MAX1611CSE データシートの表示（PDF） - Maxim Integrated

部品番号

コンポーネント説明

メーカー

MAX1611CSE

Digitally Controlled CCFL Backlight Power Supplies

Maxim Integrated 

Digitally Controlled CCFL Backlight

Power Supplies

Open-Tube Protection (OTP)

Any real transformer used in a Royer oscillator will have a

maximum-allowed secondary voltage. If the maximum-

allowed secondary voltage is exceeded, the winding

insulation can break down, leading to permanent trans-

former damage. The maximum-allowed secondary volt-

age can be exceeded either when the CCFL drive circuit

is turned on without the CCFL being in place, or when

the CCFL becomes disconnected during normal opera-

tion due to a mechanical failure. To protect against these

fault conditions, use the OTP pin to sense the voltage on

the transformer center tap (pin 2 of Figure 4). Whenever

the voltage on OTP exceeds the REF reference voltage,

the BATT-to-LX power switch is forced off.

For example, in Figure 4, the CTX110605 transformer

has a maximum-allowed continuous secondary voltage

of 1340VRMS. D1 and C5 detect the peak voltage on

the center tap of T1. R3 and R4 determine the limit on

the center tap peak voltage. The relationship between

the voltage on the center tap of T1 and the secondary

voltage is diagrammed in Figure 8. Neglecting the

Q1/Q2 saturation voltage and the voltage on the R1

current-sense resistor yields Equation 1:

VCTPK

=

VSEC 2

2N

where VSEC is the maximum root-mean-square voltage

allowed on the secondary, N is the secondary-to-prima-

ry turns ratio, and VCTPK is the peak voltage on the

transformer center tap.

Block Diagram of the Analog Section

Figure 9 shows a functional diagram of the analog cir-

cuitry in the MAX1610/MAX1611. The chips have identi-

cal analog circuitry, and differ only in their digital

interface.

Loop-Compensation Capacitor (CC)

The BATT-to-LX switch turns on at fixed frequency, and

turns off when the current-sense voltage on the CS pin

exceeds CC - REF. As the CC pin voltage rises, the CS

current limit rises as well. A transconductance amplifier

compares the voltage on CSAV to the desired regulation

voltage and outputs a current proportional to this error

to the CC pin. A capacitor from CC to GND sets the

bandwidth of this regulation loop, as shown in Equation 2:

BW = 85

2πC3

where BW is the bandwidth of the CSAV regulation loop

in kHz, and C3 is the capacitance from CC to GND

in nF.

Soft Start (SS)

Soft start prevents the triggering of OTP upon power-

up. Placing a capacitor from SS to GND soft starts the

Royer oscillator by slowly raising the CS current-limit

voltage. Internal circuitry pulls SS to GND during

power-on reset, or whenever the lamp is turned off (DAC

= 00000, shutdown mode, ON-1 = 0, or ON-0 = 0)

(Figures 10 and 11). When SS is not pulled to GND, an

internal 4µA current sources into the capacitor at the

SS pin. This pin is internally diode clamped to REF so

that it rises to a maximum voltage of about 2.7V.

Regardless of the voltage on CC, the CS current-sense

voltage is never allowed to exceed the voltage on SS

divided by 5.

Frequency Selection and Synchronization

The SYNC pin performs two functions: it sets the BATT-

to-LX switching frequency, and it allows the BATT-to-LX

switching frequency to be synchronized to an external

oscillator. SYNC tied to GND or VL sets a 145kHz

switching frequency; SYNC tied to REF sets a 290kHz

πVCT

2

NπVCT

2

-NπVCT

2

2π

2π

ω

ω

NOTE: VCT = AVERAGE VOLTAGE FROM THE T1 CENTER TO THE EMITTERS OF Q1 AND Q2 (IGNORING Q1, Q2 VCE, SAT). ω = 2πfROY.

Figure 8. Transformer Primary/Secondary Voltage Relationship

12 ______________________________________________________________________________________

Share Link: