LM3489QMM National Semiconductor, LM3489QMM Datasheet - Page 8

• Manufacturer Part Number: LM3489QMM
• Description: LM3489/LM3489Q Hysteretic PFET Buck Controller with Enable Pin; Package: MINI SOIC; No of Pins: 8; Qty per Container: 1000; Container: Reel
• Manufacturer: National Semiconductor

www.national.com
Simplified Functional Block Diagram
Functional Description
OVERVIEW
The LM3489 is a buck (step-down) DC-DC controller that us-
es 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 oth-
er architectures.
The output voltage can be programmed by two external re-
sistors. The output can be set in a wide range from 1.239V
(typical) to V
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 C
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 volt-
age reaches the upper threshold, which is the internal refer-
ence voltage plus 10mV, the output of the comparator
changes from low to high, and the PGATE responds by turn-
ing 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
IN
.
OUT
and
8
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
resistors. It can be calculated as follows:
The minimum output voltage ripple (V
lated in the same way.
FIGURE 1. Hysteretic Window
V
OUT
= 1.239 x (R1 + R2) / R2
OUT
) can be programmed by 2 external
OUT_PP
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) can be calcu-
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