LP3907 datasheet(24/57 Pages) TI1

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High PFM Threshold

~1.016 * Vout

Low1 PFM Threshold

~1.008 * Vout

PFM Mode at Light Load

PWM Mode at

Moderate to Heavy

Loads

Pfet on

until

Ipfm limit

reached

Nfet on

drains

inductor

current

until

I inductor = 0

High PFM

Voltage

Threshold

reached,

go into

sleep mode

Low PFM

Threshold,

turn on

PFET

Current load

increases,

draws Vout

towards

Low2 PFM

Threshold

Low2 PFM Threshold,

switch back to PWMmode

Load current

increases

Low2 PFM Threshold

Vout

LP3907

SNVS511N – JUNE 2007 – REVISED AUGUST 2011

www.ti.com

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 recommended 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 converters 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 VIN 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 down to the lowest possible input voltage.

When the device operates near 100% duty cycle, output voltage ripple is approximately 25mV. The minimum

input voltage needed to support the output voltage is

VIN, MIN = ILOAD * (RDSON, PFET + RINDUCTOR) + VOUT

Load current

•

ILOAD

Drain to source resistance of

•

RDSON, PFET

PFET switch in the triode region

Inductor resistance

•

RINDUCTOR

FLEXIBLE POWER SEQUENCING OF MULTIPLE POWER SUPPLIES

The LP3907 provides several options for power on sequencing. The two bucks can be individually controlled with

ENSW1 and ENSW2. The two LDOs can also be individually controlled with ENLDO1 and ENLDO2.

If the user desires a set power on sequence, he can program the chip through I2C and raise EN_T from LOW to

HIGH to activate the power on sequencing.

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LP3907 Datasheet(PDF) 24 Page - Texas Instruments

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