11
FN6828.3
December 9, 2015
Overcurrent Protection
The overcurrent protection is provided on ISL9103, ISL9103A
when overload condition happens. It is realized by monitoring
the CSA output with the OCP comparator, as shown in
Figure 22. When the current at P-Channel MOSFET is sensed
to reach the current limit, the OCP comparator is triggered to
turn off the P-Channel MOSFET immediately.
Short-Circuit Protection
ISL9103, ISL9103A has a Short-Circuit Protection (SCP)
comparator, which monitors the FB pin voltage for output
short-circuit protection. When the output voltage is sensed to
be lower than a certain threshold, the SCP comparator
reduces the PWM oscillator frequency to a much lower
frequency to protect the IC from being damaged.
Undervoltage Lockout (UVLO)
When the input voltage is below the Undervoltage Lock Out
(UVLO) threshold, ISL9103, ISL9103A is disabled.
Soft-Start
The soft-start feature eliminates the inrush current during the
circuit start-up. The soft-start block outputs a ramp reference
to both the voltage loop and the current loop. The two ramps
limit the inductor current rising speed as well as the output
voltage speed so that the output voltage rises in a controlled
fashion.
Low Dropout Operation
The ISL9103, ISL9103A features low dropout operation to
maximize the battery life. When the input voltage drops to a
level that ISL9103, ISL9103A can no longer operate under
switching regulation to maintain the output voltage, the
P-Channel MOSFET is completely turned on (100% duty
cycle). The dropout voltage under such condition is the
product of the load current and the ON-resistance of the
P-Channel MOSFET. Minimum required input voltage V
IN
under this condition is the sum of output voltage plus the
voltage drop cross the inductor and the P-Channel MOSFET
switch.
Thermal Shut Down
The ISL9103, ISL9103A provides built-in thermal protection
function. The thermal shutdown threshold temperature is
+130°C (typ) with a 30°C (typ) hysteresis. When the internal
temperature is sensed to reach +130°C, the regulator is
completely shut down and as the temperature drops to
+100°C (typ), the ISL9103, ISL9103A resumes operation
starting from the soft-start.
Applications Information
Inductor and Output Capacitor Selection
To achieve better steady state and transient response,
ISL9103, ISL9103A typically uses a 2.2µH inductor. The
peak-to-peak inductor current ripple can be expressed in
Equation 1:
In Equation 1, usually the typical values can be used but to
have a more conservative estimation, the inductance should
consider the value with worst case tolerance; and for
switching frequency f
S
, the minimum f
S
from the “Electrical
Specifications” table on page 3 can be used.
To select the inductor, its saturation current rating should be
at least higher than the sum of the maximum output current
and half of the delta calculated from Equation 1. Another
more conservative approach is to select the inductor with the
current rating higher than the P-Channel MOSFET peak
current limit.
Another consideration is the inductor DC resistance since it
directly affects the efficiency of the converter. Ideally, the
inductor with the lower DC resistance should be considered
to achieve higher efficiency.
Inductor specifications could be different from different
manufacturers so please check with each manufacturer if
additional information is needed.
For the output capacitor, a ceramic capacitor can be used
because of the low ESR values, which helps to minimize the
output voltage ripple. A typical value of 10µF ceramic
capacitor should be enough for most of the applications and
the capacitor should be X5R or X7R.
Input Capacitor Selection
The main function for the input capacitor is to provide
decoupling of the parasitic inductance and to provide filtering
function to prevent the switching current from flowing back to
the battery rail. A 10µF ceramic capacitor (X5R or X7R) is a
good starting point for the input capacitor selection.
Output Voltage Setting Resistor Selection
For ISL9103, ISL9103A adjustable output option, the voltage
resistors, R
1
and R
2
, as shown in Figure 21, set the desired
output voltage values. The output voltage can be calculated
using Equation 2:
where V
FB
is the feedback voltage (typically it is 0.8V). The
current flowing through the voltage divider resistors can be
calculated as V
O
/(R
1
+ R
2
), so larger resistance is desirable
to minimize this current. On the other hand, the FB pin has
leakage current that will cause error in the output voltage
setting. The leakage current has a typical value of 0.1µA. To
minimize the accuracy impact on the output voltage, select
the R
2
no larger than 200k.
I
V
O
1
V
O
V
IN
---------
–
Lf
S
---------------------------------------
=
(EQ. 1)
V
O
V
FB
1
R
1
R
2
-------
+
=
(EQ. 2)
ISL9103, ISL9103A
