ADP1621ARMZ-R7(RevB) 查看數據表（PDF） - Analog Devices
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ADP1621ARMZ-R7 Datasheet PDF : 32 Pages
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ADP1621
Data Sheet
EXAMPLES OF APPLICATION CIRCUITS
STANDARD BOOST CONVERTER—
DESIGN EXAMPLE
The example covered here is for the ADP1621 configured as a
standard boost converter, as shown in Figure 33, where lossless
current sensing is employed. The design parameters are VIN =
3.3 V, VOUT = 5 V, and a maximum load current of 1 A.
To begin this design, a switching frequency of 600 kHz is chosen
(by setting RFREQ to 32 kΩ, see Figure 30) so that a small inductor
and small output capacitors can be used. The duty cycle is cal-
culated from Equation 1 to be 0.4, given a forward-voltage drop of
0.5 V for the Schottky diode. The feedback resistors are calculated
to be R1 = 35.7 kΩ and R2 = 11.5 kΩ from Equation 4.
Assuming that the inductor ripple is 30% of 1/(1 − D) times
the maximum load current, the inductor size is calculated to be
about 4.4 µH, according to Equation 9. The small, magnetically
shielded 4.7 µH Toko FDV0630-4R7M inductor is selected.
Because ceramic capacitors have very low ESR (a few milliohms),
a 47 µF/6.3 V Murata GRM31CR60J476M ceramic capacitor is
chosen for the input capacitor. The output voltage ripple for a
given COUT, ESR, and ESL can be found by solving Equation 12.
By choosing an output voltage ripple equal to 1% of the output
voltage, Equation 12 yields that the minimum COUT required is
100 µF and the maximum ESR required is 25 mΩ. Other com-
binations of capacitance and ESR are possible by choosing a
much larger COUT and a larger ESR. In this case, a small 1 µF
ceramic capacitor and two 150 µF Sanyo POSCAP™ capacitors
are selected. The low ESR ceramic capacitor helps to suppress
the high frequency overshoot at the output. POSCAP has low
ESR and high capacitance in a relatively small package. Ceramic
capacitors can also be used. Generally, bigger ceramic capacitors
are more expensive.
The next step is to choose a Schottky diode. The average
and rms diode currents are calculated to be 1.0 A and 1.3 A,
respectively, using Equations 14 and 15. A Vishay SSA33L
Schottky diode meets the current and thermal requirements
and is an excellent choice.
The power MOSFET must be chosen based on threshold voltage
(VT), on resistance (RDSON), maximum voltage and current ratings,
and gate charge. The rms current through the MOSFET is given
by Equation 18 as 1.1 A. The Vishay Si7882DP is a 20 V n-channel
power MOSFET that meets the current and thermal requirements.
It comes in a PowerPAK® package and offers low RDSON and gate
charge. At VGS = 2.5 V, the on resistance, RDSON, is 8 mΩ.
The loop-compensation components are chosen to be RCOMP =
9.1 kΩ and CCOMP = 1.7 nF from Equations 30 and 31, respectively.
A roll-off capacitor of C2 = 120 pF is also added. The slope-
compensation resistor is set to be RS = 80 Ω from Equation 34.
Lastly, given the chosen components, the peak inductor current
as set by the current limit circuitry is given by Equation 35 as
IL,PK = 12 A. Thus, the maximum load current, assuming CCM
operation, is given by Equation 36 as ILOAD,MAX = 8 A, which is
safely above the 1.0 A load current requirement for this design
example. Note that the current limit is a strong function of RCS,
which can vary part to part and with temperature. In addition,
note that RCS can be implemented with an external current-
sense resistor or with the RDSON of a MOSFET. Variations in RCS
and the other parameters in Equations 35 and 36 must be taken
into account if precise current limiting is necessary. Due to the
parasitic resistance of PCB traces, RS might need to be adjusted
on the actual circuit board to achieve the desired current limit.
Keep in mind that RS must be less than 1.6 kΩ. Using a MOSFET
with a different RDSON or adjusting RCS can also set the current
limit to the desired level.
VIN = 3.3V
C3
1µF
10V
C2
120pF
L1
4.7µH
C4
0.1µF
10V
PIN IN
RCOMP
9.09kΩ
CCOMP
1.8nF
CS
ADP1621
SDSN
GATE
COMP PGND
FREQ
FB
GND
RFREQ
31.6kΩ
1%
D1
RS
R1
35.7kΩ
80Ω
1%
R2
11.5kΩ
M1
1%
COUT1
1µF
10V
VOUT = 5V
1A
COUT2
10µF
10V
COUT3
150µF
6.3V
×2
C1
47µF
6.3V
AGND
fOSC = 600kHz
C1 = MURATA GRM31CR60J476M
COUT3 = SANYO POSCAP 6TPE150M
L1 = TOKO FDV0630-4R7M
M1 = VISHAY Si7882DP
D1 = VISHAY SSA33L
Figure 33. Typical Boost Converter Application Circuit
Rev. B | Page 22 of 32
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