MAX782 MAXIM [Maxim Integrated Products], MAX782 Datasheet - Page 19

• Manufacturer Part Number: MAX782
• Description: Triple-Output Power-Supply Controller for Notebook Computers
• Manufacturer: MAXIM [Maxim Integrated Products]

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.
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.
Table 4. Surface-Mount Components
__________Applications Information
Central Semi
Coiltronics
IRC
Murata-Erie
Nihon
Siliconix
Sprague
Sumida
TDK
Transpower Tech.
Company
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[ 1] 404 736-3030
[81] 3-3494-7414
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[81] 3-3607-5144
[81] 3-3278-5358
[ 1] 702 831-3521
Efficiency Considerations
[country code]
Factory FAX
Controller for Notebook Computers
Heavy-Load Efficiency
[phone]
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USA Phone
Triple-Output Power-Supply
The major loss mechanisms under heavy loads are, in
usual order of importance:
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.
where R
the drain-source on resistance of the MOSFET, and
R
r
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
terms according to duty factor.
where VL is the MAX782’s logic supply voltage (nomi-
nally 5V) and q
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
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
where t
110ns), V
and f is the switching frequency.
DS(ON)
CS
P
PD
PD
PD
PD
I
gate-charge losses
diode-conduction losses
transition losses
capacitor-ESR losses
losses due to the operating supply current of the IC.
Efficiency = P
DIODE
2
R losses
is the current-sense resistor value. Note that the
GATE
TRAN
(I 2 R)
TOTAL
D
COIL
term assumes that identical MOSFETs are
= diode conduction losses = I
= resistive loss = (I
D
= gate driver loss = q
= transition loss = ———————————
is the diode’s conduction time (typically
is the forward voltage of the Schottky diode,
= PD
is the DC resistance of the coil, r
PD
G
(I 2 R)
CAP
PD
is sum of the gate charge for low-
OUT
TOTAL
+ PD
+ PD
IN
/P
.
IN
) x 100%
IC
GATE
R
G
LOAD 2
CS
x 100% = P
V
is simply twice the gate
IN 2
)
+ PD
G
x C
) x (R
x f x VL
LOAD
DIODE
RSS
I
DRIVE
COIL
x I
OUT
x V
+ PD
LOAD
+ r
D
/(P
x t
DS(ON)
DS(ON)
TRAN
DS(ON)
D
x f
OUT
x f
19
is
+
+
+