Datasheet

R3
(min)
=
25 mV x (R1 + R2)
R2 x I
OR (min)
= 0.24:
(V
IN(min)
± V
OUT
) x t
on(max)
L1
= 140 mA
I
OR(min)
=
L1 =
(V
IN(max)
± V
OUT
) x t
ON(min
)
I
OR(max)
= 6.67 PH
R
ON
=
V
OUT
x (V
IN(min)
- 1.5V)
F
S
x 0.565 x 10
-10
x V
IN(min)
-1.4 k: = 27.8 k:
LM34919B
LM34919B-Q1
SNVS623A –MAY 2010–REVISED FEBRUARY 2013 www.ti.com
APPLICATIONS INFORMATION
EXTERNAL COMPONENTS
The procedure for calculating the external components is illustrated with the following design example. Referring
to the Block Diagram, the circuit is to be configured for the following specifications:
- V
OUT
= 3.3V
- V
IN
= 6V to 24V
- Minimum load current = 200 mA
- Maximum load current = 600 mA
- Switching Frequency = 1.5 MHz
- Soft-start time = 5 ms
R1 and R2: These resistors set the output voltage. The ratio of the feedback resistors is calculated from:
R1/R2 = (V
OUT
/2.5V) - 1 (8)
For this example, R1/R2 = 0.32. R1 and R2 should be chosen from standard value resistors in the range of 1.0
kΩ - 10 kΩ which satisfy the above ratio. For this example, 2.49kΩ is chosen for R2 and 787Ω for R1.
R
ON
: This resistor sets the on-time, and (by default) the switching frequency. The switching frequency must be
less than 1.53 MHz to ensure the minimum forced on-time does not interfere with the circuit's proper operation at
the maximum input voltage. The R
ON
resistor is calculated from the following equation, using the minimum input
voltage.
(9)
Check that this value resistor does not set an on-time less than 90 ns at maximum V
IN
.
A standard value 28 kΩ resistor is used, resulting in a nominal frequency of 1.49 MHz. The minimum on-time is
≊129 ns at Vin = 24V, and the maximum on-time is ≊424 ns at Vin = 6V. Alternately, R
ON
can be determined
using Equation 5 if a specific on-time is required.
L1: The main parameter affected by the inductor is the inductor current ripple amplitude (I
OR
). The minimum load
current is used to determine the maximum allowable ripple in order to maintain continuous conduction mode,
where the lower peak does not reach 0 mA. This is not a requirement of the LM34919B, but serves as a
guideline for selecting L1. For this case the maximum ripple current is:
I
OR(MAX)
= 2 x I
OUT(min)
= 400 mA (10)
If the minimum load current is zero, use 20% of I
OUT(max)
for I
OUT(min)
in Equation 10. The ripple calculated in
Equation 10 is then used in the following equation:
(11)
A standard value 8.2 µH inductor is selected. The maximum ripple amplitude, which occurs at maximum V
IN
,
calculates to 325 mA p-p, and the peak current is 763 mA at maximum load current. Ensure the selected inductor
is rated for this peak current.
C2 and R3: Since the LM34919B requires a minimum of 25 mVpp 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 11, rearranged to solve for I
OR
at minimum V
IN
.
(12)
The minimum value for R3 is equal to:
(13)
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