ISL8022 Просмотр технического описания (PDF) - Intersil
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ISL8022 Datasheet PDF : 18 Pages
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ISL8022
100% Duty Cycle
The ISL8022 features 100% duty cycle operation to
maximize the battery life. When the battery voltage
drops to a level that the ISL8022 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 Shutdown
The ISL8022 has built-in thermal protection. When the
internal temperature reaches +150°C, the regulator is
completely shut down. As the temperature drops to
+130°C, the ISL8022 resumes operation by stepping
through a soft start-up.
Applications Information
Output Inductor and Capacitor Selection
To consider steady state and transient operation,
ISL8022 typically uses a 1.2µH output inductor. 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. The inductor
ripple current can be expressed as in Equation 1:
VO
•
⎛
⎜1
⎝
–
V-V----I-O-N--⎠⎟⎞
ΔI
=
--------------------------------------
L • fS
(EQ. 1)
The inductor’s saturation current rating needs to be at
least larger than the peak current. The ISL8022 protects
the typical peak current 3.2A/2.8A. The saturation
current needs to be over 3.6A for maximum output
current application.
ISL8022 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 minimum output capacitor values are
shown in Table 1 for the ISL8022 on page 2.
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
“Typical Applications” on page 2 Figure 1.
The output voltage programming resistor, R2 (or R5 in
Channel 2), will depend on the desired output voltage of
the regulator. The value for the feedback resistor is
typically between 0Ω and 750kΩ.
Let R3 = 100kΩ, then R2 will be as shown in Equation 2:
R2
=
R3
⎛
⎜
⎝
V-----O----U----T--
VFB
–
⎞
1⎟
⎠
(EQ. 2)
If the output voltage desired is 0.6V, then R3 is left
unpopulated and short R2. For better performance, add
10pF in parallel to R2.
Input Capacitor Selection
The main functions for the input capacitor is to provide
decoupling of the parasitic inductance and to provide
filtering function to prevent the switching current flowing
back to the battery rail. One 10µF X5R or X7R ceramic
capacitor is a good starting point for the input capacitor
selection per channel. An optional input inductor can be
used before the ceramic capacitor to limit switching
noise. It is recommended to limit the inductance less
than 0.15µH.
PCB Layout Recommendation
The PCB layout is a very important converter design step
to make sure the designed converter works well. Refer to
ISL8022 design procedure for suggestions. For ISL8022,
the power loop is composed of the output inductor L’s,
the output capacitor COUT1 and COUT2, the LX’s pins, and
the GND pin. It is necessary to make the power loop as
small as possible and the connecting traces among them
should be direct, short and wide. The switching node of
the converter, the LX_ pins, and the traces connected to
the node are very noisy, so keep the voltage feedback
trace away from these noisy traces. The input capacitor
should be placed to VIN pin as close as possible and the
ground of input and output capacitors should be
connected as close as possible. The heat of the IC is
mainly dissipated through the thermal pad. Maximizing
the copper area connected to the thermal pad is
preferable. In addition, a solid ground plane is helpful for
better EMI performance. It is recommended to add at
least 5 vias ground connection within the pad for the best
thermal relief.
16
FN7650.1
August 25, 2010
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