ADP3159 データシートの表示（PDF） - Analog Devices
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ADP3159 Datasheet PDF : 16 Pages
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ADP3159/ADP3179
The current comparator threshold sets the peak of the inductor
current yielding a maximum output current, IO, which equals
twice the peak inductor current value less half of the peak-to-
peak inductor ripple current. From this the maximum value of
RSENSE is calculated as:
RSENSE
≤ VCS(CL )( MIN )
IO
+
IL(RIPPLE )
2
= 69 mV = 4 mΩ
15 A + 1.9 A
(8)
In this case, 4 mΩ was chosen as the closest standard value.
Once RSENSE has been chosen, the output current at the point
where current limit is reached, IOUT(CL), can be calculated using
the maximum current sense threshold of 87 mV:
IOUT (CL )
= VCS(CL )( MAX )
RSENSE
–
IL(RIPPLE )
2
= 87 mV – 3.8 A ≈ 20 A
(9)
4 mΩ 2
At output voltages below 450 mV, the current sense threshold is
reduced to 54 mV, and the ripple current is negligible. There-
fore, at dead short the output current is reduced to:
IOUT (SC )
= 54 mV
4 mΩ
= 13.5 A
(10)
To safely carry the current under maximum load conditions, the
sense resistor must have a power rating of at least:
PRSENSE = (IO )2 × RSENSE = (20 A)2 × 4 mΩ = 1.6 W (11)
Power MOSFETs
Two external N-channel power MOSFETs must be selected for
use with the ADP3159, one for the main switch and an identical
one for the synchronous switch. The main selection parameters
for the power MOSFETs are the threshold voltage (VGS(TH)) and
the ON-resistance (RDS(ON)).
The minimum input voltage dictates whether standard threshold
or logic-level threshold MOSFETs must be used. For VIN > 8 V,
standard threshold MOSFETs (VGS(TH) < 4 V) may be used. If
VIN is expected to drop below 8 V, logic-level threshold MOSFETs
(VGS(TH) < 2.5 V) are strongly recommended. Only logic-level
MOSFETs with VGS ratings higher than the absolute maximum
value of VCC should be used.
The maximum output current IO(MAX) determines the RDS(ON)
requirement for the two power MOSFETs. When the ADP3159
is operating in continuous mode, the simplifying assumption can
be made that one of the two MOSFETs is always conducting
the average load current. For VIN = 5 V and VOUT = 1.65 V, the
maximum duty ratio of the high-side FET is:
DHSF ( MAX ) = 1 – ( fMIN × tOFF )
( ) DHSF ( MAX ) = 1 – 195 kHz × 3.3 µs = 36%
(12)
The maximum duty ratio of the low-side (synchronous rectifier)
MOSFET is:
DLSF ( MAX ) = 1 – DHSF ( MAX ) = 54%
(13)
The maximum rms current of the high-side MOSFET is:
IRMSHSF =
IRMSHSF =
DHSF ( MAX )
×
I2
L(VALLEY )
+ (IL(VALLEY )
3
×
IL(PEAK ) ) +
IL(PEAK
2
)
36% × 13.1 A2 + (13.1 A × 16.1 A) + 16.1 A2 = 8.8 A rms (14)
3
The maximum rms current of the low-side MOSFET is:
IRMSLSF =
IRMSLSF =
DLSF ( MAX )
×
I2
L(VALLEY )
+
IL(VALLEY )
3
×
IL(PEAK )
+
IL(
PEAK
2
)
54% × 13.1 A2 + (13.1 A × 16.1 A) + 16.1 A2
(15)
= 10.8 A rms
3
The RDS(ON) for each MOSFET can be derived from the allowable
dissipation. If 10% of the maximum output power is allowed for
MOSFET dissipation, the total dissipation will be:
PD(FETs) = 0.1 × VOUT × IOUT ( MAX ) = 2.26 W
(16)
Allocating half of the total dissipation for the high-side MOSFET
and half for the low-side MOSFET and assuming that switching
losses are small relative to the dc conduction losses, the required
minimum MOSFET resistances will be:
RDS(ON )HSF
≤
PHSF
I HSF 2
= 1.13 W
8.8 A2
= 15 mΩ
(17)
RDS(ON )LSF
≤
PLSF
I LSF 2
= 1.13 W
10.8 A2
= 10 mΩ
(18)
Note that there is a trade-off between converter efficiency and
cost. Larger MOSFETs reduce the conduction losses and allow
higher efficiency, but increase the system cost. If efficiency is not a
major concern, a Vishay-Siliconix SUB45N03-13L (RDS(ON) =
10 mΩ nominal, 16 mΩ worst-case) for the high-side and a
Vishay-Siliconix SUB75N03-07 (RDS(ON) = 6 mΩ nominal,
10 mΩ worst-case) for the low-side are good choices.
The high-side MOSFET dissipation is:
PDHSF
=
I RMSHSF 2
× RDS(ON ) + VIN
× IL(PEAK ) × QG
2 × IG
×
f MIN
PDHSF
=
8.8
A2
× 16 mΩ +
5V
× 15
A × 70 nC
2 ×1A
× 195 kHz
= 1.75 W
(19)
where the second term represents the turn-off loss of the
MOSFET. In the second term, QG is the gate charge to be removed
from the gate for turn-off and IG is the gate current. From the
data sheet, QG is 70 nC and the gate drive current provided by
the ADP3159 is about 1 A.
The low-side MOSFET dissipation is:
PDLSF = IRMSLSF 2 × RDS(ON )
PDLSF = 10.8 A2 × 10 mΩ = 1.08 W
(20)
Note that there are no switching losses in the low-side MOSFET.
REV. A
–9–
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