Rainbow Electronics MAX782 - Manual (Page 19)

MAX782
Triple-Output Power-Supply
Controller for Notebook Computers
______________________________________________________________________________________ 19
__________Applications Information
Efficiency Considerations
Achieving outstanding efficiency over a wide range of
loads is a result of balanced design rather than brute-
force overkill, particularly with regard to selecting the
power MOSFETs. Generally, the best approach is to
design for two loading conditions, light load and heavy
load (corresponding to suspend and run modes in the
host computer), at some nominal battery voltage (such
as 1.2V/cell for NiCd or NiMH). Efficiency improves as
the input voltage is reduced, as long as the high-side
switch saturation voltage is low relative to the input volt-
age. If there is a choice, use the lowest-voltage battery
pack possible, but with at least six cells.
Heavy-Load Efficiency
Losses due to parasitic resistances in the switches,
coil, and sense resistor dominate at high load-current
levels. Under heavy loads, the MAX782 operates in the
continuous-conduction mode, where there is a large
DC offset to the inductor current plus a small sawtooth
AC component (see the
+3.3V Inductor section). This
DC current is exactly equal to the load current – a fact
that makes it easy to estimate resistive losses through
the assumption that total inductor current is equal to
this DC offset current.
The major loss mechanisms under heavy loads are, in
usual order of importance:
● I 2 R losses
● gate-charge losses
● diode-conduction losses
● transition losses
● capacitor-ESR losses
● losses due to the operating supply current of the IC.
Inductor core losses are fairly low at heavy loads
because the inductor current’s AC component is small.
Therefore, they are not accounted for in this analysis.
Efficiency = P
OUT
/P
IN
x 100% = P
OUT
/(P
OUT
+
PD
TOTAL
) x 100%
PD
TOTAL
= PD
(I
2
R)
+ PD
GATE
+ PD
DIODE
+ PD
TRAN
+
PD
CAP
+ PD
IC
PD
(I
2
R)
= resistive loss = (I
LOAD
2 ) x (R
COIL
+ r
DS(ON)
+
R
CS
)
where R
COIL
is the DC resistance of the coil, r
DS(ON)
is
the drain-source on resistance of the MOSFET, and
R
CS
is the current-sense resistor value. Note that the
r
DS(ON)
term assumes that identical MOSFETs are
employed for both the synchronous rectifier and high-
side switch, because they time-share the inductor cur-
rent. If the MOSFETs are not identical, losses can be
estimated by averaging the two individual r
DS(ON)
terms according to duty factor.
PD
GATE
= gate driver loss = q
G
x f x VL
where VL is the MAX782’s logic supply voltage (nomi-
nally 5V) and q
G
is sum of the gate charge for low-
side and high-side switches. Note that gate charge
losses are dissipated in the IC, not the MOSFETs,
and therefore contribute to package temperature rise.
For matched MOSFETs, q
G
is simply twice the gate
charge of a single MOSFET (a data sheet specifica-
tion). If the +5V buck SMPS is turned off, replace VL
in this equation with V
IN
.
P
DIODE
= diode conduction losses = I
LOAD
x V
D
x t
D
x f
where t
D
is the diode’s conduction time (typically
110ns), V
D
is the forward voltage of the Schottky diode,
and f is the switching frequency.
V
IN
2 x C
RSS
x I
LOAD
x f
PD
TRAN
= transition loss = ———————————
I
DRIVE
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