Rainbow-electronics MAX1701 Manual de usuario descargar pdf (Pagina 10)

MAX1700/MAX1701

Step-Up Converter

The step-up switching DC-DC converter generates an

adjustable output from 2.5V to 5.5V. During the first part

of each cycle, the internal N-channel MOSFET switch is

turned on. This allows current to ramp up in the induc-

tor and store energy in a magnetic field. During the

second part of each cycle, when the MOSFET is turned

off, the voltage across the inductor reverses and forces

current through the diode and synchronous rectifier to

the output filter capacitor and load. As the energy

stored in the inductor is depleted, the current ramps

down and the output diode and synchronous rectifier

turn off. Voltage across the load is regulated using

either low-noise PWM or low-power operation, depend-

ing on the CLK/SEL pin setting (Table 2).

Low-Noise PWM Operation

When CLK/SEL is pulled high, the MAX1700/MAX1701

operate in a higher power, low-noise pulse-width-

modulation (PWM) mode. During PWM operation, they

switch at a constant frequency (300kHz) and then mod-

ulate the MOSFET switch pulse width to control the

power transferred per cycle and regulate the voltage

across the load. In PWM mode the devices can output

up to 800mA. Switching harmonics generated by fixed-

frequency operation are consistent and easily filtered.

See the Noise Spectrum Plot in the

Typical Operating

Characteristics.

During PWM operation, each rising edge of the internal

clock sets a flip-flop, which turns on the N-channel

MOSFET switch (Figure 3). The switch is turned off

when the sum of the voltage-error, slope compensation,

and current-feedback signals trips a multi-input com-

parator and resets the flip-flop; the switch remains off

for the rest of the cycle. When a change occurs in the

output-voltage error signal into the comparator, it shifts

the level to which the inductor current is allowed to

ramp during each cycle and modulates the MOSFET

switch pulse width. A second comparator enforces an

inductor current limit of 1.6A max.

1-Cell to 3-Cell, High-Power (1A),

Low-Noise, Step-Up DC-DC Converters

10 ______________________________________________________________________________________

PGNDGND

POUT

OUT

OUTPUT

OUT

FIXED

OUTPUT

(GND)

MBR0520L

2 x

100µF

ADJUSTABLE

L1 10µH

REF

CLK/SEL

ONA

ONB

0.22µF

22µF

10Ω

0.7V TO 5.5V

MAX1700

MAX1701

Figure 2. Fixed or Adjustable Output (PWM mode).

CLK/SEL MODE FEATURES

0 Low Power Low supply current

1 PWM

Low noise,

high output current

External Clock

(200kHz to 400kHz)

Synchronized

PWM

Low noise,

high output current

Table 2. Selecting the Operating Mode

Table 1. Typical Available Output Current

POUT

PGND

REF

1.3A CURRENT

LIMIT

OSC

Figure 3. Simplified PWM Controller Block Diagram

NUMBER

OF CELLS

INPUT

VOLTAGE (V)

OUTPUT

CURRENT (mA)

1 NiCd/NiMH 1.2 300

2 NiCd/NiMH 2.4 750

2 NiCd/NiMH 2.4 525

OUTPUT

VOLTAGE (V)

3.3

5.0

3 NiCd/NiMH

or 1 Li-Ion

3.6 8505.0

1 2 ... 5 6 7 8 9 10 11 12 13 14 15 16

MAX1700/MAX1701 1

Comentarios a estos manuales

Sin comentarios

Rainbow-electronics MAX1701 Manual de usuario Pagina 10

Comentarios a estos manuales

Relacionado con productos y manuales para Sensores Rainbow-electronics MAX1701