Rainbow-electronics MAX1712 Manual de usuario descargar pdf (Pagina 20)

input voltage don’t exceed the package ratings or violate

the overall thermal budget.

Choose a low-side MOSFET (Q2) that has the lowest

possible R

DS(ON)

, comes in a moderate to small pack-

age (i.e., SO-8), and is reasonably priced. Ensure that

the MAX1710/MAX1711/MAX1712 DL gate driver can

drive Q2; in other words, check that the gate isn’t pulled

up by the high-side switch turning on due to parasitic

drain-to-gate capacitance, causing cross-conduction

problems. Switching losses aren’t an issue for the low-

side MOSFET since it’s a zero-voltage switched device

when used in the buck topology.

MOSFET Power Dissipation

Worst-case conduction losses occur at the duty factor

extremes. For the high-side MOSFET, the worst-case

power dissipation due to resistance occurs at minimum

battery voltage:

PD(Q1) = (V

OUT

/ V

BATT(MIN)

)

LOAD

DS(ON)

Generally, a small high-side MOSFET is desired in order

to reduce switching losses at high input voltages.

However, the R

DS(ON)

required to stay within package

power-dissipation limits often limits how small the MOS-

FET can be. Again, the optimum occurs when the switch-

ing (AC) losses equal the conduction (R

DS(ON)

) losses.

High-side switching losses don’t usually become an

issue until the input is greater than approximately 15V.

Switching losses in the high-side MOSFET can become

an insidious heat problem when maximum AC adapter

voltages are applied, due to the squared term in the

F switching loss equation. If the high-side MOSFET

you’ve chosen for adequate R

DS(ON)

at low battery volt-

ages becomes extraordinarily hot when subjected to

BATT(MAX)

, you must reconsider your choice of MOS-

FET.

Calculating the power dissipation in Q1 due to switching

losses is difficult, since it must allow for difficult-to-quanti-

MAX1710/MAX1711/MAX1712

High-Speed, Digitally Adjusted

Step-Down Controllers for Notebook CPUs

20 ______________________________________________________________________________________

V+ V

= 7V TO 24V*

SHDN

SKIP

REF

DAC

INPUTS

ON/OFF

0.22µF

470pF

BST

5Ω

PGND

GND

+5V

20Ω

0.1µF

1µF

0.5µH

0.1µF

1nF

C2 CPU

1.6V AT 7A

FBS

GNDS

MAX1711

MAX1712

C1 = 4 x 4.7µF/25V TAIYO YUDEN (TMK325BJ475K)

C2 = 6 x 47µF/10V TAIYO YUDEN (LMK550BJ476KM)

R1 + R2 = 5mΩ MINIMUM OF PC BOARD TRACE RESISTANCE (TOTAL)

TON

*FOR HIGHER MINIMUM INPUT VOLTAGE,

*LESS OUTPUT CAPACITANCE IS REQUIRED.

Figure 7. All-Ceramic-Capacitor Application

TON

SETTING

(kHz)

APPROXIMATE

K-FACTOR

ERROR (%)

MIN V

BATT

AT V

OUT

= 2V

(V)

200 ±10 2.6

300 ±10 2.9

400 ±12.5 3.2

550 ±12.5 3.6

FACTOR

(µs-V)

3.3

2.5

1.8

Table 6. Approximate K-Factors Errors

1 2 ... 15 16 17 18 19 20 21 22 23 24 25 26 27 28

MAX1710/MAX1711/MAX1712 1
MAX1710/MAX1711 14

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Rainbow-electronics MAX1712 Manual de usuario Pagina 20

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