RT8032
Application Information
Output Voltage Programming
The output voltage is set by an external resistive divider
according to the following equation :
V
OUT
=
V
REF
× ⎛
1
+
R1
⎞
⎜
⎟
⎝
R2
⎠
where V
REF
equals to 0.8V typical.
The resistive divider allows the FB pin to sense a fraction
of the output voltage as shown in Figure 1.
V
OUT
R1
FB
RT8032
GND
R2
the main switch to remain on for more than one cycle and
eventually reaching 100% duty cycle. The output voltage
will then be determined by the input voltage minus the
voltage drop across the internal P-MOSFET and the
inductor.
Inductor Selection
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current
ΔI
L
increases with higher V
IN
and decreases
with higher inductance.
⎡
V
⎤⎡
V
⎤
Δ
I
L
= ⎢
OUT
⎥ ⎢
1
−
OUT
⎥
V
IN
⎦
⎣
f
×
L
⎦⎣
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite or mollypermalloy
cores. Actual core loss is independent of core size for a
fixed inductor value but it is very dependent on the
inductance selected. As the inductance increases, core
losses decrease. Unfortunately, increased inductance
requires more turns of wire and therefore copper losses
will increase.
Ferrite designs have very low core losses and are preferred
at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates
“hard”,
which means that
inductance collapses abruptly when the peak design
current is exceeded.
This result in an abrupt increase in inductor ripple current
and consequent output voltage ripple.
Do not allow the core to saturate!
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and don't radiate energy but generally cost more
than powdered iron core inductors with similar
characteristics. The choice of which style inductor to use
mainly depends on the price vs. size requirements and
any radiated field/EMI requirements.
DS8032-02 March 2011
Figure 1. Setting the Output Voltage
Input Average Current Limit Setting
The input current limit circuit is programmed by an external
resistor on ISET. This allows the user to program a
maximum average input current. For applications such
as USB that the current from the bus must be limited, the
value of R
LIM
and C
C1
can be calculated as following
equation.
R
LIM
= 0.8 / (70 x 10
-6
x I
IN
(A))
C
C1
= 16 x 10
-6
/ R
LIM
,
, R
C1
= 1kΩ
Soft-Start
The soft-start function is combined with shutdown. When
the SHDN/SS pin is brought above 1V (typ.), the IC will
be enabled. The components of R
SS
and C
SS
provide a
slow ramping voltage on the SHDN/SS pin to provide a
soft-start function.
Input Over Voltage Protection
The RT8032 equips input over voltage protection function.
When the input voltage exceeds 6V, the next switching
cycle of the IC will be terminated. Once the input voltage
is lower than 6V, the IC will enter normal operation again.
100% Duty Cycle Operation
When the input supply voltage decreases toward the output
voltage, the duty cycle increases toward the maximum
on-time. Further reduction of the supply voltage forces
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