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LP3907_14 Datasheet, PDF (22/57 Pages) Texas Instruments – Dual High-Current Step-Down DC/DC and Dual Linear Regulator with I2C-Compatible Interface
LP3907
SNVS511N – JUNE 2007 – REVISED AUGUST 2011
www.ti.com
There are three modes of operation depending on the current required - PWM, PFM, and shutdown. PWM mode
handles current loads of approximately 70mA or higher, delivering voltage precision of ±3% with 90% efficiency
or better. Lighter output current loads cause the device to automatically switch into PFM for reduced current
consumption (IQ = 15µA typ.) and a longer battery life. The Standby operating mode turns off the device, offering
the lowest current consumption. PWM or PFM mode is selected automatically or PWM mode can be forced
through the setting of the buck control register.
Both SW1 and SW2 can operate up to a 100% duty cycle (PMOS switch always on) for low drop out control of
the output voltage. In this way the output voltage will be controlled down to the lowest possible input voltage.
Additional features include soft-start, under-voltage lock-out, current overload protection, and thermal overload
protection.
CIRCUIT OPERATION DESCRIPTION
A buck converter contains a control block, a switching PFET connected between input and output, a synchronous
rectifying NFET connected between the output and ground (BCKGND pin) and a feedback path. During the first
portion of each switching cycle, the control block 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
(1)
by storing energy in a magnetic field. During the second portion of each cycle, the control block 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
(2)
The output filter stores charge when the inductor current is high, and releases it when low, smoothing the voltage
across the load.
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 voltage inversely proportional to the input
voltage is introduced.
INTERNAL SYNCHRONOUS RECTIFICATION
While in PWM mode, the buck uses an internal NFET as a synchronous rectifier to reduce rectifier forward
voltage drop and associated power loss. Synchronous rectification provides a significant improvement in
efficiency whenever the output voltage is relatively low compared to the voltage drop across an ordinary rectifier
diode.
CURRENT LIMITING
A current limit feature allows the converter to protect itself and external components during overload conditions.
PWM mode implements current limiting using an internal comparator that trips at 1.5A for Buck1 and at 1.0A for
Buck2 (typ). If the output is shorted to ground the device enters a timed current limit mode where the NFET is
turned on for a longer duration until the inductor current falls below a low threshold, ensuring inductor current has
more time to decay, thereby preventing runaway.
PFM OPERATION
At very light loads, the converter enters PFM mode and operates with reduced switching frequency and supply
current to maintain high efficiency.
The part will automatically transition into PFM mode when either of two conditions occurs for a duration of 32 or
more clock cycles:
A. The inductor current becomes discontinuous
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