Features

The supercharged section

–4.5 V to 36 V input voltage range

- Internal power MOSFET

- Internal++ internal+ 5 V LDO, for equipment power supply

-The output voltage is as high as 36 V

-The constant frequency peak current mode

control

–250 kHz to 1 MHz adjustable switch

frequency

- Multi -device external synchronization

Application

- The pulse under the light below the power saving mode

[

123] - Programmable soft start

- programmable OVP protection

- Ceramic output capacitor stable

- Hot shutdown

The backlight driver part

- Six -row, maximum current 30 ma

Capability (adjustable)

Course with higher current

- ]-The minimum dimension time is less than 500 nan seconds

(20:00 minimum lighting duty cycle is 1%kilo)

-The current current matching ± 2%

] - LED failure (open road and short circuit)

Investigation

Application

LCD display and TV panel

PDA panel backlight

GPS Panel backlight

Instructions

LED7706 has high -efficiency single -chip booster converters and six -control special design to provide LED array LCD panels for backlight lighting. The device can manage the output voltage up to 36 V (that is, 10 white LEDs per row). The generator can be external programming to absorb as high as 30 mAh, and it can be managed by PWM Dark Signal (1%of the light adjustment ratio at 20 kHz). The device allows detecting and managing LED roads and short circuit faults, and floating unused lines. Basic protection (output over -voltage, internal MOSFET over current and heat off) is provided.

Electric characteristics

(vin u003d 12 v; ta u003d 0 ° C to 85 ° C, LDO5 is connected to AVCC, unless there are other regulations) [ 123]

Electric characteristics (continued)

(vin u003d 12 v; ta u003d 0 ° C to 85 ° C, LDO5 is connected to AVCC, unless there are other regulations, there are other regulations )

1. The accuracy of the current is #8710; IROWX, 1 u003d (IROWX IROWX, NOM)/IROWX, nom

2. The calculation of the current loss is #8710; IROWX, 2 u003d | IROWX IRowy |/IROWX, NOM

Operation Instructions-Mertilization Part

LED7706 is a single for LCD panel backlight. The LED driver includes a booster converter and six PWM -adjustable light current generators. Based on the constant switching frequency and peak current mode building. Control the voltage output voltage to make the lowest line voltage, and refer to SGND, which is equal to internal reference voltage (400 MV typical values). The input voltage range from 4.5 to the output voltage. In addition, LED7706 also has an internal LDO to power the internal circuit of the device and provides a current of up to 40 mAh. This LDO input is a VIN pin. The LDO5 pin is the LDO output, and it is also the power time of the power MOSFET drive. The AVCC pin is the power supply of analog circuit, which should be connected to the LDO output through a simple RC filter.

The two circuits are used to adjust the current of the generator.

The main circuit is related to the voltage regulator, and the constant frequency peak current mode structure is adopted (see Table 7), while the internal current circuit is based on the set value (Rilim Pin) per line. Special circuit automatically selects the minimum voltage drop in all rows to provide this voltage to the main circuit, and then adjust the output voltage. In fact, the benchmark generator was detected at one time, and the error amplifier dropped its voltage and internal reference voltage and changed the compensation output. The voltage at the compensation pin determines the peak current of the inductors of each switch cycle. As a result, the voltage regulator is determined by the general positive voltage of the LED string:

Among them, the first item represents the maximum general positive direction of the N event, and the second One is the voltage drop on the main motor (a typical value of 400 MV). The device continues to automatically monitor the voltage reduction of all rows to the current generator with the lowest voltage drop.

Enable function

LED7706 is enabled by EN pin. The pin is activated at a high level. When the SGND is forced, the device has been closed. The pin is connected to a permanently activated 2.5μA current source; when the device needs to be opened suddenly when power is powered, the pin must be kept floating or connected to a delay capacitor. When closed, LED7706 quickly releases soft startup capacitors, off the power MOSFET, current generator and LDO. Therefore, the power consumption is only reduced to 20μA. In the application of opening and closing the device with the lighting signal, the EN pin is shown in Figure 5, which can be connected to the size salesEssence

Soft activation

requires the soft startup function to correctly start the system, control the output capacitor charging and avoid the influx of the influx required for the output voltage. The duration of the soft start is the setting of the soft start and the setting between the external capacitors. The capacitor is charged with a constant current of 5μA, forcing the voltage on the SS to increase the nail on the slope. When this voltage increases from 0 to close to 1.2 V, the current limit power MOSFET is released to its final value in proportion. In addition, the frequency of the switch booster converter is reduced to half of the nominal value to avoid the saturation of the inductor caused by the current loss control; the nominal switch frequency is the SS pin voltage exceeding 0.8 V.

