LM3489_15 Datasheet, PDF (9/25 Pages) Texas Instruments – Hysteretic PFET Buck Controller With Enable Pin

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LM3489
LM3489-Q1
SNVS443B – MAY 2006 – REVISED FEBRUARY 2013
SIMPLIFIED FUNCTIONAL BLOCK DIAGRAM
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 V_IN.
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_OUT 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.
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Product Folder Links: LM3489 LM3489-Q1
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