Datasheet

R3
(min)
=
25 mV x (R1 + R2)
R2 x I
OR (min)
I
OR (min)
=
V
OUT
x (V
IN (min)
- V
OUT
)
L1 x F
S
x V
IN (min)
L1 =
V
OUT
x (V
IN (max)
- V
OUT
)
I
OR (max)
x F
S
x V
IN (max)
R
ON
t
100 ns x (V
IN(MAX)
± 1.5V)
1.15 x 10
-10
- 1.4 k:
LM34914
SNVS453B –MAY 2006–REVISED MARCH 2013 www.ti.com
APPLICATIONS INFORMATION
EXTERNAL COMPONENTS
The following guidelines can be used to select the external components (see the Block Diagram). First determine
the following operating parameters:
- Output voltage (V
OUT
)
- Minimum and maximum input voltage (V
IN(min)
and V
IN(max)
)
- Minimum and maximum load current (I
OUT(min)
and I
OUT(max)
)
- Switching Frequency (F
S
)
R1 and R2: These resistors set the output voltage. The ratio of these resistors is calculated from:
R1/R2 = (V
OUT
/2.5V) - 1 (7)
R1 and R2 should be chosen from standard value resistors in the range of 1.0 kΩ - 10 kΩ which satisfy the
above ratio.
R
ON
: The resistor sets the on-time, and consequently, the switching frequency. Its value can be determined using
Equation 6 based on the frequency, or Equation 5 if a specific on-time is required. The minimum allowed value
for R
ON
is calculated from:
(8)
L1: The main parameter affected by the inductor is the output current ripple amplitude (I
OR
). The minimum load
current is used to determine the maximum allowable ripple. In order to maintain continuous conduction mode the
valley should not reach 0 mA. This is not a requirement of the LM34914, but serves as a guideline for selecting
L1. For this case, the maximum ripple current is:
I
OR(MAX)
= 2 x I
OUT(min)
(9)
If the minimum load current is zero, use 20% of I
OUT(max)
for I
OUT(min)
in Equation 9. The ripple calculated in
Equation 6 is then used in the following equation:
(10)
where Fs is the switching frequency. This provides a minimum value for L1. The next larger standard value
should be used, and L1 should be rated for the peak current level, equal to I
OUT(max)
+ I
OR(max)
/2.
C2 and R3: Since the LM34914 requires a minimum of 25 mVp-p of ripple at the FB pin for proper operation, the
required ripple at V
OUT
is increased by R1 and R2. This necessary ripple is created by the inductor ripple current
flowing through R3, and to a lesser extent by C2 and its ESR. The minimum inductor ripple current is calculated
using Equation 10, rearranged to solve for I
OR
at minimum V
IN
.
(11)
The minimum value for R3 is then equal to:
(12)
Typically R3 is less than 5Ω. C2 should generally be no smaller than 3.3 µF, although that is dependent on the
frequency and the desired output characteristics. C2 should be a low ESR good quality ceramic capacitor.
Experimentation is usually necessary to determine the minimum value for C2, as the nature of the load may
require a larger value. A load which creates significant transients requires a larger value for C2 than a non-
varying load.
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