Rainbow-electronics MAX17061 Manual do Utilizador descarregar pdf (Página 20)

MAX17061

To ensure the stable operation, the MAX17061 includes

slope compensation, which sets the minimum inductor

value. In continuous-conduction mode (CCM), the mini-

mum inductor value is calculated with the following

equation:

where 24.7mV is a scale factor from the slope compen-

sation, the L

CCM(MIN)

is the minimum inductor value for

stable operation in CCM, and R

=12mΩ (typ) is the

equivalent sensing scale factor from the controller’s

internal current-sense circuit.

The controller can also operate in discontinuous con-

duction mode (DCM). In this mode, the inductor value

can be lower, but the peak inductor current is higher

than in CCM. In DCM, the maximum inductor value is

calculated with the following equation:

where the L

DCM(MAX)

is the maximum inductor value

for DCM, η is the nominal regulator efficiency (85%),

and I

OUT(MAX)

is the maximum output current.

The output current capability of the step-up converter is

a function of current limit, input voltage, operating fre-

quency, and inductor value. Because the slope com-

pensation is used to stabilize the feedback loop, the

inductor current limit depends on the duty cycle, and is

determined with the following equation:

where 24.7mV is the scale factor from the slope com-

pensation, 1.9A is a typical current limit at 75% duty

cycle, and D is the duty cycle.

The output current capability depends on the current-

limit value and operating mode. The maximum output

current in CCM is governed by the following equation:

where I

LIM

is the current limit calculated above, η is the

nominal regulator efficiency (85%), and D is the duty

cycle. The corresponding duty cycle for this current is:

where V

DIODE

is the forward voltage of the rectifier

diode and R

is the internal MOSFET’s on-resistance

(0.15Ω typ).

The maximum output current in DCM is governed by

the following equation:

Inductor Selection

The inductance, peak current rating, series resistance,

and physical size should all be considered when selecting

an inductor. These factors affect the converter’s operating

mode, efficiency, maximum output load capability, tran-

sient response time, output voltage ripple, and cost.

The maximum output current, input voltage, output volt-

age, and switching frequency determine the inductor

value. Very high inductance minimizes the current rip-

ple, and therefore reduces the peak current, which

decreases core losses in the inductor and I

R losses in

the entire power path. However, large inductor values

also require more energy storage and more turns of

wire, which increases physical size and I

R copper

losses. Low inductor values decrease the physical size,

but increase the current ripple and peak current.

Finding the best inductor involves the compromises

among circuit efficiency, inductor size, and cost.

In choosing an inductor, the first step is to determine

the operating mode: continuous conduction mode

(CCM) or discontinuous conduction mode (DCM). The

MAX17061 has a fixed internal slope compensation that

requires minimum inductor value. When CCM mode is

chosen, the ripple current and the peak current of the

inductor can be minimized. If a small-size inductor is

required, DCM mode can be chosen. In DCM mode,

the inductor value and size can be minimized, but the

inductor ripple current and peak current are higher

than those in CCM. The controller can be stable, inde-

pendent of the internal slope compensation mode, but

there is a maximum inductor value requirement to

ensure the DCM operating mode.

LI f

VV V

OUT DCM MAX

LIM OSC

OUT DIODE IN

_()

×××

×+ −

()

VVV

VIRV

OUT IN DIODE

OUT LIM ON DIODE

−+

−× +

OUT CCM MAX LIM

OSC

_()

=−

××

⎛

⎝

⎜

⎞

⎠

⎟

mV D

LIM

×−

()

24 7 0 75

LDCM

fVI

MAX

IN MIN

OUT MAX DIODE

IN MIN

OSC MAX OUT MAX OUT MAX

()

() () ()

=−

⎛

⎝

⎜

⎞

⎠

⎟

×× ×

VVVR

mV f

CCM MIN

OUT MAX DIODE IN MIN S

OSC MIN

()

() ()

()

+−×

()

××

2247

8-String White LED Driver with

SMBus for LCD Panel Applications

20 ______________________________________________________________________________________

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

MAX17061 1

Comentários a estes Manuais

Sem comentários

Rainbow-electronics MAX17061 Manual do Utilizador Página 20

Comentários a estes Manuais

Manuais e produtos relacionados com Sensores Rainbow-electronics MAX17061