LM3489_14 Datasheet, PDF (13/23 Pages) Texas Instruments – Hysteretic PFET Buck Controller with Enable Pin

LM3489
www.ti.com
SNVS443A – MAY 2006 – REVISED JUNE 2008
V_{IN(UVLO_HYST)} = V_{EN_HYST} x
1+
R4
R3
(8)
where V_EN is the enable rising threshold voltage and V_{EN_HYST} is the enable threshold hysteresis.
V_IN
R4
V_EN
R3
1
7
ISENSE
PGATE
5
8
3
ADJ
VIN
LM3489
FB
GND
4
2
EN
PGND
6
Figure 7. 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 National's 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 voltgae 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.
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