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THEORY OF OPERATIONS: Continued
Zero Crossing Detection
In some applications, it may be undesirable to have
negative conduction current in the inductor. This
situation happens when the ripple current in the
inductor is higher than the load current. Therefore,
the SP6120 provides an option for “discontinu-
ous” operation. If the Program Logic (see next
section) is set for discontinuous mode, then the
Driver Logic looks at the Zero Crossing Compara-
tor and the state of the lower gate driver. If the low
side MOSFET was “on” and V(ISP)-V(ISN) < 0
then the low side MOSFET is immediately turned
off and held off until the high side MOSFET is
turned “on”. When the high side MOSFET turns
“on” , the Driver Logic is reset. The following
figures show continuous and discontinuous opera-
tion for a converter with an NFET high side
MOSFET.
Discontinuous vs. Continuous Mode
The discontinuous mode is used when better light
load efficiency is desired, for example in portable
applications. Additionally, for power supply se-
quencing in some applications the DC-DC con-
verter output is pre-charged to a voltage through a
switch at start-up, and discontinuous operation
would be required to prevent reverse inductor
current from discharging the pre-charge voltage.
The continuous mode is preferable for lower noise
and EMI applications since the discontinuous mode
can cause ringing of the switch node voltage when
it turns both switches off. Another example where
continuous mode could be required is one where
the inductor has an extra winding used for an over-
winding regulator and thus continuous conduction
is necessary to produce this second output voltage.
Program Logic
The Program pin (PROG) of the SP6120 adds a
new level of flexibility to the design of DC/DC
Continuous
Load
Current
0A
GH, GL
Voltage
0V
0V
Discontinuous
Load
Current
0A
GH, GL
Voltage
0V
0V
TIME
Program Logic: continued
the voltage drop across the resistor signals the
SP6120 to put the Driver Logic in “discontinuous”
mode. With one pin and a 68k ? resistor, the
SP6120 can be configured for a variety of operat-
ing modes:
PROGRAM LOG IC TRUTH T ABLE
converters. A 10 μ A current flows either into or out
P R O G R A M P I N
N F E T O R P F E T
M O D E
of the Program pin depending on the initial poten-
tial presented to the pin. If no resistor is present, the
Program Logic simply looks at the potential on the
pin, sets the mode to “continuous” and programs
NFET or PFET high side drive accordingly. If the
68k ? resistor is present,
S h o r t t o G N D
6 8 k ? t o G N D
S h o r t t o V C C
6 8 k ? t o V C C
N F E T
N F E T
P F E T
P F E T
C o n t i n u o u s
D i s c o n t i n u o u s
C o n t i n u o u s
D i s c o n t i n o u s
Date: 1/21/05
SP6120 Low Voltage, AnyFET TM , Synchronous, Buck Controller
11
? Copyright 2005 Sipex Corporation
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