ADP3158 データシートの表示（PDF） - Analog Devices
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ADP3158 Datasheet PDF : 16 Pages
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ADP3158/ADP3178
RSENSE
The value of RSENSE is based on the maximum required output
current. The current comparators of the ADP3158 and ADP3178
have a minimum current limit threshold of 69 mV. Note that the
69 mV value cannot be used for the maximum specified nominal
current, as headroom is needed for ripple current and tolerances.
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 induc-
tor 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 ADP3158 and ADP3178, 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 ADP3158
and ADP3178 are 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 = 10.8 A rms (15)
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
IHSF 2
=
1.13 W
8.8 A2
= 15 mΩ
(17)
RDS(ON )LSF
≤
PLSF
ILSF 2
=
1.13 W
10.8 A2
= 10 mΩ
(18)
REV. A
–9–
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