MAX8790A(2007) 查看數據表（PDF） - Maxim Integrated

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MAX8790A
(Rev.:2007)

Six-String White LED Driver with Active Current Balancing for LCD Panel Applications

Maxim Integrated 

Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

Output Capacitor Selection

The total output voltage ripple has two components: the

capacitive ripple caused by the charging and discharging

on the output capacitor, and the ohmic ripple due to the

capacitor’s equivalent series resistance (ESR):

VRIPPLE = VRIPPLE(C) + VRIPPLE(ESR)

VRIPPLE(C)

≈

IOUT(MAX)

COUT

⎛

⎜

⎝

VOUT(MAX) − VIN(MIN)

VOUT(MAX)fOSC

⎞

⎟

⎠

and:

VRIPPLE(ESR) ≈ IPEAKRESR(COUT)

where IPEAK is the peak inductor current (see the

Inductor Selection section).

The output voltage-ripple voltage should be low

enough for the FB_ current-source regulation. The rip-

ple voltage should be less than 200mVP-P. For ceramic

capacitors, the output-voltage ripple is typically domi-

nated by VRIPPLE(C). The voltage rating and tempera-

ture characteristics of the output capacitor must also

be considered.

External MOSFET Selection

The MAX8790A’s step-up converter uses an external

MOSFET to enable applications with scalable output

voltage and output power. The boost switching architec-

ture is simple and ensures that the controller is never

exposed to high voltage. Only the external MOSFET,

diode, and inductor are exposed to the output voltage

plus one Schottky diode forward voltage:

VBV = N × VF _LED + VF _ SCHOTTKY + VFB_

The MOSFET’s breakdown ratings should be higher

than VBV with sufficient margin to ensure long-term relia-

bility. A conservative rule of thumb, a minimum 30%

margin would be recommended for MOSFET break-

down voltage. The external MOSFET should have a cur-

rent rating of no less than the IPEAK derived from the

Inductor Selection section. To improve efficiency,

choose a MOSFET with low RDS(ON). The MAX8790A’s

gate-drive linear regulator can provide 10mA. Select the

external MOSFET with a total gate charge so the aver-

age current to drive the MOSFET at maximum switching

frequency is less than 10mA:

Qg(MAX) × fOSC < 10mA

For example, the Si3458DV is specified with 16nC of

max total gate charge at Vg = 10V. For 5V of gate

drive, the required gate charge is 8nC, which equates

to 8mA at 1MHz.

The MOSFET conduction loss or resistive loss is

caused by the MOSFET’s on-resistance (RDS(ON)). This

power loss can be estimated as:

PDRES(MAX)

=

RDS(ON) × L × fOSC ×

3 × VIN(MIN)

IPEAK3

For the above Si3458DV, the estimated conduction loss is:

PDRES(MAX)

=

0.1Ω × 4.7μH × 750kHz ×1.35A3

3 × 7V

= 0.04W

The approximate maximum switching loss can be cal-

culated as:

PDSW(MAX)

=

t turn−off

× IPEAK ×

2

VOUT

×

fOSC

For the above Si3458DV, the approximate switching

loss is:

PDSW(MAX)

=

10ns

× 1.35A

× 28.72V

2

× 750kHz

=

0.145W

Rectifier Diode Selection

The MAX8790A’s high switching frequency demands a

high-speed rectifier. Schottky diodes are recommended

for most applications because of their fast recovery

time and low forward voltage. The diode should be

rated to handle the output voltage and the peak switch

current. Make sure that the diode’s peak current rating

is at least IPEAK calculated in the Inductor Selection

section and that its breakdown voltage exceeds the

output voltage.

Setting the Overvoltage Protection Limit

The OV protection circuit should ensure the circuit safe

operation; therefore, the controller should limit the out-

put voltage within the ratings of all MOSFET, diode, and

output capacitor components, while providing sufficient

output voltage for LED current regulation. The OV pin is

connected to the center tap of a resistive voltage-

divider (R1 and R2 in Figure 1) from the high-voltage

output. When the controller detects the OV pin voltage

reaching the threshold VOV_TH, typically 1.23V, OV pro-

tection is activated. Hence, the step-up converter out-

put overvoltage protection point is:

VOUT(OVP)

=

VOV

_ TH

×

(1+

R1)

R2

In Figure 1, the output OVP voltage is set to:

VOUT(OVP)

=

1.23V

×

(1+

1MΩ

37.4kΩ

)

=

34.1V

______________________________________________________________________________________ 19

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