Rainbow-electronics MAX1845 Instrukcja Użytkownika Pobierz pdf (Strona 12)

MAX1845

Two external factors that influence switching-frequency

accuracy are resistive drops in the two conduction

loops (including inductor and PC board resistance) and

the dead-time effect. These effects are the largest con-

tributors to the change of frequency with changing load

current. The dead-time effect increases the effective

on-time, reducing the switching frequency as one or

both dead times. It occurs only in PWM mode (SKIP =

high) when the inductor current reverses at light or neg-

ative load currents. With reversed inductor current, the

inductor’s EMF causes LX to go high earlier than nor-

mal, extending the on-time by a period equal to the

low-to-high dead time.

For loads above the critical conduction point, the actual

switching frequency is:

where V

DROP

1 is the sum of the parasitic voltage drops

in the inductor discharge path, including synchronous

rectifier, inductor, and PC board resistances; V

DROP2

the sum of the resistances in the charging path; and

is the on-time calculated by the MAX1845.

Automatic Pulse-Skipping Switchover

In skip mode (SKIP = GND), an inherent automatic

switchover to PFM takes place at light loads. This

switchover is effected by a comparator that truncates

the low-side switch on-time at the inductor current’s

zero crossing. This mechanism causes the threshold

between pulse-skipping PFM and nonskipping PWM

operation to coincide with the boundary between con-

tinuous and discontinuous inductor-current operation

(also known as the critical conduction point). For a 7V

to 24V battery range, of this threshold is relatively con-

stant, with only a minor dependence on battery voltage:

V-V

LOAD(SKIP)

OUT_ IN OUT_

≈













tV V

OUT DROP

ON IN DROP

()

Dual, High-Efficiency, Step-Down

Controller with Accurate Current Limit

12 ______________________________________________________________________________________

Table 1. Component Selection for

Standard Applications

Table 2. Component Suppliers

*Distributor

COMPONENT

SIDE 1: 1.8V AT 8A/

SIDE 2: 2.5V AT 4A

Input Range 4.5V to 28V

Q1 High-Side MOSFET

Fairchild Semiconductor

FDS6612A or

International Rectifier

IRF7807

Q2 Low-Side MOSFET

Fairchild Semiconductor

FDS6670A or

International Rectifier

IRF7805

Q3, Q4 High/Low-Side

MOSFETs

Fairchild Semiconductor

FDS6982A

D1, D2 Rectifier Nihon EP10QY03

D3 Rectifier

Central Semiconductor

CMPSH-3A

L1 Inductor

2.2µH

Panasonic ETQP6F2R2SFA

Sumida CDRH127-2R4

L2 Inductor

4.7µH

Sumida CDRH124-4R7MC

C1 (3), C2 (2) Input

Capacitor

10µF, 25V

Taiyo Yuden

TMK432BJ106KM or

TDK C4532X5R1E106M

C3 (3), C4 Output Capacitor

470µF, 6V

Kemet T510X477M006AS or

Sanyo 6TPB330M

SENSE1

5mΩ, ±1%, 1W

IRC LR2512-01-R005-F or

ALE

WSL-2512-R005F

SENSE2

10mΩ, ±1%, 0.5W

IRC LR2010-01-R010-F or

ALE

WSL-2010-R010F

MANUFACTURER USA PHONE

FACTORY FAX

[Country Code]

Central Semiconductor 516-435-1110 [1] 516-435-1824

Dale/Vishay 203-452-5664 [1] 203-452-5670

Fairchild Semiconductor 408-822-2181 [1] 408-721-1635

International Rectifier 310-322-3331 [1] 310-322-3332

IRC 800-752-8708 [1] 828-264-7204

Kemet 408-986-0424 [1] 408-986-1442

NIEC (Nihon) 805-867-2555* [81] 3-3494-7414

Sanyo 619-661-6835 [81] 7-2070-1174

Siliconix

408-988-8000

800-554-5565

[1] 408-970-3950

Sumida 847-956-0666 [81] 3-3607-5144

Taiyo Yuden 408-573-4150 [1] 408-573-4159

TDK 847-390-4461 [1] 847-390-4405

1 2 ... 7 8 9 10 11 12 13 14 15 16 17 ... 23 24

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