Rainbow-electronics MAX1845 Manual do Utilizador descarregar pdf (Página 13)

MAX1845

Dual, High-Efficiency, Step-Down

Controller with Accurate Current Limit

______________________________________________________________________________________ 13

where K is the on-time scale factor (Table 4). The load-

current level at which PFM/PWM crossover occurs,

LOAD(SKIP)

, is equal to 1/2 the peak-to-peak ripple cur-

rent, which is a function of the inductor value (Figure

4). For example, in the standard application circuit with

OUT1

= 2.5V, V

= 15V, and K = 2.96µs (Table 4),

switchover to pulse-skipping operation occurs at I

LOAD

= 0.7A or about 1/6 full load. The crossover point

occurs at an even lower value if a swinging (soft-satu-

ration) inductor is used.

The switching waveforms may appear noisy and asyn-

chronous when light loading causes pulse-skipping

operation, but this is a normal operating condition that

results in high light-load efficiency. Trade-offs in PFM

noise vs. light-load efficiency are made by varying the

inductor value. Generally, low inductor values produce

a broader efficiency vs. load curve, while higher values

result in higher full-load efficiency (assuming that the

coil resistance remains fixed) and less output voltage

ripple. Penalties for using higher inductor values

FB2

OUT 2

PWM

CONTROLLER

(FIGURE 3)

V+

REF

GND

FAULT1

FAULT2

REF

20Ω

OUT1

UVP

OVP

FB1

SKIP

TON

ON1

ON2

5V INPUT

DL1

LX1

CS1

DH1

BST1

V+

PWM

CONTROLLER

(FIGURE 3)

MAX1845

PGOOD

- 1V

0.5V

LIM1

LIM2

DL2

LX2

CS2

DH2

BST2

V+

- 1V

0.5V

2V TO 28V

Figure 2. Functional Diagram

1 2 ... 8 9 10 11 12 13 14 15 16 17 18 ... 23 24

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