ISL8023AIRTAJZ-T7A Intersil, ISL8023AIRTAJZ-T7A Datasheet - Page 16

• Manufacturer Part Number: ISL8023AIRTAJZ-T7A
• Description: ISL8023 Series 2.7 - 5.5 V 3 A 4 Mhz Buck Regulator - TQFN-16
• Manufacturer: Intersil

Soft Start-Up
The soft start-up reduces the in-rush 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 0.1V at the
beginning of the soft-start, the switching frequency is reduced to
200kHz so that the output can start-up smoothly at light load
condition. During soft-start, the IC operates in the SKIP mode to
support pre-biased output condition.
Tie SS to SGND for internal soft-start approximately 1ms.
Connect a capacitor from SS to SGND to adjust the soft-start
time. This capacitor, along with an internal 1.6µA current source,
sets the soft-start interval of the converter, T
Equation 2.
C
C
fault 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-FET is typically 45mΩ and the
ON-resistance for the N-FET is typically 19mΩ.
100% Duty Cycle
The ISL8023, ISL8024 features 100% duty cycle operation to
maximize the battery life. When the battery voltage drops to a
level that the ISL8023, ISL8024 can no longer maintain the
regulation at the output, the regulator completely turns on the
P-FET. The maximum dropout voltage under the 100% duty-cycle
operation is the product of the load current and the
ON-resistance of the P-FET.
Thermal Shut-Down
The ISL8023, ISL8024 has built-in thermal protection. When the
internal temperature reaches +150°C, the regulator is completely
shut down. As the temperature drops to +125°C, the ISL8023,
ISL8024 resumes operation by stepping through the soft-start.
ss
SS
must be less than 33nF to insure proper soft-start reset after
[
μF
]
=
3.33 T
⋅
SS
s [ ]
16
SS
as shown by
ISL8023, ISL8024
(EQ. 2)
Applications Information
Output Inductor and Capacitor Selection
To consider steady state and transient operations, ISL8023,
ISL8024 typically uses a 1.0µH 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 ripple inductor current
approximately 30% of the maximum output current for optimized
performance. The inductor ripple current can be expressed as
shown in Equation 3:
The inductor’s saturation current rating needs to be at least
larger than the peak current. The ISL8023, ISL8024 protects the
typical peak current 4.8A/6.5A. The saturation current needs be
over 7A for maximum output current application.
ISL8023, ISL8024 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. Additional output capacitance should be added
for better performances in applications where high load transient
or low output ripple is required. It is recommended to check the
system level performance along with the simulation model.
Output Voltage Selection
The output voltage of the regulator can be programmed via an
external resistor divider that is used to scale the output voltage
relative to the internal reference voltage and feed it back to the
inverting input of the error amplifier. Refer to Figure 3.
The output voltage programming resistor, R
value chosen for the feedback resistor and the desired output
voltage of the regulator. The value for the feedback resistor is
typically between 10kΩ and 100kΩ, as shown in Equation 4.
If the output voltage desired is 0.6V, then R
and R
It is recommended to preload the output with 10µA minimum.
For better performance, add 15pF in parallel with R
Check loop analysis before use in application.
Input Capacitor Selection
The main functions for the input capacitor are to provide
decoupling of the parasitic inductance and to provide filtering
function to prevent the switching current flowing back to the
battery rail. At least two 22µF X5R or X7R ceramic capacitors are
a good starting point for the input capacitor selection.
ΔI
R
2
=
=
V
-------------------------------------- -
2
R
O
is shorted. There is a leakage current from VIN to PHASE.
3
•
⎛
⎝
L f
----------- - 1
VFB
⎛
⎜
⎝
V
1
•
O
–
S
---------
V
V
–
IN
O
⎞
⎠
⎞
⎟
⎠
3
2
, will depend on the
is left unpopulated
2
(100kΩ).
May 17, 2012
FN7812.2
(EQ. 4)
(EQ. 3)