Rainbow-electronics MAX1813 Manual do Utilizador descarregar pdf (Página 15)

MAX1813

Dynamically-Adjustable, Synchronous Step-Down

Controller with Integrated Voltage Positioning

______________________________________________________________________________________ 15

(Figure 2). This architecture relies on the output ripple

voltage to provide the PWM ramp signal. Thus, the out-

put filter capacitor’s equivalent series resistance (ESR)

acts as a feedback resistor. The control algorithm is

simple: the high-side switch on-time is determined sole-

ly by a one-shot whose period is inversely proportional

to input voltage and directly proportional to output volt-

age (see On-Time One-Shot). Another one-shot sets a

minimum off-time (400ns typ). The on-time one-shot is

triggered if the error comparator is low, the low-side

switch current is below the current-limit threshold, and

the minimum off-time one-shot has timed out.

On-Time One-Shot (TON)

The heart of the PWM core is the one-shot that sets the

high-side switch on-time. This fast, low-jitter, adjustable

one-shot includes circuitry that varies the on-time in

response to the input and output voltages. The high-

side switch on-time is inversely proportional to V+ and

directly proportional to the output voltage as set by the

DAC code. This algorithm results in a nearly constant

switching frequency despite the lack of a fixed-frequen-

cy clock generator. The benefits of a constant switch-

ing frequency are twofold: first, the frequency can be

selected to avoid noise-sensitive regions such as the

455kHz IF band; second, the inductor ripple-current

operating point remains relatively constant, resulting in

COMPONENT

CIRCUIT 1

(FIGURE 1)

Output Voltage 0.6V to 1.75V

Input Voltage Range 7V to 24V

Maximum Load Current 22A

Inductor

0.68µH

Sumida CDEP134H-0R6 or

Panasonic ETQP6F0R6BFA

Frequency 300kHz (TON = float)

High-Side MOSFET

International Rectifier

(2) IRF7811A

Low-Side MOSFET

Fairchild (3) FDS7764A or

International Rectifier

(3) IRF7822A

Input Capacitor

(6) 10µF, 25V

Taiyo Yuden TMK432BJ106 or

TDK C4532X5R1E106M

Output Capacitor

(6) 220µF

Panasonic

EEFUE0E221R

Current-Sense Resistor

1.5mΩ Dale WSL 2512, plus

0.5mΩ copper PC board trace

ILIM Level V

(Default)

Voltage-Positioning Gain

Resistor

150kΩ

M A NUF ACT URER

PH ONE

[ C OUN TRY

CO DE]

WEBSITE

MOSFETs

Fairchild

Semiconductor

[1] 888-

522-5372

www.fairchildsemi.com

International

Rectifier

[1] 310-

322-3331

www.irf.com

Siliconix

[1] 203-

268-6261

www.vishay.com

Capacitors

Kemet

[1] 408-

986-0424

www.kemet.com

Panasonic

[1] 847-

468-5624

www.panasonic.com

Sanyo

[ 65] 281-

3226

( S i ng ap or e)

[ 1] 408-

749- 9714

www.secc.co.jp

Taiyo Yuden

[03] 3667-

3408

(Japan)

[1] 408-

573-4150

www.t-yuden.com

Inductors

Coilcraft

[1] 800-

322-2645

www.coilcraft.com

Coiltronics

[1] 561-

752-5000

www.coiltronics.com

Sumida

[1] 408-

982-9660

www.sumida.com

Table 1. Component Selection for

Standard Applications

Table 2. Component Suppliers

1 2 ... 10 11 12 13 14 15 16 17 18 19 20 ... 37 38

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