During the soft start phase, floating detection was also performed. When the number of people's faces or floating columns is large, the error amplifier is unbalanced and the output voltage increases; when the floating line detection (FRD) threshold (95%of the OVP threshold) Page 25 Section 8). At the SS voltage reaches 2.4V threshold, the start is over, and all the protection devices rotate positive. Soft startup capacitor CSS can be calculated based on type 2.

Overvoltage protection

Provide adjustable overvoltage protection. It can be set to a partition with an output voltage. The voltage of the hip tap of the frequency division is compared with a fixed 1.234V threshold. When the voltage on the OVSEL pin exceeds OV, the fault pins are tied (see p. 25), the device is closed; this situation is locked, and the power -on reset by switching EN pins or executing (when LDO is when LDO When the output is lower than the lower limit, the PORUVLO threshold occurs, and then cross the UVLO threshold upper limit of the input voltage during the rise). Generally, the high -voltage -side resistance value of the pressure division must be selected as high as possible (but below 1 m ) to reduce the discharge when the output capacitance is turned off (at the closing phase of the lighting cycle). This R2/R1 ratio is calculated to trigger the maximum value of the OVP circuit higher than the given LED string when the output voltage is 2V (see equations 3). The other two may require filter capacitors C10 and C13 to improve the noise suppression capacity of OVSEL as shown in Figure 7. The typical value of the C10 is within the range of 100 PF-330 PF, while the C13 value is given by Formula 4.

Switch frequency selection and synchronization

The switching frequency of the booster converter can be set in the following ways FSW pin ground. The calculation of the whole resistance is made of equation 5, and it should not exceed 100 k -400 k .

In addition, when the FSW pin is connected to AVCC, the LED7706 uses the default 660 KHz fixed switch frequency to allow a resistance to the minimum component count. The FSW pin can also be used as synchronous input, allowing LED7706 to work as a main device or from the device. If the clock signal with a minimum frequency of 220 kHz locks 270 millivoltors (synchronized to the device). As long as the external clock frequency is greater than 220kHz, synchronization is allowed. Keeping the FSW pin voltage at a time below 270mV exceeds 6 μs, which will cause the device to turn off. Once the FSW rises to the above threshold, the order of repeated soft starting order is resumed. Synchronous pipes are synchronized output. When LED7706 is used as the master or copy of the FSW, 35%(typical) duty clock is used as a device. It is used to connect multiple devices in the chrysanthemum chain configuration or synchronize other switch converters running in the system (main operation) with LED7706.

When the external synchronization clock is applied to the FSW pin, the internal oscillator is driven: each switching cycle starts from the clock rising edge, and the slope compensate the slope from the same signal of the same signal. The falling edge starts. Therefore, a clock with a maximum occupy ratio of 40%is required to work in a synchronous converter in a continuous pitch mode (CCM). The synchronous pulse that allows the detected pulse width is 270ns.

System stability

The boost part of the LED7706 is a fixed frequency, a current mode converter. During the normal operation, the minimum voltage selection circuit will compare the active current generator of all voltage drops, and provide the minimum current generator to the error amplifier. The output voltage of this error amplifier determines the peak current of the inductor to maintain its peak current to convert the input voltage to 270 MV (typical value). The compensation network includes a COMP pin and grounding between a simple RC series (RCOMP-CCMP). The calculations of RCOMP and CCOMP are closely related to the stability of the optimal circuit and the dynamic performance of the boost converter and the working state.

Circle compensation

Compensation network can be calculated quickly with Formula 6 to 10. Both RCOMP and CCOMP have been determined, and they may need to obtain the best dynamic performance from the application. First of all, the parameter is the switching frequency. Generally, the high switching frequency allows the dimensions of the inductors, but it will increase the switching loss and the dynamic response to the frequency frequency. For most applications, the value (660 kHz) represents a good balance response between power consumption and dynamic performance, allowing a external resistance at the same time. In low-key applications, the value of inductors is usually kept at a lower level to reduce the number of turns; the range of 4.7 μH-15 μH is a good starting choice. Even BOOThe loop bandwidth of the ST converter should be as large as possible, and it can also be set to 20%of the switch frequency. Be careful not to exceed the right half plane of the CCM mode (RHPZ).