SP6120B Datasheet, PDF (11/22 Pages) Sipex Corporation – Low Voltage, AnyFETTM, Synchronous ,Buck Controller Ideal for 2A to 10A, High Performance, DC-DC Power Converters

Over Current Protection: continued
providing continuous current sensing and flex-
ible MOSFET selection. An internal, 10µs filter
conditions the over current signal from tran-
sients generated during load steps. In addition,
because the over current inputs, ISP and ISN, are
capable of rail to rail operation, the SP6120B
provides excellent over current protection dur-
ing conditions where V_IN approaches V_OUT.
Zero Crossing Detection
In some applications, it may be undesirable to
have negative conduction current in the induc-
tor. This situation happens when the ripple cur-
rent in the inductor is higher than the load cur-
rent. Therefore, the SP6120B provides an option
for “discontinuous” operation. If the Program
Logic (see next section) is set for discontinuous
mode, then the Driver Logic looks at the Zero
Crossing Comparator 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 operation
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 sequencing in some applications the DC-
DC converter 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 regu-
lator and thus continuous conduction is neces-
sary to produce this second output voltage.
Continuous
Load
Current
0A
GH, GL
Voltage
0V
0V
Discontinuous
Load
Current
0A
GH, GL
Voltage
0V
0V
TIME
Program Logic
The Program pin (PROG) of the SP6120B adds
a new level of flexibility to the design of DC/DC
converters. A 10µA current flows either into or
out of the Program pin depending on the initial
potential presented to the pin. If no resistor is
present, the Program Logic simply looks at the
potential on the pin, sets the mode to “continu-
ous” and programs NFET or PFET high side
drive accordingly. If the 68kΩ resistor is present,
the voltage drop across the resistor signals the
SP6120B to put the Driver Logic in “discontinu-
ous” mode. With one pin and a 68kΩ resistor, the
SP6120B can be configured for a variety of
operating modes:
Date: 5/25/04
SP6120B Low Voltage, AnyFET^TM, Synchronous, Buck Controller
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© Copyright 2004 Sipex Corporation