RAMP
RAMP
and RAMP CAPACITOR C
S
. For this example, the value of
to adjust the maximum output
CS
S
and C
. A larger value resistor can
S
CS
OUT(MAX)
S
can be calcu-
) should be
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RAMP