Rainbow-electronics MAX1844 Manual de usuario descargar pdf (Pagina 19)

pation limits often limits how small the MOSFET can be.

Again, the optimum occurs when the switching (AC)

losses equal the conduction (R

DS(ON)

) losses. High-side

switching losses do not 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

chosen for adequate R

DS(ON)

at low battery voltages

becomes extraordinarily hot when subjected to

IN(MAX)

, reconsider the choice of MOSFET.

Calculating the power dissipation in Q1 due to switching

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

fy factors that influence the turn-on and turn-off times.

These factors include the internal gate resistance, gate

charge, threshold voltage, source inductance, and PC

board layout characteristics. The following switching loss

calculation provides only a very rough estimate and is no

substitute for breadboard evaluation, preferably including

a sanity check using a thermocouple mounted on Q1.

where C

RSS

is the reverse transfer capacitance of Q1,

and I

GATE

is the peak gate-drive source/sink current (1A

typ).

For the low-side MOSFET, Q2, the worst-case power dis-

sipation always occurs at maximum battery voltage:

PD(Q2) = (1 - V

OUT

/ V

IN(MAX)

)

LOAD

DS(ON)

The absolute worst case for MOSFET power dissipation

occurs under heavy overloads that are greater than

LOAD(MAX)

but are not quite high enough to exceed the

current limit. To protect against this possibility, you must

“overdesign” the circuit to tolerate I

LOAD

= I

LIMIT(HIGH)

[(LIR / 2)

LOAD(MAX)

], where I

LIMIT(HIGH)

is the maxi-

mum valley current allowed by the current-limit circuit,

including threshold tolerance and sense-resistance vari-

ation. If short-circuit protection without overload protec-

tion is adequate, enable undervoltage protection, and

use I

LOAD(MAX

) to calculate component stresses.

Choose a Schottky diode D1 having a forward voltage

drop low enough to prevent the Q2 MOSFET body diode

from turning on during the dead time. As a general rule,

a diode having a DC current rating equal to 1/3 of the

load current is sufficient. This diode is optional, and if

efficiency isn’t critical it can be removed.

Applications Information

Dropout Performance

The output voltage adjust range for continuous-conduc-

tion operation is restricted by the nonadjustable 500ns

(max) minimum off-time one-shot. For best dropout per-

formance, use the slower (200kHz) on-time settings.

When working with low input voltages, the duty-factor

limit must be calculated using worst-case values for on-

and off-times. Manufacturing tolerances and internal

propagation delays introduce an error to the TON K-

factor. This error is greater at higher frequencies (Table

5). Also, keep in mind that transient response perfor-

mance of buck regulators operated close to dropout is

poor, and bulk output capacitance must often be

added (see the V

SAG

equation in the Transient

Response section).

The absolute point of dropout is when the inductor cur-

rent ramps down during the minimum off-time (∆I

DOWN

)

as much as it ramps up during the on-time (∆I

). The

ratio h = ∆I

/∆I

DOWN

indicates the circuit’s ability to

slew the inductor current higher in response to

increased load, and must always be greater than 1. As

h approaches 1, the absolute minimum dropout point,

the inductor current will be less able to increase during

each switching cycle, and V

SAG

will greatly increase

unless additional output capacitance is used.

A reasonable minimum value for h is 1.5, but this may

be adjusted up or down to allow trade-offs between

SAG

, output capacitance, and minimum operating

voltage. For a given value of h, the minimum operating

voltage can be calculated as:

where V

DROP1

and V

DROP2

are the parasitic voltage

drops in the discharge and charge paths, t

OFF(MIN)

from the Electrical Characteristics table, and K is taken-

from Table 5. The absolute minimum input voltage is cal-

culated with h = 1.

If the calculated V

IN(MIN)

is greater than the required

minimum input voltage, then operating frequency must

be reduced or output capacitance added to obtain an

acceptable V

SAG

. If operation near dropout is anticipat-

ed, calculate V

SAG

to be sure of adequate transient

response.

IN MIN

OUT DROP

OFF MIN

DROP DROP()

()













PD(Q1 switching)

CV fI

RSS IN(MAX)

LOAD

GATE

×××

MAX1844

High-Speed Step-Down Controller with

Accurate Current Limit for Notebook Computers

______________________________________________________________________________________ 19

1 2 ... 14 15 16 17 18 19 20 21 22 23

Comentarios a estos manuales

Sin comentarios

Rainbow-electronics MAX1844 Manual de usuario Pagina 19

Comentarios a estos manuales

Relacionado con productos y manuales para Sensores Rainbow-electronics MAX1844