ADP1875 Analog Devices, ADP1875 Datasheet - Page 32

• Manufacturer Part Number: ADP1875
• Description: Synchronous Buck Controller with Constant On-Time, Valley Current Mode, and Power Saving Mode
• Manufacturer: Analog Devices

ADP1874/ADP1875
For example, if the external MOSFET characteristics are θ
(16-lead QSOP) = 104°C/W, f
3.3 nF, C
power loss is
The rise in package temperature (for 16-lead QSOP) is
Assuming a maximum ambient temperature environment of 85°C,
which is below the maximum junction temperature of 125°C.
DESIGN EXAMPLE
The ADP1874/ADP1875 are easy to use, requiring only a few
design criteria. For example, the example outlined in this section
uses only four design criteria: V
V
Input Capacitor
The maximum input voltage ripple is usually 1% of the
minimum input voltage (11.8 V × 0.01 = 120 mV).
Choose five 22 μF ceramic capacitors. The overall ESR of five
22 μF ceramic capacitors is less than 1 mΩ.
IN
= 12 V (typical), and f
= 57.12 mW
= 55.6 mW
= 132.73 mW
= 13.7°C
T
V
V
= 120 mV − (15 A × 0.001) = 45 mV
= 120 μF
I
P
[
5 (
T
C
P
=
P
=
P
=
=
VREG
RMS
J
CIN
DISS
R
RIPP
MAX,RIPPLE
DR
DISS
IN,min
104
(
0 .
(
77
= T
13
. 4
lowerFET
=
(
= I
×
= (I
LOSS
.
(
62
(
= 120 mV
TOTAL
13
θ
LDO
°C
(
V
R
JA
300
LOAD
=
×
× T
×
)
−
mW
RMS
)
×
×
(
= 3.3 nF, V
(
=
4
5
300
f
=
)
= V
P
132
×
/2 = 7.5 A
A
)
SW
f
[
V
=
DR
2
V
SW
10
(
= 13.7°C + 85°C = 98.7°C
I
V
× ESR = (7.5 A)
+
)
DR
C
P
LOAD
(
×
RIPP
.
V
×
LOSS
3
IN
DISS
05
55
lowerFET
10
MAX
×
(
×
300
−
6 .
mW
− (I
3
,
(
3
(
)
MAX
LDO
VREG
3 .
f
×
DR
,
mW
SW
RIPPLE
×
×
3
SW
LOAD,MAX
VREG
)
= 4.62 V, and VREG = 5.0 V, then the
3 .
10
C
10
+
SW
= 300 kHz.
upperFET
×
3
P
−
)
OUT
10
= 300 kHz, I
=
9
DR
×
×
×
(
3
(
4
+
−
= 1.8 V, I
LOSS
2
5
f
3 .
× ESR)
9
×
SW
× 1 mΩ = 56.25 mW
I
0 .
V
×
×
300
BIAS
)
DR
. 4
+
10
×
62
. 0
C
−
+
)
×
]
002
9
total
15
10
+
LOAD
I
×
BIAS
BIAS
. 0
5
3
A
))
×
002
×
+
= 2 mA, C
VREG
= 15 A (pulsing),
)
105
. 0
]
+
))
002
+
mV
)
+
I
upperFET
BIAS
JA
)
Rev. 0 | Page 32 of 44
=
Inductor
Determine inductor ripple current amplitude as follows:
Therefore, calculating for the inductor value
The inductor peak current is approximately
Therefore, an appropriate inductor selection is 1.0 μH with
DCR = 3.3 mΩ (Würth Elektronik 7443552100) from Table 10
with peak current handling of 20 A.
Current Limit Programming
The valley current is approximately
Assuming a lower-side MOSFET R
the valley current limit from Table 7 and Figure 71 indicates, a
programming resistor (RES) of 100 kΩ corresponds to an A
of 24 V/V.
Choose a programmable resistor of R
sense gain of 24 V/V.
Output Capacitor
Assume that a load step of 15 A occurs at the output and no more
than 5% output deviation is allowed from the steady state
operating point. In this case, the ADP1874 advantage is that,
because the frequency is pseudo-fixed, the converter is able to
respond quickly because of the immediate, though temporary,
increase in switching frequency.
Assuming that the overall ESR of the output capacitor ranges
from 5 mΩ to 10 mΩ,
Therefore, an appropriate inductor selection is five 270 μF
polymer capacitors with a combined ESR of 3.5 mΩ.
= 1.03 μH
15 A + (5 A × 0.5) = 17.5 A
= 0.003 × (15 A)
15 A − (5 A × 0.5) = 12.5 A
Δ V
= 1.11 mF
Δ
C
L
=
P
=
DCR
I ≈
OUT
2
=
(
L
5
13
DROOP
×
(
V
(
V
LOSS
2 .
300
=
×
IN,MAX
I
Δ
LOAD
V
2
300
= 0.05 × 1.8 V = 90 mV
)
3
I
×
×
−
=
L
10
1
f
×
×
DCR
15
8 .
SW
= 5 A
−
3
10
f
V
×
SW
V
A
×
Δ
3
2
(
OUT
)
90
×
(
= 675 mW
I
×
Δ
LOAD
I
V
13
)
2
L
mV
1
×
DROOP
8 .
2 .
V
)
V
V
V
IN,MAX
OUT
)
ON
RES
of 4.5 mΩ and 13 A as
= 100 kΩ for a current-
CS