LP3907 Datasheet, PDF (18/42 Pages) National Semiconductor (TI) – Dual High-Current Step-Down DC/DC and Dual Linear Regulator with I2C Compatible Interface

tions. When the output drops below the ‘low’ PFM threshold,
the cycle repeats to restore the output voltage to ~1.6% above
the nominal PWM output voltage.
If the load current should increase during PFM mode (see
figure below) causing the output voltage to fall below the
‘low2’ PFM threshold, the part will automatically transition into
fixed-frequency PWM mode.
SW1, SW2 OPERATION
SW1 and SW2 have selectable output voltages ranging from
0.8V to 3.5V (typ.). Both SW1 and SW2 in the LP3907 are
I²C register controlled and are enabled by default through the
internal state machine of the LP3907 following a Power-On
event that moves the operating mode to the Active state. (see
Power On Sequence). The SW1 and SW2 output voltages
revert to default values when the power on sequence has
been completed. The default output voltage for each buck
converter is factory programmable. (See Application Notes).
SW1, SW2 CONTROL REGISTERS
SW1, SW2 can be enabled/disabled through the correspond-
ing control register.
The Modulation mode PWM/PFM is by default automatic and
depends on the load as described above in the functional de-
scription. The modulation mode can be overridden by setting
I²C bit to a logic 1 in the corresponding buck control register,
forcing the buck to operate in PWM mode regardless of the
load condition.
SHUTDOWN MODE
During shutdown the PFET switch, reference, control and
bias circuitry of the converters are turned off. The NFET
switch will be on in shutdown to discharge the output. When
the converter is enabled, soft start is activated. It is recom-
mended to disable the converter during the system power up
and under voltage conditions when the supply is less than
2.8V.
SOFT START
The soft-start feature allows the power converter to gradually
reach the initial steady state operating point, thus reducing
start-up stresses and surges. The two LP3907 buck convert-
ers have a soft-start circuit that limits in-rush current during
start-up. During start-up the switch current limit is increased
in steps. Soft start is activated only if EN goes from logic low
to logic high after V_IN reaches 2.8V. Soft start is implemented
by increasing switch current limit in steps of 180mA, 300mA,
and 720mA for Buck1; 161mA, 300mA and 536mA for Buck2
(typ. Switch current limit). The start-up time thereby depends
on the output capacitor and load current demanded at start-
up.
LOW DROPOUT OPERATION
The LP3907 can operate at 100% duty cycle (no switching;
PMOS switch completely on) for low drop out support of the
output voltage. In this way the output voltage will be controlled
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down to the lowest possible input voltage. When the device
operates near 100% duty cycle, output voltage ripple is ap-
proximately 25mV. The minimum input voltage needed to
support the output voltage is
V_{IN, MIN} = I_LOAD * (R_{DSON, PFET} + R_INDUCTOR) + V_OUT
— ^ILOAD
Load current
— ^RDSON, PFET
Drain to source resistance of
PFET switch in the triode region
— ^RINDUCTOR
Inductor resistance
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