LP3907QTLX-VXSS/NOPB National Semiconductor, LP3907QTLX-VXSS/NOPB Datasheet - Page 21

• Manufacturer Part Number: LP3907QTLX-VXSS/NOPB
• Description: IC REG BUCK SYNC-2 LDO-2 25MSMD
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LP3907QTLX-VXSS/NOPB

• Topology: Step-Down (Buck) Synchronous (2), Linear (LDO) (2)
• Function: Any Function
• Number Of Outputs: 4
• Frequency - Switching: 2.1MHz
• Voltage/current - Output 1: 1.8V, 1A
• Voltage/current - Output 2: 3.3V, 600mA
• Voltage/current - Output 3: 2.8V, 300mA
• W/led Driver: No
• W/supervisor: No
• W/sequencer: Yes
• Voltage - Supply: 2.8 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 25-UFBGA
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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
I
internal state machine of the LP3907 following a Power-On
event that moves the operating mode to the Active state. (see
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
startup stresses and surges. The two LP3907 buck convert-
ers have a soft-start circuit that limits in-rush current during
startup. During startup the switch current limit is increased in
steps. Soft start is activated only if EN goes from logic low to
logic high after V
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.
2
C register controlled and are enabled by default through the
IN
reaches 2.8V. Soft start is implemented by
21
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
I
forcing the buck to operate in PWM mode regardless of the
load condition.
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
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
— I
— R
— R
2
C bit to a logic 1 in the corresponding buck control register,
IN, MIN
LOAD
DSON, PFET
INDUCTOR
= I
LOAD
* (R
Load current
Drain to source resistance of
Inductor resistance
DSON, PFET
PFET switch in the triode region
+ R
INDUCTOR
) + V
30017814
OUT
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