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MAX17480GTL Datasheet PDF : 48 Pages
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AMD 2-/3-Output Mobile Serial
VID Controller
Core Power-MOSFET Selection
Most of the following MOSFET guidelines focus on the
challenge of obtaining high-load-current capability
when using high-voltage (> 20V) AC adapters. Low-
current applications usually require less attention.
The high-side MOSFET (NH) must be able to dissipate
the resistive losses plus the switching losses at both
VIN(MIN) and VIN(MAX). Calculate both of these sums.
Ideally, the losses at VIN(MIN) should be roughly equal to
losses at VIN(MAX), with lower losses in between. If the
losses at VIN(MIN) are significantly higher than the losses
at VIN(MAX), consider increasing the size of NH (reducing
RDS(ON) but with higher CGATE). Conversely, if the loss-
es at VIN(MAX) are significantly higher than the losses at
VIN(MIN), consider reducing the size of NH (increasing
RDS(ON) to lower CGATE). If VIN does not vary over a
wide range, the minimum power dissipation occurs
where the resistive losses equal the switching losses.
Choose a low-side MOSFET that has the lowest possible
on-resistance (RDS(ON)), comes in a moderate-sized
package (i.e., one or two 8-pin SOs, DPAK, or D2PAK),
and is reasonably priced. Make sure that the DL gate dri-
ver can supply sufficient current to support the gate
charge and the current injected into the parasitic gate-to-
drain capacitor caused by the high-side MOSFET turning
on; otherwise, cross-conduction problems might occur
(see the Core SMPS MOSFET Gate Drivers section).
Core MOSFET Power Dissipation
Worst-case conduction losses occur at the duty factor
extremes. For the high-side MOSFET (NH), the worst-
case power dissipation due to resistance occurs at the
minimum input voltage:
PD
(NH
Resistive)
⎛
= ⎝⎜
VOUT
VIN
⎞
⎠⎟
ILOAD2RDS(ON)
where ILOAD is the per-phase current.
Generally, a small high-side MOSFET is desired to
reduce switching losses at high input voltages.
However, the RDS(ON) required to stay within package
power dissipation often limits how small the MOSFET
can be. Again, the optimum occurs when the switching
losses equal the conduction (RDS(ON)) losses. High-
side switching losses do not usually become an issue
until the input is greater than approximately 15V.
Calculating the power dissipation in the high-side
MOSFET (NH) due to switching losses is difficult since it
must allow for difficult quantifying factors that influence
the turn-on and turn-off times. These factors include the
internal gate resistance, gate charge, threshold voltage,
source inductance, and PCB layout characteristics. The
following switching-loss calculation provides only a very
rough estimate and is no substitute for breadboard
evaluation, preferably including verification using a
thermocouple mounted on NH:
( ) :
PD (NH Switching) =
VIN(MAX)
2 ⎛ CRSSfSW ⎞
⎝⎜ IGATE ⎠⎟
ILOAD
where CRSS is the reverse transfer capacitance of NH,
IGATE is the peak gate-drive source/sink current (1A,
typ), and ILOAD is the per-phase current.
Switching losses in the high-side MOSFET can become
an insidious heat problem when maximum AC adapter
voltages are applied, due to the squared term in the C
x VIN2 x fSW switching-loss equation. If the high-side
MOSFET chosen for adequate RDS(ON) at low battery
voltages becomes extraordinarily hot when biased from
VIN(MAX), consider choosing another MOSFET with
lower parasitic capacitance.
For the low-side MOSFET (NL), the worst-case power
dissipation always occurs at maximum input voltage:
PD
(NL
Resistive)
=
⎡
⎢1−
⎣⎢
⎛
⎝⎜⎜
VOUT
VIN(MAX)
⎞⎠⎟⎟⎤⎦⎥⎥⎛⎝⎜
ILOAD
ηTOTAL
⎞2
⎟
⎠
RDS(ON)
The worst case for MOSFET power dissipation occurs
under heavy overloads that are greater than
ILOAD(MAX), but are not quite high enough to exceed
the current limit and cause the fault latch to trip. To pro-
tect against this possibility, the circuit can be “overde-
signed” to tolerate:
ILOAD(MAX)=
IPEAK(MAX)−
∆IINDUCTOR
2
=
IPEAK(MAX)−
⎛⎝⎜⎜ILOAD(M2AX)LIR
⎞
⎠⎟⎟
where IPEAK(MAX) is the maximum valley current
allowed by the current-limit circuit, including threshold
tolerance and on-resistance variation. The MOSFETs
must have a good-sized heatsink to handle the over-
load power dissipation.
Choose a Schottky diode (DL) with a forward voltage
low enough to prevent the low-side MOSFET body
diode from turning on during the dead time. As a gen-
eral rule, select a diode with a DC current rating equal
to 1/3 the load current per phase. This diode is optional
and can be removed if efficiency is not critical.
Core Boost Capacitors
The boost capacitors (CBST) must be selected large
enough to handle the gate-charging requirements of
the high-side MOSFETs. Typically, 0.1µF ceramic
capacitors work well for low-power applications driving
medium-sized MOSFETs. However, high-current appli-
cations driving large, high-side MOSFETs require boost
capacitors larger than 0.1µF. For these applications,
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