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ACT4921QI Datasheet(PDF) 26 Page - Active-Semi, Inc

No. de pieza ACT4921QI
Descripción Electrónicos  Power Loss Protection with 6A eFuse
Download  35 Pages
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Fabricante Electrónico  ACTIVE-SEMI [Active-Semi, Inc]
Página de inicio  http://www.active-semi.com
Logo ACTIVE-SEMI - Active-Semi, Inc

ACT4921QI Datasheet(HTML) 26 Page - Active-Semi, Inc

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ACT4921QI
Rev 1.0, 18-Jan-2018
Innovative PowerTM
www.active-semi.com
ActiveSwitcherTM is a trademark of Active-Semi
Copyright © 2018 Active-Semi, Inc.
26
Inductor Selection
The Buck regulator utilizes current-mode control and a
proprietary
internal
compensation
scheme
to
simultaneously simplify external component selection
and optimize transient performance over their full
operating range. The ACT4921 is optimized for
operation with 1uH to 3.3uH inductors. Choose an
inductor with a low DC-resistance, and avoid inductor
saturation by choosing inductors with DC ratings that
exceed the maximum output current by at least 30%.
Due to the requirement for the buck converter to start
up as quickly as possible, the inductor should be
designed to give a maximum ripple current, ΔIL, of 50%
to 60% of the maximum output current. The following
equation calculates the recommended inductor value.
∗∆
Equation 7
Where L is the inductor value in µH, VBUCK is the output
voltage, VSTR is the maximum storage voltage, FSW is
the switching frequency in Hz, and ΔIL is the desired
ripple current in Amperes.
Output Capacitor Selection
The buck converter is designed to take advantage of the
benefits of ceramic capacitors, namely small size and
very-low ESR. The buck converter is designed to
operate with 44µF output capacitor over most of its
operating ranges, although more capacitance may be
desired depending on the duty cycle and load step
requirements. Choose a ripple voltage that is
approximately 1% of the output voltage setpoint. Note
that the output capacitance must be placed at the output
of
the
buck
converter. Additional
downstream
capacitance will be placed at the loads, but this
capacitance should not be considered when calculating
the buck output capacitance. However, the downstream
capacitance should be considered when compensating
the power supply. The following equation calculates the
output voltage ripple as a function of output capacitance.
Note that the worst case ripple voltage occurs at the
beginning of supplement mode when the storage
capacitors are fully charged.
COUT
Equation 8
Where Vripple is the desired output ripple voltage, FSW is
the switching frequency in Hz, and ΔIL is the maximum
ripple current in Amperes.
As with the input capacitor selection, use X5R or X7R
dielectrics and be sure to consider the capacitor’s DC
bias effects.
Storage Boost Converter
General Description
The ACT4921 contains an integrated peak current-
mode, synchronous boost converter. It minimizes
system level costs by using the same components as
the buck converter. The peak current is adjustable
between 250mA and 950mA, allowing for system
optimization. The output voltage is adjustable between
5V and 28V via I2C registers and an external resistor.
The peak current-mode control topology eliminates the
need for compensation. The boost automatically
charges up the storage capacitors from the input
voltage so they are ready to provide backup power in
the event of a system fault.
Startup
The boost converter automatically starts when the IC
exits the SOFTSTART state and the ENB pin is pulled
high. Note that the IC exits the SOFTSTART state 10ms
after the output voltage is within 200mV of the input
voltage. Most systems wait until the system is up and
running before enabling the buck and boost with the
ENB pin.
When the storage voltage reaches regulation, the boost
enters standby mode and monitors the storage voltage.
It automatically turns back on and “tops off” the storage
capacitors when the storage voltage drops below 95%
of the programmed voltage. The boost converter
automatically turns off if it is “topping off” the storage
capacitors when the IC enters supplement mode.
The average input current when the boost charges the
storage capacitors is approximately ½ of the peak
switching current. The peak switch current is programed
by register BST_CLIM[1:0].
Table 6: Boost Peak Current Settings
BST_CLIM[1:0]
(mA)
00
250
01
500
10
950
11
1500


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