Datasheet

Select the next smaller standard value of resistor and
then calculate the compensation capacitor required to
cancel out the output-capacitor-induced pole (P
OUT1
)
determined previously.
Choose the next larger standard value of capacitor.
In order for p
COMP
to compensate the loop, the open-
loop gain must reach unity at a lower frequency than the
right-half-plane zero or the second output pole, whichever
is lower in frequency. If the second output pole and the
right-half-plane zero are close together in frequency, the
higher resulting phase shift at unity gain may require
a lower crossover frequency. For duty cycles greater
than 50%, slope compensation reduces A
DC
, reducing
the actual crossover frequency from f
CROS
. It is also a
good practice to reduce noise on COMP with a capacitor
(C
COMP2
) to ground. To avoid adding extra phase margin
at the crossover, the capacitor (C
COMP2
) should roll-off
noise at five times the crossover frequency. The value for
C
COMP2
can be found using:
It might require a couple iterations to obtain a suitable
combination of compensation components.
Finally, the zero introduced by the output capacitor's
ESR must be compensated. This compensation is
accomplished by placing a capacitor between REF
and FB creating a pole directly in the feedback loop.
Calculate the value of this capacitor using the frequency
of z
ESR
and the selected feedback resistor values with
the formula:
12
FB ESR OUT
12
RR
C R xC x
R xR
+
=
When using low-ESR, ceramic chip capacitors (MLCCs)
at the output, calculate the value of C
FB
as fows:
12
FB
OSC 1 2
RR
C
2 3.14 f R R
+
=
× × ××
Applications Information
Maximum Output Power
The maximum output power that the MAX1846/MAX1847
can provide depends on the maximum input power avail-
able and the circuit's efficiency:
P
OUT(MAX)
= Efficiency × P
IN(MAX)
Furthermore, the efficiency and input power are both
functions of component selection. Efficiency losses can
be divided into three categories: 1) resistive losses across
the inductor, MOSFET on-resistance, current-sense resis-
tor, rectification diode, and the ESR of the input and out-
put capacitors; 2) switching losses due to the MOSFET's
transition region, and charging the MOSFET's gate
capacitance; and 3) inductor core losses. Typically, 80%
efficiency can be assumed for initial calculations. The
required input power depends on the inductor current
limit, input voltage, output voltage, output current, induc-
tor value, and the switching frequency. The maximum
output power is approximated by the following formula:
P
MAX
= [V
IN
- (V
LIM
+ I
LIM
x R
DS(ON)
)] x I
LIM
x
[1 - (LIR / 2)] x [(-V
OUT
+ V
D
) / (V
IN
- V
SW
- V
LIM
- V
OUT
+ V
D
)]
where I
LIM
is the peak current limit and LIR is the inductor
current-ripple ratio and is calculated by:
LIR = I
LPP
/ I
LDC
Again, remember that V
OUT
for the MAX1846/MAX1847
is negative.
Diode Selection
The MAX1846/MAX1847's high-switching frequency
demands a high-speed rectifier. Schottky diodes are
recommended for most applications because of their
fast recovery time and low forward voltage. Ensure that
the diode's average current rating exceeds the peak
inductor current by using the diode manufacturer's data.
Additionally, the diode's reverse breakdown voltage must
exceed the potential difference between V
OUT
and the
input voltage plus the leakage-inductance spikes. For
high output voltages (-50V or more), Schottky diodes
may not be practical because of this voltage requirement.
In these cases, use an ultrafast recovery diode with ade-
quate reverse-breakdown voltage.
COMP
OUT1 COMP
1
C
6.28 x P xR
=
O COMP
COMP2
CROS O COMP
R R
C
5 x 6.28 x f x R x R
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Maxim Integrated │ 16
MAX1846–MAX1847 High-Efciency, Current-Mode,
Inverting PWM Controller