14 FN6309.1
December 27, 2007
PG
During power-up, the open-drain power good output holds low
for about 1ms after VOUT reaches the regulation voltage. The
PG output also serves as a 1ms delayed the Power Good
signal when the pull-up resistor R1 is installed.
UVLO
When the input voltage is below the undervoltage lock-out
(UVLO) threshold, the regulator is disabled.
Soft Start-Up
The soft-start-up reduces the inrush current during the start-
up. The soft-start block outputs a ramp reference to the input
of the error amplifier. This voltage ramp limits the inductor
current as well as the output voltage speed so that the output
voltage rises in a controlled fashion. When VFB is less than
0.2V at the beginning of the soft-start, the switching
frequency is reduced to 1/3 of the nominal value so that the
output can start up smoothly at light load condition. During
soft-st art, the IC operates in the SKIP mode to support
pre-biased output condition .
Enable
The enable (EN) input allows the user to control the turning
on or off the regulator for purposes such as power-up
sequencing. When the regulator is enabled, there is typically
a 600µs delay for waking up the bandgap reference and then
the soft-start-up begins.
Discharge Mode (Soft-Stop)
When a transition to shutdown mode occurs or the VIN
UVLO is set, the outputs discharge to GND through an
internal 100Ω switch.
Power MOSFETs
The power MOSFETs are optimized for best efficiency. The
ON-resistance for the P-MOSFET is typically 50mΩ and the
ON-resistance for the N-MOSFET is typically 50mΩ.
100% Duty Cycle
The ISL8013 features 100% duty cycle operation to
maximize the battery life. When the battery voltage drops to
a level that the ISL8013 can no longer maintain the
regulation at the output, the regulator completely turns on
the P-MOSFET. The maximum dropout voltage under the
100% duty-cycle operation is the product of the load current
and the ON-resistance of the P-MOSFET.
Thermal Shut-Down
The ISL8013 has built-in thermal protection. When the internal
temperature reaches +140°C, the regulator is completely shut
down. As the temperature drops to +115°C, the ISL8013
resumes operation by stepping through the soft-start.
Applications Information
Output Inductor and Capacitor Selection
To consider steady state and transient operations, ISL8013
typically uses a 1.5µH output inductor. The higher or lower
inductor value can be used to optimize the total converter
system performance. For example, for higher output voltage
3.3V application, in order to decrease the inductor current
ripple and output voltage ripple, the output inductor value
can be increased. It is recommended to set the ri pple
inductor current approximately 30% of the maximum output
current for optimized performance. The inductor ripple
current can be expressed as shown in Equation 1:
The inductor’s saturation current rating needs to be at least
larger than the peak current. The ISL8013 protects the
typical peak current 4.8A. The saturation current needs be
over 5.5A for maximum output current application.
ISL8013 uses internal compensation network and the output
capacitor value is dependent on the output voltage. The
ceramic capacitor is recommended to be X5R or X7R. The
recommended X5R or X7R minimum output capacitor
values are shown in Table 1.
In Table 1, the minimum output capacitor value is given for
the different output voltage to make sure that the whole
converter system is stable.
TABLE 1. OUTPUT CAPACITOR VALUE vs VOUT
VOUT COUT L
0.8V 2 x 22µF 1.0µH~2.2µH
1.2V 2 x 22µF 1.0µH~2.2µH
1.5V 2 x 22µF 1.5µH~3.3µH
1.8V 2 x 22µF 1.5µH~3.3µH
2.5V 2 x 22µF 1.5µH~3.3µH
3.3V 2 x 22µF 2.2µH~4.7µH
3.6V 2 x 22µF 2.2µH~4.7µH
ΔI
VO1VO
VIN
---------
–
⎝⎠
⎜⎟
⎛⎞
•
Lf
S
•
---------------------------------------
=
(EQ. 1)
ISL8013