Rainbow Electronics MAX786 - Manual (Page 14)

MAX786
Dual-Output Power-Supply
Controller for Notebook Computers
14 ______________________________________________________________________________________
V
IN(MAX)
, the maximum input (battery) voltage. This
value should include the worst-case conditions under
which the power supply is expected to function, such
as no-load (standby) operation when a battery charger
is connected but no battery is installed. V
IN(MAX)
cannot
exceed 30V.
V
IN(MIN)
, the minimum input (battery) voltage. This
value should be taken at the full-load operating cur-
rent under the lowest battery conditions. If V
IN(MIN)
is
below about 6.5V, the filter capacitance required to
maintain good AC load regulation increases, and the
current limit for the +5V supply has to be increased
for the same load level.
Inductor (L1, L2)
Three inductor parameters are required: the inductance
value (L), the peak inductor current (I
LPEAK
), and the
coil resistance (R
L
). The inductance is:
(V
OUT
) (V
IN(MAX)
- V
OUT
)
L = ————————————
(V
IN(MAX)
) (f) (I
OUT
) (LIR)
where: V
OUT
= output voltage (3.3V or 5V);
V
IN(MAX)
= maximum input voltage (V);
f = switching frequency, normally 300kHz;
I
OUT
= maximum DC load current (A);
LIR = ratio of inductor peak-to-peak AC
current to average DC load current, typically 0.3.
A higher value of LIR allows smaller inductance, but
results in higher losses and higher ripple.
The highest peak inductor current (I
LPEAK
) equals the DC
load current (I
OUT
) plus half the peak-to-peak AC inductor
current (I
LPP
). The peak-to-peak AC inductor current is
typically chosen as 30% of the maximum DC load cur-
rent, so the peak inductor current is 1.15 times I
OUT
.
The peak inductor current at full load is given by:
(V
OUT
) (V
IN(MAX)
- V
OUT
)
I
LPEAK
= I
OUT
+ —————————————.
(2) (f) (L) (V
IN(MAX)
)
The coil resistance should be as low as possible,
preferably in the low milliohms. The coil is effectively in
series with the load at all times, so the wire losses alone
are approximately:
Power loss = (I
OUT
2 ) (R
L
).
In general, select a standard inductor that meets the L,
I
LPEAK
, and R
L
requirements (see Tables 1 and 2). If a
standard inductor is unavailable, choose a core with an
LI
2 parameter greater than (L) (I
LPEAK
2 ), and use the
largest wire that will fit the core.
Current-Sense Resistors (R1, R2)
The sense resistors must carry the peak current in the
inductor, which exceeds the full DC load current. The
internal current limiting starts when the voltage across
the sense resistors exceeds 100mV nominally, 80mV
minimum. Use the minimum value to ensure adequate
output current capability: For the +3.3V supply, R1 =
80mV / (1.15 x I
OUT
); for the +5V supply, R2 =
80mV/(1.15 x I
OUT
), assuming that LIR = 0.3.
Since the sense resistance values (e.g., R1 = 25m
Ω
for
I
OUT
= 3A) are similar to a few centimeters of narrow
traces on a printed circuit board, trace resistance can
contribute significant errors. To prevent this, Kelvin con-
nect the CS_ and FB_ pins to the sense resistors; i.e.,
use separate traces not carrying any of the inductor or
load current, as shown in Figure 5.
Run these traces parallel at minimum spacing from one
another. The wiring layout for these traces is critical for
stable, low-ripple outputs (see the
Layout and
Grounding section).
MOSFET Switches (N1-N4)
The four N-channel power MOSFETs are usually iden-
tical and must be “logic-level” FETs; that is, they must
be fully on (have low r
DS(ON)
) with only 4V gate-
source drive voltage. The MOSFET r
DS(ON)
should
ideally be about twice the value of the sense resistor.
MOSFETs with even lower r
DS(ON)
have higher gate
capacitance, which increases switching time and
transition losses.
MOSFETs with low gate-threshold voltage specifica-
tions (i.e., maximum V
GS(TH)
= 2V rather than 3V) are
preferred, especially for high-current (5A) applications.
Output Filter Capacitors (C6, C7, C12)
The output filter capacitors determine the loop stability
and output ripple voltage. To ensure stability, the mini-
mum capacitance and maximum ESR values are:
V
REF
C F > —————————————
(V
OUT
) (R
CS
) (2) (
π
) (GBWP)
and,
(V
OUT
) (R
CS
)
ESR
CF
< ——————
V
REF
where: C
F
= output filter capacitance (F);
V
REF
= reference voltage, 3.3V;
V
OUT
= output voltage, 3.3V or 5V;
R
CS
= sense resistor (
Ω
);
GBWP = gain-bandwidth product, 60kHz;
ESR
CF
= output filter capacitor ESR (
Ω
).