. . Regulator A includes a pin to disable the
'lllhronous rectifier driver (GUT2A) to prevent pulling
.."t out from the output, allowing the inductor current
lllfIcome discontinuous at light loads.
iktion of the external MOSFETs, output inductor and
QlllCitor determine the output capabilities of the
,.lator. Output voltage is set by RF1 and RF2 where
APPLICATIONS
BUCK REGULATOR INDUCTOR
Inductors are specified with three main parameters;
inductance
(l.),
maximum current (I0UTIMAX)), and
Dc
resistance. (R
l .)
Inductance for a given set of requirements can be
calculated with the following:
IJ,
A
ir
h
~
1
v.
OUT -
-
2.5x(RF1+RF2)
RF2
(1)
lIf5hort circuit current limit is set by external resistor Rs.
L
l
=
(v.
IN-
v.
OUT
)
V1NxFxAI
VOUT
'SHORT CKT
=
0.2
R
s
(2)
Assuming that the desired ripple current:
(fiG
discharged whin tha regulator is off or when the
~e
across Rs exceeds 200mV. F2 ensures that Css is
tiJrdischarged. This circuit provides reliable output
_ circuit protection with very little power wasted in the
_ng element. The error amplifier's output voltage is
!iaiIed
to the voltage on the SOFT START pin. When Css
i>tiK:harged, the regulator's duty cycle is
O.
lOQC
TO MOSFET GATE DRIVERS
1lIis
section provides a convenient translation stage for
~g
on low cost external N-Channel power MOSFETs
By choosing the ripple current to
be
40% of maximum
output current (IMAX), below 20% of 'MAX, the inductor
will actually pull current out of the output capacitor
during part of the on time of the low side MOSFET.
The
result of this current "shuttling" is a slight increase in
losses. Buck regulator A includes the ability to disable the
synchronous rectifier output to avoid current shuttling at
light loads and thereby operate using conventional
rectification.
To avoid inductor saturation, the maximum output current
of the regulator should not exceed 80% of the current
rating of the inductor, especially when using ferrites,
which have a "hard" saturation characteristic. Powdered
iron cores saturate more softly and may therefore
be
pushed closer to their rated currents.
DC resistance of the inductor sets up its conduction loss.
For the same size package DC resistance decreases as
inductance is decreased. It is a good rule of thumb to
select the DC resistance of the inductor to be 1.4 of the
sum of the on resistance of the two output MOSFETs. This
setsup conduction losses evenly among the power
components.
_power management. SOUT sources current when SW
ilhigh
and sinks current when SW is low. These outputs
. .limited to small currents to switch these MOSFETs
liewIy,
reducing the transients to the main regulator
OIIIput from switching discharged capacities. Additional
slow
down of the switching may be achieved by adding
GJPaCitance form the SOUT pins to GND.
MTJERY DETECTION AND ADAPTER SWITCH
These
two functions are provided by comparators. The
V"DAPTER comparator goes high when the VBAT pin is
lligher than VADAPTER, so that the system can run from the
battery
without the loss associated with a diode. When
!he
AC adapter is plugged in, the voltage on pin 2 goes
fNgh, VG4 switches low, and the system runs from the AC
~pter.
A low battery comparator with an open collector
ClUtput
is also provided to monitor battery level.
nv
UNEAR REGULATOR
The 12V regulator includes a shut-off pin. Since this
~ulator
takes its input from VCTcare should
be
taken to
ensurethat the regulator does not cause excessive heat in
the
IC when used with high values of VIN.
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
