Datasheet

ADP1621 Data Sheet
Rev. B | Page 22 of 32
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 V
IN
=
3.3 V, V
OUT
= 5 V, and a maximum load current of 1 A.
To begin this design, a switching frequency of 600 kHz is chosen
(by setting R
FREQ
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 C
OUT
, 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 C
OUT
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 C
OUT
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
(V
T
), on resistance (R
DSON
), 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 R
DSON
and gate
charge. At V
GS
= 2.5 V, the on resistance, R
DSON
, is 8 mΩ.
The loop-compensation components are chosen to be R
COMP
=
9.1 kΩ and C
COMP
= 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 R
S
= 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
I
L,PK
= 12 A. Thus, the maximum load current, assuming CCM
operation, is given by Equation 36 as I
LOAD,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 R
CS
,
which can vary part to part and with temperature. In addition,
note that R
CS
can be implemented with an external current-
sense resistor or with the R
DSON
of a MOSFET. Variations in R
CS
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, R
S
might need to be adjusted
on the actual circuit board to achieve the desired current limit.
Keep in mind that R
S
must be less than 1.6 kΩ. Using a MOSFET
with a different R
DSON
or adjusting R
CS
can also set the current
limit to the desired level.
ADP1621
IN
GATE
PGND
AGND
FB
SDSN
COMP
FREQ
GND
PIN
CS
R1
35.7kΩ
1%
R2
11.5kΩ
1%
C1
47µF
6.3V
L1
4.7µH
M1
C
COMP
1.8nF
C
OUT1
1µF
10V
f
OSC
= 600kHz
C1 = MURATA GRM31CR60J476M
C
OUT3
= SANYO POSCAP 6TPE150M
L1 = TOKO FDV0630-4R7M
M1 = VISHAY Si7882DP
D1 = VISHAY SSA33L
R
S
80Ω
C
OUT2
10µF
10V
R
FREQ
31.6kΩ
1%
C
OUT3
150µF
6.3V
×2
D1
C3
1µF
10V
C4
0.1µF
10V
C2
120pF
R
COMP
9.09kΩ
V
OUT
= 5V
1A
V
IN
= 3.3V
06090-032
Figure 33. Typical Boost Converter Application Circuit