Rainbow-electronics MAX1845 Uživatelský manuál stáhnout pdf (Strana 11)

MAX1845

Dual, High-Efficiency, Step-Down

Controller with Accurate Current Limit

______________________________________________________________________________________ 11

= 5V

BIAS SUPPLY

POWER-GOOD

INDICATOR

MAX1845

OUTPUT1

1.8V, 8A

7V TO 24V

CMPSH-3A

ILIM1

DL1

TON

CS1

OUT1

GND

470µF

LX1

DH1

0.1µF

0.22µF

FB1

UVP

1µF

10µF

C11

1µF

2.2µH

4.7µH

BST1

ILIM2

REF

ON1

ON2

OVP

DL2

CS2

100kΩ

OUT2

LX2

DH2

FB2

PGOOD

V+

BST2

SKIP

4.7µF

20Ω

5mΩ

OUTPUT2

2.5V, 4A

10mΩ

ON/OFF

CONTROLS

Figure 1. Standard Application Circuit

On-Time One-Shot (TON)

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

high-side switch on-time for both controllers. This fast,

low-jitter, adjustable one-shot includes circuitry that

varies the on-time in response to battery and output

voltage. The high-side switch on-time is inversely pro-

portional to the battery voltage as measured by the V+

input, and proportional to the output voltage. This algo-

rithm results in a nearly constant switching frequency

despite the lack of a fixed-frequency clock generator.

The benefits of a constant switching 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 easy design

methodology and predictable output voltage ripple.

The on-times for side 1 are set 35% higher than the on-

times for side 2. This is done to prevent audio-frequen-

cy “beating” between the two sides, which switch asyn-

chronously for each side. The on-time is given by:

On-Time = K (V

OUT

+ 0.075V) / V

where K is set by the TON pin-strap connection (Table

4), and 0.075V is an approximation to accommodate

for the expected drop across the low-side MOSFET

switch. One-shot timing error increases for the shorter

on-time settings due to fixed propagation delays; it is

approximately ±12.5% at higher frequencies and ±10%

at lower frequencies. This translates to reduced switch-

ing-frequency accuracy at higher frequencies (Table

4). Switching frequency increases as a function of load

current due to the increasing drop across the low-side

MOSFET, which causes a faster inductor-current dis-

charge ramp. The on-times guaranteed in the Electrical

Characteristics tables are influenced by switching

delays in the external high-side power MOSFET.

1 2 ... 6 7 8 9 10 11 12 13 14 15 16 ... 23 24

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