Rainbow-electronics MAX1791 Manual de usuario descargar pdf (Pagina 17)

MAX1762/MAX1791

High-Efficiency, 10-Pin µMAX, Step-Down

Controllers for Notebooks

______________________________________________________________________________________ 17

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 battery volt-

age is applied, due to the squared term in the CV

switching loss equation. If the high-side MOSFET cho-

sen for adequate R

DS(ON)

at low battery voltages

becomes extraordinarily hot when subjected to

VP(MAX)

, reconsider your choice of high-side MOS-

FET.

Calculating the power dissipation in Q1 due to switch-

ing losses is difficult since it must allow for difficult

quantifying factors that influence the turn-on and turn-

off times. These factors include the internal gate resis-

tance, gate charge, threshold voltage, source induc-

tance, and PC board layout characteristics. The follow-

ing switching loss calculation provides only a very

rough estimate and is no substitute for breadboard

evaluation, preferably including a verification 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.

For the low-side MOSFET, the worst-case power dissi-

pation always occurs at maximum battery voltage:

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 and cause the fault latch to trip. To pro-

tect against this possibility, the circuit must be overde-

signed to tolerate:

LOAD

= I

LIMIT(HIGH)

+ (LIR / 2 )

LOAD(MAX)

where I

LIMIT(HIGH)

is the maximum valley current

allowed by the current-limit circuit, including threshold

tolerance and on-resistance variation. This means that

the MOSFET must be very well heatsinked. If short-cir-

cuit protection without overload protection is enough, a

normal I

LOAD

value can be used for calculating compo-

nent stresses.

During the period when the high-side switch is off, cur-

rent circulates from ground to the junction of both FETs

and the inductor. As a consequence, the polarity of the

switching node is negative with respect to ground. If

unchanged, this voltage will be approximately 0.7V (a

diode drop) at both transition edges while both switch-

es are off. In between the edges, the low-side switch

conducts; the drop is I

DS(ON)

. If a Schottky clamp

is connected across the low-side switch, the initial and

final voltage drops will be reduced, improving efficien-

cy slightly.

Choose a Schottky diode (D1) having a forward voltage

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 can

be removed if efficiency isn’t critical.

Applications Issues

Dropout Performance

The output voltage adjust range for continuous-conduc-

tion operation is restricted by the nonadjustable 500ns

(max) minimum off-time one-shot. When working with

low input voltages, the duty-factor limit must be calcu-

lated using worst-case values for on- and off-times.

Manufacturing tolerances and internal propagation

delays introduce an error to the t

K-factor. Also,

keep in mind that transient response performance of

buck regulators operating close to dropout is poor, and

bulk output capacitance must often be added.

Dropout design example: V

= 7V (min), V

OUT

= 5V, f

= 300kHz. The required duty cycle is :

The worst-case on-time is:

The maximum IC duty factor based on timing con-

straints of the MAX1762/MAX1792 is:

which meets the required duty cycle. Remember to

include inductor resistance and MOSFET on-state volt-

age drops (V

) when doing worst-case dropout duty-

factor calculations.

Fixed Output Voltages

The MAX1762/MAX1791 Dual Mode operation allows

the selection of common voltages without requiring

external components (Figure 9). Connect FB to GND for

Duty

t+t

ON(MIN)

ON(MIN) OFF(MAX)

218

218 05

082

µµ

V +0.075

5V+0.075

ON(MIN)

OUT

=×=×

×=

ss335 90 218.%.µµ

V+V

V-V

5V+0.1V

7V - 0.1V

REQ

OUT SW

VP SW

===074.

PD(Q2) -

OUT

VP(MAX)

LOAD DS













××1

PD (Q1 switching)

CV I

RSS VP(MAX) LOAD

GATE

××ƒ×













1 2 ... 12 13 14 15 16 17 18 19 20

Comentarios a estos manuales

Sin comentarios

Rainbow-electronics MAX1791 Manual de usuario Pagina 17

Comentarios a estos manuales

Relacionado con productos y manuales para Sensores Rainbow-electronics MAX1791