LM3485_15 Datasheet, PDF (9/24 Pages) Texas Instruments – Hysteretic PFET Buck Controller

LM3485
www.ti.com
SNVS178G – JANUARY 2002 – REVISED FEBRUARY 2013
The LM3485 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. Next cycle starts when the FB voltage reaches the internal voltage. Until
then, the inductor current remains zero. Operating frequency is lower and switching losses reduce. 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 = 1.242* ( R1 + R2 ) / R2
(1)
Figure 2. 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* ( 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 LM3485 propagation delay time and the PFET delay time. The propagation delay is 90ns
typically (see Figure 3).
Copyright © 2002–2013, Texas Instruments Incorporated
Product Folder Links: LM3485
Submit Documentation Feedback
9