LM8801
SNVS597H –APRIL 2009–REVISED MAY 2013
www.ti.com
OPERATION DESCRIPTION
DEVICE INFORMATION
The LM8801, a high-efficiency, step-down DC-DC switching buck converter, delivers a constant voltage from
either a single Li-Ion or three cell NiMH/NiCd battery to portable devices such as cell phones and PDAs. Using a
voltage mode architecture with synchronous rectification, the LM8801 has the ability to deliver up to 600 mA
depending on the input voltage and output voltage, ambient temperature, and the inductor chosen.
There are three modes of operation depending on the current required - PWM (Pulse Width Modulation), PFM
(Pulse Frequency Modulation), and shutdown. The device operates in PWM mode at load currents of
approximately 80 mA or higher, having voltage precision of ±1.5% with 90% efficiency or better. Lighter output
current loads cause the device to automatically switch into PFM for reduced current consumption (IQ = 27 µA
typ.) and a longer battery life. Shutdown mode turns off the device, offering the lowest current consumption
(ISHUTDOWN = 0.08 µA typ.).
Additional features include soft-start, under voltage protection, current overload protection, and thermal shutdown
protection. As shown in the Typical Application Circuit, only three external power components are required for
implementation.
CIRCUIT OPERATION
The LM8801 operates as follows. During the first portion of each switching cycle, the control block in the LM8801
turns on the internal PFET switch. This allows current to flow from the input through the inductor to the output
filter capacitor and load. The inductor limits the current to a ramp with a slope of (VIN − VOUT)/L, by storing energy
in a magnetic field. During the second portion of each cycle, the controller turns the PFET switch off, blocking
current flow from the input, and then turns the NFET synchronous rectifier on. The inductor draws current from
ground through the NFET to the output filter capacitor and load, which ramps the inductor current down with a
slope of –VOUT/L.
The output filter capacitor stores charge when the inductor current is high, and releases it when low, smoothing
the voltage across the load.
The output voltage is regulated by modulating the PFET switch on time to control the average current sent to the
load. The effect is identical to sending a duty-cycle modulated rectangular wave formed by the switch and
synchronous rectifier at the SW pin to a low-pass filter formed by the inductor and output filter capacitor. The
output voltage is equal to the average voltage at the SW pin.
PWM OPERATION
During PWM operation, the converter operates as a voltage-mode controller with input voltage feed forward. This
allows the converter to achieve excellent load and line regulation. The DC gain of the power stage is proportional
to the input voltage. To eliminate this dependence, feed forward inversely proportional to the input voltage is
introduced.
While in PWM mode, the output voltage is regulated by switching at a constant frequency and then modulating
the energy per cycle to control power to the load. At the beginning of each clock cycle the PFET switch is turned
on and the inductor current ramps up until the comparator trips and the control logic turns off the switch. The
current limit comparator can also turn off the switch in case the current limit of the PFET is exceeded. Then the
NFET switch is turned on and the inductor current ramps down. The next cycle is initiated by the clock turning off
the NFET and turning on the PFET.
AUTO MODE OPERATION
Setting Mode pin low places the LM8801 in Auto mode. By doing so the device will automatically switch between
PFM state and PWM (Pulse Width Modulation) state based on load demand. At light loads (less than 50 mA), the
device enters PFM mode and operates with reduced switching cycle and supply current to maintain high
efficiency. During PFM operation, the converter positions the output voltage slightly higher (+15 mV typ.) than the
nominal output voltage during PWM operation, allowing additional headroom for voltage drop during a load
transient from light to heavy load.
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