Applications information, Main application circuits – Rainbow Electronics MAX1847 User Manual
Page 16
MAX1846/MAX1847
High-Efficiency, Current-Mode,
Inverting PWM Controller
16
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may require a larger compensation capacitor than cal-
culated. It might take more than a couple of iterations to
obtain a suitable combination.
Finally, the zero introduced by the output capacitor’s
ESR must be compensated. This 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:
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 resistor, and the ESR of the input and output
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 cur-
rent limit, input voltage, output voltage, output current,
inductor value, and the switching frequency. The maxi-
mum 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.
Input Filter Capacitor
The input capacitor (C
IN
) in inverting converter designs
reduces the current peaks drawn from the input supply
and reduces noise injection. The source impedance of
the input supply largely determines the value of C
IN
.
High source impedance requires high input capaci-
tance, particularly as the input voltage falls. Since
inverting converters act as “constant-power” loads to
their input supply, input current rises as the input volt-
age falls. Consequently, in low-input-voltage designs,
increasing C
IN
and/or lowering its ESR can add as
much as 5% to the conversion efficiency.
Bypass Capacitor
In addition to C
IN
and C
OUT
, other ceramic bypass
capacitors are required with the MAX1846/MAX1847.
Bypass REF to GND with a 0.1µF or larger capacitor.
Bypass V
L
to GND with a 0.47µF or larger capacitor. All
bypass capacitors should be located as close to their
respective pins as possible.
PC Board Layout Guidelines
Good PC board layout and routing are required in high-
frequency-switching power supplies to achieve good
regulation, high efficiency, and stability. It is strongly
recommended that the evaluation kit PC board layouts
be followed as closely as possible. Place power com-
ponents as close together as possible, keeping their
traces short, direct, and wide. Avoid interconnecting
the ground pins of the power components using vias
through an internal ground plane. Instead, keep the
power components close together and route them in a
“star” ground configuration using component-side cop-
per, then connect the star ground to internal ground
using multiple vias.
Main Application Circuits
The MAX1846/MAX1847 are extremely versatile devices.
Figure 2 shows a generic schematic of the MAX1846.
Table 1 lists component values for several typical appli-
cations. These component values also apply to the
MAX1847. The first two applications are featured in the
MAX1846/MAX1847 EV Kit.
C
R
C
R
R
R
R
FB
ESR
OUT
=
Ч
Ч
+
Ч
1
2
1
2