lm8801tmx National Semiconductor Corporation, lm8801tmx Datasheet - Page 9

• Manufacturer Part Number: lm8801tmx
• Description: High Precision 6mhz, 600 Ma Synchronous Step-down Dc-dc Converter For Mobile Applications
• Manufacturer: National Semiconductor Corporation

Operation Description
DEVICE INFORMATION
The LM8801, a high-efficiency, step-down DC-DC switching
buck converter, delivers a constant voltage from either a sin-
gle Li-Ion or three cell NiMH/NiCd battery to portable devices
such as cell phones and PDAs. Using a voltage mode archi-
tecture 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 Fre-
quency Modulation), and shutdown. The device operates in
PWM mode at load currents of approximately 80 mA or high-
er, having voltage precision of ±1.5% with 90% efficiency or
better. Lighter output current loads cause the device to auto-
matically switch into PFM for reduced current consumption
(I
turns off the device, offering the lowest current consumption
(I
Additional features include soft-start, under voltage protec-
tion, current overload protection, and thermal shutdown pro-
tection. 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
(V
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 ca-
pacitor and load, which ramps the inductor current down with
a slope of –V
The output filter capacitor stores charge when the inductor
current is high, and releases it when low, smoothing the volt-
age across the load.
Q
SHUTDOWN
IN
= 27 µA typ.) and a longer battery life. Shutdown mode
− V
OUT
= 0.08 µA typ.).
)/L, by storing energy in a magnetic field. During
OUT
/L.
FIGURE 3. Operation in PFM Mode and Transfer to PWM Mode
9
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 rect-
angular 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 av-
erage 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 in-
ductor 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 frequency
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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