Datasheet

I
pk
=
I
OUT
+
'i
2
x 1.1
I
pk
=
I
OUT
+
'i
2
x 1.1
V
IN
- V
DS
- V
OUT
L =
'i
x
D
f
ISENSE
ADJ
VIN
PGATE
FB
EN
LM3489
GND
PGND
1
2
3
4
5
6
7
8
V
IN
V
EN
R3
R4
LM3489
LM3489-Q1
SNVS443B –MAY 2006–REVISED FEBRUARY 2013 www.ti.com
Figure 29. Adjustable UVLO
DESIGN INFORMATION
Hysteretic control is a simple control scheme. However the operating frequency and other performance
characteristics highly depend on external conditions and components. If either the inductance, output
capacitance, ESR, V
IN
, or Cff is changed, there will be a change in the operating frequency and output ripple.
The best approach is to determine what operating frequency is desirable in the application and then begin with
the selection of the inductor and C
OUT
ESR.
INDUCTOR SELECTION (L)
The important parameters for the inductor are the inductance and the current rating. The LM3489 operates over
a wide frequency range and can use a wide range of inductance values. A rule of thumb is to use the equations
used for Simple Switchers
®
. The equation for inductor ripple (Δi) as a function of output current (I
OUT
) is:
for I
out
< 2.0Amps
Δi ≤ I
out
x I
out
−0.366726
for I
out
> 2.0Amps
Δi ≤ I
out
x 0.3
The inductance can be calculated based upon the desired operating frequency where:
(9)
And
(10)
where D is the duty cycle, V
D
is the diode forward voltage, and V
DS
is the voltage drop across the PFET.
The inductor should be rated to the following:
(11)
The inductance value and the resulting ripple is one of the key parameters controlling operating frequency. The
second is the inductor ESR that contribute to the steady state power loss due to current flowing through the
inductor.
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