LM3489_15 Datasheet, PDF (10/25 Pages) Texas Instruments – Hysteretic PFET Buck Controller With Enable Pin

LM3489
LM3489-Q1
SNVS443B – MAY 2006 – REVISED FEBRUARY 2013
www.ti.com
The LM3489 operates in discontinuous conduction mode at light load current or continuous conduction mode at
heavy load current. In discontinuous conduction mode, current through the inductor starts at zero and ramps up
to the peak, then ramps down to zero. The next cycle starts when the FB voltage reaches the reference voltage.
Until then, the inductor current remains zero and the output capacitor supplies the load. The operating frequency
is lower and switching losses reduced. In continuous conduction mode, current always flows through the inductor
and never ramps down to zero.
The output voltage (V_OUT) can be programmed by 2 external resistors. It can be calculated as follows:
V_OUT = 1.239 x (R1 + R2) / R2
(1)
Figure 24. Hysteretic Window
The minimum output voltage ripple (V_{OUT_PP}) can be calculated in the same way.
V_{OUT_PP} = V_HYST (R1 + R2) / R2
(2)
For example, with V_OUT set to 3.3V, V_{OUT_PP} is 26.6mV
V_{OUT_PP} = 0.01 x (33K + 20k) / 20k = 0.0266V
(3)
Operating frequency (F) is determined by knowing the input voltage, output voltage, inductor, V_HYST, ESR
(Equivalent Series Resistance) of output capacitor, and the delay. It can be approximately calculated using the
formula:
(4)
where:
α: (R1 + R2) / R2
delay: It includes the LM3489 propagation delay time and the PFET delay time. The propagation delay is 90ns
typically (see Figure 25).
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