ADP1621ARMZ-R7(RevB) View Datasheet(PDF) - Analog Devices
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Description
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ADP1621ARMZ-R7 Datasheet PDF : 32 Pages
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ADP1621
Data Sheet
APPLICATION INFORMATION: BOOST CONVERTER
In this section, an analysis of a boost converter is presented,
along with guidelines for component selection. A typical boost-
converter application circuit is shown in Figure 1.
ADIsimPower DESIGN TOOL
The ADP1621 is supported by ADIsimPower design tool set.
ADIsimPower is a collection of tools that produce complete
power designs optimized for a specific design goal. The tools
enable the user to generate a full schematic, bill of materials,
and calculate performance in minutes. ADIsimPower can
optimize designs for cost, area, efficiency, and parts count
while taking into consideration the operating conditions and
limitations of the IC and all real external components. For
more information about ADIsimPower design tools, refer to
www.analog.com/ADIsimPower. The tool set is available from
this website, and users can also request an unpopulated board
through the tool.
SETTING THE OUTPUT VOLTAGE
The output voltage is set through a voltage divider from the
output voltage to the FB input. The feedback resistor ratio sets
the output voltage of the system. The regulation voltage at FB is
1.215 V. The output voltage is given by (see Figure 1)
VOUT
= 1.215 V × 1 +
R1
R2
(4)
The input bias current into FB is 25 nA typical, 70 nA
maximum. For a 0.1% degradation in regulation voltage and
with 70 nA bias current, R2 must be less than 18 kΩ, which
results in 68 µA of divider current. Choose the value of R1 to set
the output voltage. Using higher values for R2 results in reduced
output voltage accuracy due to the input bias current at the FB
pin, whereas lower values cause increased quiescent current
consumption.
DUTY CYCLE
INDUCTOR CURRENT RIPPLE
To determine the worst-case inductor current ripple, output voltage
ripple, and slope-compensation factor, it is first necessary to
determine the system duty cycle. The duty cycle in continuous
conduction mode (CCM) is calculated by the equation
Choose a peak-to-peak inductor ripple current between 20%
and 40% of the average inductor current. A good starting point
for a design is to choose the peak-to-peak ripple current to be
30% of 1/(1 − D) times the maximum load current:
D = VOUT + VD − VIN
(1)
VOUT + VD
where VOUT is the desired output voltage, VIN is the input
voltage, and VD is the forward-voltage drop of the diode. A
typical Schottky diode has a forward-voltage drop of 0.5 V.
∆I L
=
0.3 ×
I LOAD,MAX
1−D
(5)
where ΔIL is the peak-to-peak inductor ripple current, and ILOAD,MAX
is the maximum load current required by the application.
INDUCTOR SELECTION
The GATE minimum on and off times determine the minimum
and maximum duty cycles, respectively. The minimum on and
off times are typically 180 ns and 190 ns, respectively. The
minimum and maximum duty cycles are given by
D MIN
= tON ,MIN
t SW
= tON ,MIN × fSW
(2)
D MAX
= 1 − tOFF,MIN
t SW
= 1 − (tOFF,MIN × fSW )
(3)
where DMIN is the minimum duty cycle, DMAX is the maximum duty
cycle, tON,MIN is the minimum on time, tOFF,MIN is the minimum off
time, tSW is the switching period, and fSW is the switching frequency.
Note that when the converter tries to operate at a duty cycle
lower than DMIN, pulse-skipping modulation occurs to maintain
the output voltage regulation (see the Light Load Operation
section).
The inductor value choice is important because it dictates
the inductor current ripple and therefore the voltage ripple
at the output.
The average inductor current, IL,AVE, is given by
I L,AVE
=
I LOAD
1−D
(6)
and the peak-to-peak inductor ripple current is inversely
proportional to the inductor value:
∆I L
=
VIN × D
fSW × L
(7)
where fSW is the switching frequency, and L is the inductor value.
Assuming continuous conduction mode (CCM) operation, the
peak inductor current is given by
I L,PK
=
I LOAD
1−D
+
∆I L
2
=
I LOAD
1−D
+ VIN × D
2 × fSW × L
(8)
Smaller inductor values are typically smaller in size and usually
less expensive, but increase the ripple current. Larger ripple current
also increases the power loss in the inductor core. Too large an
inductor value results in added expense and may impede load
transient responses because it reduces the effect of slope
compensation.
Rev. B | Page 14 of 32
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