ACT4910QW
Rev 1.0, 15-Sept-2017
Table 3: Buck Current Limit
BK_CLIM[1:0] ILIMSET (A)
00
5.0
from the capacitor to PGND should as short and wide
as possible.
Inductor Selection
01
10
11
6.0
7.0
9.0
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. These ACT4910 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.
A short circuit condition that results in the peak switch
current being 122.5% of BK_CLIM immediately shuts
down the supply and asserts nIRQ low. A buck
overcurrent, undervoltage, or overvoltage condition
moves the IC into the UV/POR state.
Compensation
The Buck regulator utilizes type
2
external
compensation placed on the COMP pin. Contact the
factory for compensation details.
ꢐ
ꢐ
ꢠꢡꢢ
ꢝꢢꢑ
꣄∗ꢀꢟ
ꢠꢡꢢ
ꢀ ꢘꢀꣂꢲꣃ
Equation 6
ꢩ
∗∆ꢋ
ꢃ
ꢝꣅ
Input Capacitor Selection
Where L is the inductor value in µH, VOUT 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.
The STR pin is the input voltage to the buck converter.
It requires a dedicated high quality, low-ESR, ceramic
input capacitor that is optimally placed to minimize the
power routing. For optimal PCB layout considerations,
1206 or 1210 sized input capacitors are recommended.
A 22uF capacitor is typically suitable, but the actual
value is application dependent. The input capacitor can
be increased without limit. Choose the input capacitor
value to keep the input voltage ripple less than 50mV
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
ꢐ
ꢐ
ꢠꢡꢢ
ꢝꢢꢑ
ꢠꢡꢢ
∗ꣂꢲꣃ
꣄
ꢀ ꢒꢋꢽ ꢘ ꢾꢿꣀꣁ ∗ ꢐꢝꢢꢑ
Equation 5
ꢐ
ꢩ
∗ꢟ
ꢝꣅ ꢌ
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.
Where Iout is the maximum eFuse load current in
Amperes, VSTR is the maximum storage voltage, FSW is
the switching frequency, and Vripple is the maximum
allowable ripple voltage on the input of the buck
converter. Note that the storage capacitor values should
not be considered when calculating the input voltage
ripple because they are not typically designed for high
frequency functionality.
Be sure to consider the input capacitor’s DC bias effects.
A capacitor’s actual capacitance is strongly affected by
its DC bias characteristics. The input capacitor is
typically an X5R, X7R, or similar dielectric. Use of Y5U,
Z5U, or similar dielectrics is not recommended. Input
capacitor placement is critical for proper operation. The
buck’s input capacitor must be placed as close to the IC
as possible. The traces from STR to the capacitor and
∆ꢋ
ꢃ
COUT ꢘꢀꢀꢳ∗ꢩ ∗ꢟ
ꢀ
Equation 7
ꢝꣅ ꢌ
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