1-Cell to 3-Cell, High-Power,

Low-Noise, Step-Up DC-DC Converters

12

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When using two counters of the same length, as shown

in Figure 8, one counter (A) just counts the A/D clock

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 100m

Ω. 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

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.

DATA

GIVES YOU 2-BIT RESOLUTION

COUNTING FOUR PULSES

Figure 7. Bit Stream at 1/2 Full Scale

EN

CLR

CLK

RC

8-BIT COUNTER

CLR

CLK

EN

8-BIT COUNTER

LATCH

÷2

CLOCK/SEL

OR LX

CLEAR

CARRY OUTPUT

DATA OUTPUT

A

B

Figure 8. Discrete Hardware Solution for Counting A/D Output

Data Pulses