LM3489_15 Datasheet, PDF (14/25 Pages) Texas Instruments – Hysteretic PFET Buck Controller With Enable Pin

LM3489
LM3489-Q1
SNVS443B – MAY 2006 – REVISED FEBRUARY 2013
V_IN
R4
V_EN
R3
1
ISENSE
7
PGATE
5
8
3
ADJ
VIN
LM3489
FB
GND
4
2
EN
PGND
6
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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:
V_IN - V_DS - V_{OUT D}
L=
x
'i
f
(9)
And
I_pk =
'i
I_OUT + ₂
x 1.1
(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:
I_pk =
'i
I_OUT + ₂
x 1.1
(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.
14
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