lp8765rlx National Semiconductor Corporation, lp8765rlx Datasheet - Page 16

• Manufacturer Part Number: lp8765rlx
• Description: High Performance Power Management Unit For Handset Applications
• Manufacturer: National Semiconductor Corporation

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INTERNAL SYNCHRONOUS RECTIFICATION
While in PWM mode, the converter 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 circuit to protect itself and
external components during overload conditions PWM mode
implements cycle-by-cycle current limiting using an internal
comparator that trips at 1000 mA (typ.) for Buck1 and Buck2.
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.
This ensures the inductor current has more time to decay,
thereby preventing runaway.
Forced PWM BUCKx
0
1
Auto mode PFM/PWM
Forced PWM mode
PFM/ PWM Mode
16
SOFT-START
The LP8765 buck regulators have a soft start circuit which
limits in-rush current during startup.
LOW-DROPOUT OPERATION
The buck converter can operate at 100% duty cycle (no
switching, PFET switch completely on) for low-dropout sup-
port of the output voltage. In this way the output voltage will
be controlled down to the lowest possible input voltage. The
minimum input voltage needed to support the output voltage
is:
•
•
•
•
V
I
R
region
R
LOAD
IN,MIN
DSON(P)
INDUCTOR
: I
: =I
LOAD
: PFET drain-source resistance in the triode
LOAD
: Inductor resistance
current
* (R
DSON(P)
+ R
INDUCTOR
) + V
30162007
OUT