LM5010 EVAL/NOPB National Semiconductor, LM5010 EVAL/NOPB Datasheet - Page 13

• Manufacturer Part Number: LM5010 EVAL/NOPB
• Manufacturer: National Semiconductor

Specifications of LM5010 EVAL/NOPB

• Lead Free Status / Rohs Status: Compliant

Thermal Shutdown
heating. When the junction temperature reduces below
155˚C (typical hysteresis = 20˚C), the Softstart pin is re-
leased and normal operation resumes.
Applications Information
EXTERNAL COMPONENTS
The procedure for calculating the external components is
illustrated with a design example. The circuit in Figure 1 is to
be configured for the following specifications:
R1 and R2:The ratio of these resistors is calculated from:
R1/R2 calculates to 3.0. The resistors should be chosen
from standard value resistors in the range of 1.0 kΩ - 10 kΩ.
Values of 3.0 kΩ for R1, and 1.0 kΩ for R2 will be used.
To keep the circuit in continuous conduction mode, the maxi-
mum allowed ripple current is twice the minimum load cur-
rent, or 300 mAp-p. Using this value of ripple current, the
inductor (L1) is calculated using the following:
where F
This provides a minimum value for L1 - the next higher
standard value (100 µH) will be used. L1 must be rated for
the peak current (I
rent occurs at maximum load current with maximum ripple.
The maximum ripple is calculated by re-arranging Equation
8 using V
based on the manufacturer’s tolerance. Assume, for this
exercise, the inductor’s tolerance is
• V
• V
• F
• Minimum load current = 150 mA
• Maximum load current = 1.0A
• Softstart time = 5 ms.
S
OUT
IN
= 625 kHz
= 15V to 75V
S(min)
= 10V
IN(max)
is the minimum frequency (F
, F
R1/R2 = (V
PK+
S(min)
) to prevent saturation. The peak cur-
, and the minimum inductor value,
OUT
/2.5V) - 1
(Continued)
±
20%.
S
- 25%).
FIGURE 13. Inductor Current
(7)
(8)
13
R
Equation 2 to set a nominal frequency, or from Equation 5 if
the on-time at a particular V
quency generally means a smaller inductor and capacitors
(value, size and cost), but higher switching losses. A lower
frequency means a higher efficiency, but with larger compo-
nents. If PC board space is tight, a higher frequency is better.
The resulting on-time and frequency have a
ance. Re-arranging Equation 2 ,
The next larger standard value (137 kΩ) is chosen for R
yielding a nominal frequency of 618 kHz.
L1: The inductor value is determined based on the load
current, ripple current, and the minimum and maximum input
voltage (V
R
current waveform does not exceed 1.0A at maximum load
current (see Figure 13), it is not necessary to increase the
current limit threshold. Therefore R
exercise. For applications where the lower peak exceeds
1.0A, see the section below on increasing the current limit
threshold.
C2 and R3: Since the LM5010 requires a minimum of 25
mVp-p of ripple at the FB pin for proper operation, the
required ripple at V
necessary ripple is created by the inductor ripple current
acting on C2’s ESR + R3. First, the minimum ripple current is
determined.
ON
CL
: Since it is obvious that the lower peak of the inductor
, F
S
: R
IN(min)
ON
I
PK+
sets the on-time, and can be chosen using
, V
= 1.0A + 0.234A / 2 = 1.117A
IN(max)
OUT1
). Refer to Figure 13 .
is increased by R1 and R2. This
20119922
IN
is important. A higher fre-
CL
is not needed for this
±
www.national.com
25% toler-
ON
(9)
,