Rainbow-electronics MAX1644 Bedienungsanleitung PDF herunterladen (Seite 8)

MAX1644

__________________Design Procedure

For typical applications, use the recommended compo-

nent values in Table 1. For other applications, take the

following steps:

1) Select the desired PWM-mode switching frequency;

300kHz is a good starting point.

2) Select the constant-off-time as a function of input

voltage, output voltage, and switching frequency.

3) Select R

TOFF

as a function of off-time.

4) Select the inductor as a function of output voltage,

off-time, and peak-to-peak inductor current.

Setting the Output Voltage

The output of the MAX1644 is selectable between one

of two preset output voltages: (2.5V or 3.3V) with a 2%

AC load-regulation error, or an adjustable output volt-

age from the reference voltage (nominally 1.1V) up to

with a 1% or 2% AC load-regulation error. For a

preset output voltage, connect FB to the output voltage,

and connect FBSEL to V

(2.5V output voltage) or

leave unconnected (3.3V output voltage). Internal resis-

tor-dividers divide down the output voltage, regulating

the divided voltage to the internal reference voltage.

For output voltages other than 2.5V or 3.3V, or for

tighter AC load regulation, connect FBSEL to GND (1%

regulation) or to REF (2% regulation), and connect FB

to a resistor divider between the output voltage and

ground (Figure 2). Regulation is maintained for

adjustable output voltages when V

equals V

REF

. Use

50kΩ for R1. R2 is given by the equation:

where V

REF

is typically 1.1V.

Programming the Switching Frequency

and Off-Time

The MAX1644 features a programmable PWM mode

switching frequency, which is set by the input and out-

put voltage and the value of R

TOFF

, connected from

TOFF to GND. R

TOFF

sets the PMOS power switch off-

time in PWM mode. Use the following equation to select

the off-time according to your desired switching fre-

quency in PWM mode (I

OUT

> 0.2A):

where: t

OFF

= the programmed off-time

= the input voltage

OUT

= the output voltage

NMOS

= the voltage drop across the internal

PMOS power switch

PMOS

= the voltage drop across the internal

NMOS synchronous-rectifier switch

VV V

fVV V

OFF

IN OUT PMOS

PWM IN PMOS NMOS

–

−

()

−+

()

R2 R1

OUT

REF

=−













2A, Low-Voltage, Step-Down Regulator with

Synchronous Rectification and Internal Switches

8 _______________________________________________________________________________________

OUT

(V)

TOFF

(kΩ)

6.0 120

(µH)

5 3.3

6.8

(V)

180

6.8 2405 1.8

3.3 823.3 2.5

4.7 1803.3 1.8

4.7 2003.3 1.5

5 2.5

Table 2. Output Voltage and AC Load-

Regulation Selection

2.5 2V

Output

Voltage

3.3 2

Adjustable 2REF

Resistor

Divider

Adjustable 1GND

Resistor

Divider

Unconnected

Output

Voltage

AC LOAD-

REGULATION

ERROR (%)

OUTPUT

VOLTAGE

(V)

FBSEL

R1 = 50kΩ

R2 = R1(V

OUT

/ V

REF

- 1)

REF

= 1.1V

OUT

MAX1644

Figure 2. Adjustable Output Voltage

Table 1. Recommended Component

Values (I

OUT

= 2A, f

PWM

= 300kHz)

6.0 2705 1.5

1 2 3 4 5 6 7 8 9 10 11

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