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MAX1897 Datasheet

  • MAX1897

  • "Quick- PWM Slave Controllers for Multiphase, Step-Down Supp...

  • 1018.18KB

  • Maxim   Maxim

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Quick-PWM Slave Controllers for
Multiphase, Step-Down Supplies
MAX1887/MAX1897
Input Capacitor Selection
The input capacitor must meet the ripple current
requirement (I
RMS
) imposed by the switching currents.
The MAX1887/MAX1897 multiphase slave controllers
operate out-of-phase (MAX1897 POL = V
CC
or float),
staggering the turn-on times of each phase. This mini-
mizes the input ripple current by dividing the load cur-
rent among independent phases:
錚?/div>
錚?/div>
錚?/div>
I
LOAD
錚?/div>
V
OUT
(
V
IN
鈭?/div>
V
OUT
)
錚?/div>
I
RMS
= 錚?/div>
錚鳳，
V
IN
錚?/div>
錚?畏 錚革，
錚?/div>
錚?/div>
for out-of-phase operation.
When operating the MAX1897 in-phase (POL = GND),
the high-side MOSFETs turn on simultaneously, so
input capacitors must support the combined input rip-
ple currents of each phase:
錚?/div>
V
錚?/div>
OUT
(
V
IN
鈭?/div>
V
OUT
)
錚?/div>
I
RMS
=
I
LOAD
錚?/div>
V
IN
錚?/div>
錚?/div>
錚?/div>
錚?/div>
for in-phase operation.
For most applications, nontantalum chemistries (ceram-
ic, aluminum, or OS-CON) are preferred because of
their resistance to inrush surge currents typical of sys-
tems with a mechanical switch or connector in series
with the input. If the MAX1887/MAX1897 is operated as
the second stage of a two-stage power-conversion sys-
tem, tantalum input capacitors are acceptable. In either
configuration, choose an input capacitor that exhibits
less than +10擄C temperature rise at the RMS input cur-
rent for optimal circuit longevity.
the minimum power dissipation occurs where the resis-
tive losses equal the switching losses.
Choose a low-side MOSFET that has the lowest possi-
ble on-resistance (R
DS(ON)
), comes in a moderate-
sized package (i.e., one or two SO-8s, DPAK or
D
2
PAK), and is reasonably priced. Make sure that the
DL gate driver can supply sufficient current to support
the gate charge and the current injected into the para-
sitic gate-to-drain capacitor caused by the high-side
MOSFET turning on; otherwise, cross-conduction prob-
lems may occur.
MOSFET Power Dissipation
Worst-case conduction losses occur at the duty factor
extremes. For the high-side MOSFET (N
H
), the worst-
case power dissipation due to resistance occurs at the
minimum input voltage:
錚?/div>
V
錚訛＋
I
錚?/div>
PD (N
H
Re sistive)
= 錚?/div>
OUT
錚?錚?/div>
LOAD
錚?/div>
R
DS(ON)
錚?/div>
V
IN
錚?錚?畏 錚?/div>
Generally, a small high-side MOSFET is desired to
reduce switching losses at high input voltages.
However, the R
DS(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 (R
DS(ON)
) losses. High-
side switching losses don鈥檛 usually become an issue
until the input is greater than approximately 15V.
Calculating the power dissipation of the high-side
MOSFET (N
H
) 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 volt-
age, source inductance, and PC board layout charac-
teristics. The following switching-loss calculation
provides only a very rough estimate and is no substi-
tute for breadboard evaluation, preferably including
verification using a thermocouple mounted on N
H
:
PD (N
H
Switching)
=
2
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-cur-
rent applications usually require less attention.
The high-side MOSFET (N
H
) must be able to dissipate
the resistive losses plus the switching losses at both
V
IN(MIN)
and V
IN(MAX)
. Calculate both of these sums.
Ideally, the losses at V
IN(MIN)
should be roughly equal
to losses at V
IN(MAX)
, with lower losses in between. If
the losses at V
IN(MIN)
are significantly higher than the
losses at V
IN(MAX)
, consider increasing the size of N
H
.
Conversely, if the losses at V
IN(MAX)
are significantly
higher than the losses at V
IN(MIN)
, consider reducing
the size of N
H
. If V
IN
does not vary over a wide range,
24
(
V
IN(MAX)
)
2
C
RSS
f
SW
I
LOAD
I
GATE
畏
where C
RSS
is the reverse transfer capacitance of N
H
and I
GATE
is the peak gate-drive source/sink current
(1A typ).
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
2
鉁?/div>
V
IN
鉁?/div>
fSW
switching-loss equation. If the high-side
MOSFET chosen for adequate R
DS(ON)
at low battery
voltages becomes extraordinarily hot when biased from
______________________________________________________________________________________

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