MAX848/MAX849
1-Cell to 3-Cell, High-Power,
Low-Noise, Step-Up DC-DC Converters
12 ______________________________________________________________________________________
When using two counters of the same length, as shown
in Figure 8, one counter (A) just counts the A/D clock
pulses (f
OSC
/2), and the other counter (B) counts DATA
output pulses. When counter A overflows (for example,
after 256 clock cycles for an 8-bit counter), counter B is
disabled. The controller reads the counter B output
data and calculates the analog voltage present at the
ADC’s input.
All µC Implementation
This implementation uses a µC timer and a counter.
The timer and the counter are reset at the same time.
The counter counts data-output pulses applied at its
input. When the timer times out, an interrupt is assert-
ed. The µC then reads the state of the counter register.
The interrupt-handling overhead can cause the counter
to count more pulses than desired. Accuracy depends
on how long the µC needs to read the counter. No
errors will occur if the counter is disabled within one
clock period. Interrupt latency reduces accuracy. The
main advantage of this implementation is that no exter-
nal hardware is required.
__________________Design Procedure
Inductor Selection
The MAX848/MAX849’s high switching frequency allows
the use of a small inductor. Use a 10µH inductor for the
MAX849 and a 22µH inductor for the MAX848. Inductors
with a ferrite core or equivalent are recommended; pow-
der iron cores are not recommended for use with high
switching frequencies. Make sure the inductor’s satura-
tion rating (the current at which the core begins to satu-
rate and inductance starts to fall) exceeds the internal
current limit: 0.8A for the MAX848 and 1.4A for the
MAX849. However, it is generally acceptable to bias the
inductor into saturation by approximately 20% (the point
where the inductance is 20% below the nominal value).
For highest efficiency, use a coil with low DC resistance,
preferably under 100mΩ. To minimize radiated noise,
use a toroid, pot core, or shielded inductor. See Table 5
for a list of suggested inductor suppliers.
Diode Selection
The MAX848/MAX849’s high switching frequency
demands a high-speed rectifier. Schottky diodes, such
as the 1N5817 or MBR0520L, are recommended. Make
sure the diode’s current rating exceeds the maximum
load current and that its breakdown voltage exceeds
V
OUT
.
The Schottky rectifier diode carries load currents only in
the PFM operating mode, since the P-channel synchro-
nous rectifier is disabled. Therefore, the current rating
need not be high (0.5A is sufficient). In PFM mode, the
voltage drop across the rectifier diode causes efficien-
cy loss. However, when operating in PWM mode, the
internal P-channel synchronous rectifier is active and
efficiency loss due to the rectifier diode is minimized.
For high-temperature applications, Schottky diodes
may be inadequate due to their high leakage currents;
use high-speed silicon diodes such as the MUR105 or
EC11FS1. At heavy loads and high temperatures, the
benefits of a Schottky diode’s low forward voltage may
outweigh the disadvantage of high leakage current.
See Table 4 for a list of suggested diode suppliers.
