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LM3489 Datasheet(PDF) 9 Page - Texas Instruments

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No. de pieza LM3489
Descripción Electrónicos  Hysteretic PFET Buck Controller with Enable Pin
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Fabricante Electrónico  TI1 [Texas Instruments]
Página de inicio  http://www.ti.com
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LM3489
www.ti.com
SNVS443A – MAY 2006 – REVISED JUNE 2008
Functional Description
OVERVIEW
The LM3489 is a buck (step-down) DC-DC controller that uses a hysteretic control scheme. The control
comparator is designed with approximately 10mV of hysteresis. In response to the voltage at the FB pin, the gate
drive (PGATE pin) turns the external PFET on or off. When the inductor current is too high, the current limit
protection circuit engages and turns the PFET off for approximately 9µs.
Hysteretic control does not require an internal oscillator. Switching frequency depends on the external
components and operating conditions. The operating frequency reduces at light loads resulting in excellent
efficiency compared to other architectures.
The output voltage can be programmed by two external resistors. The output can be set in a wide range from
1.239V (typical) to VIN.
HYSTERETIC CONTROL CIRCUIT
When the FB input to the control comparator falls below the reference voltage (1.239V), the output of the
comparator switches to a low state. This results in the driver output, PGATE, pulling the gate of the PFET low
and turning on the PFET. With the PFET on, the input supply charges COUT and supplies current to the load via
the series path through the PFET and the inductor. Current through the Inductor ramps up linearly and the output
voltage increases. As the FB voltage reaches the upper threshold, which is the internal reference voltage plus
10mV, the output of the comparator changes from low to high, and the PGATE responds by turning the PFET off.
As the PFET turns off, the inductor voltage reverses, the catch diode turns on, and the current through the
inductor ramps down. Then, as the output voltage reaches the internal reference voltage again, the next cycle
starts.
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 (VOUT) can be programmed by 2 external resistors. It can be calculated as follows:
VOUT = 1.239 x (R1 + R2) / R2
(1)
Figure 2. Hysteretic Window
The minimum output voltage ripple (VOUT_PP) can be calculated in the same way.
VOUT_PP = VHYST (R1 + R2) / R2
(2)
For example, with VOUT set to 3.3V, VOUT_PP is 26.6mV
VOUT_PP = 0.01 x (33K + 20k) / 20k = 0.0266V
(3)
Copyright © 2006–2008, Texas Instruments Incorporated
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