SP6685 Semiconductor Flash Driving Circuit for Digital Camera and Camera Phone

SP6685 is a constant current charging pump used to drive semiconductor flash in digital cameras and camera phones, and can also be used for camera interruption high brightness lighting. The charging pump can set two different constant current values to drive the camera flash and the camera light, respectively. SP6685 can automatically convert the boost and lower voltage operation to ensure that the working current of the semiconductor light (LED) is independent of the forward voltage. The current required for this circuit is at a low reference voltage (50mV), and a surface mounted resistor with a small resistance pump value can be selected. The output current of the charging pump can reach 700mA in flash state, and can be adjusted by PWM mode to meet the requirements of intermittent illumination of the camera. The operating frequency of the rechargeable pump is 2.4MHz. In the off state, the current of the circuit is only 1 μ A. In addition, the circuit also has very perfect protection functions, such as output overvoltage protection, output current protection and overheat protection.

1 pin function

SP6685 uses a space-saving 10 pin DFN package. The pin arrangement is shown in Figure 1. The functions of each pin are as follows:

1 foot VIN: Charge pump input voltage foot. The foot should be attached to more than 1 μ F decoupled ceramic capacitor, in order to improve the decoupling effect, the capacitor should be as close as possible to the VIN foot when actually wiring.

2,3-foot C1,C2: Two connecting pins of an external floating capacitor should be connected with an external 1 when in use μ F Ceramic capacitor. And the capacitor should be as close as possible to C1, C2.

4-foot FLASH: Logical input that triggers flash and intermittent illumination state transition. In intermittent illumination, the feedback foot (FB) voltage is stable at the internal 50mV reference voltage. In the flash state, changing the current setting resistor RSET between the RSET foot and the ground can change the reference voltage of the feedback foot (FB). Under intermittent illumination, the resistance value of the external current sampling resistor RSENSE can be selected according to the current required by the semiconductor lamp. It should be noted that in the flash state, the flash duration should be limited to 200ms in order to prevent the device from overheating.

5foot FN: Turn off the control input foot. The EN foot should be connected to the VIN foot in normal working condition and grounded in off state.

6 foot RSET: Flash current setting foot. Connect a resistor between the foot and the ground to set the flash current. In flash state (FLASH foot high level). The resistor can adjust the constant current value by setting the feedback voltage VFB according to the following:

VFB= (1.26V/RSET) × 11.2 K

7foot FB: Feedback input foot of current control circuit. The foot should be connected directly to the current sampling resistor.

8-foot SGND: The internal control circuit ground foot, from which the control current is returned to the power supply.

9 foot PGND: Power grounding foot. The current of the floating capacitor returns to the current from this foot.

10 feet VOUT: Charging pump output feet. To reduce the output ripple, the foot should be attached 1 μ For capacitors above F, the larger the capacity of the capacitor, the smaller the ripple of the output voltage.

2 Main technical parameters

The main technical parameters of SP6685 are as follows:

The working voltage is 2.7V~4.3V;

The output pulse current (flash) is 1A

The output continuous current is 0.4A;

The FB foot reference voltage (flash) is 150 mV;

The FB foot reference voltage (continued lighting) is 50 mV;

.FB foot CURRENT 0.5 μ F;

1. Off-current μ A;

.EN, FLASH foot logic low level is 0.4V;

.EN, FLASH foot logic high level is 1.3V;

.EN, FLASH foot current 5 μ A;

The overheat shut-off temperature is 145 C.

Output voltage on time is 250 μ S;

Storage temperature ranged from -65 to +150 C.

The operating temperature ranges from -40 C to + 85 C.

Figure 2

3 Application Circuit

3.1 Structural Principles

The SP6685 rechargeable pump is powered by a single lithium-ion battery, and the input voltage varies from 2.7V to 4.2V. The output current drives the power semiconductor lamp (white LED) in the digital camera. The circuit consists of RSET unit, FB comparator, SET main controller, charging pump, clock, overtemperature comparator, output switch, etc. The internal structure block diagram is shown in Figure 2. SP6685 has two working modes: pulsed current flash and intermittent current illumination. In the flash state, the output current pulse width of SP6685 is about 200 to 300 ms, which can produce a very strong flash for use in digital cameras. The continuous current output from SP6685 is smaller than that from flash, but the output current lasts for several seconds, which is more suitable for digital cameras.

SP6685 output current has two control states, 1X state and 2X state. After the EN foot is connected to the logical high level, SP6685 starts to work and then passes through 200 μ S Establishes the reference voltage. After that, SP6685 enters a soft start state, which reduces the surge current generated during start-up. After the soft start process, SP6685 enters a 1X current control state similar to a linear regulator, which controls the output current by continuously monitoring the voltage of the feedback foot FB. In 1X control state, if SP6685 automatically detects the output drop state, that is, the time when the FB foot voltage is lower than the stability point voltage is greater than the 32-bit cycle of the internal clock, SP6685 will automatically switch to 2X control state. SP6685 remains in 2X control as long as none of the following four conditions occurs: enabling foot EN to be triggered; FLASH foot changes from high level to low level; The VIN is cut off and then plugged in again. Overheating failure occurred. The 2X current control state is the charging pump state, and the output voltage can rise to twice the input voltage, but the output voltage will not exceed the voltage of SP6685, which is limited to 4.3V inside the circuit. In the 2X current control state, as in the 1X state, the output current can be stabilized by detecting the voltage of the feedback foot (FB).

In intermittent lighting (FLASH foot grounding), the FLASH foot is set to a logical low level, while the FB foot voltage of SP6685 is stable at 50 mV, that is

VFB=50mV (intermittent illumination)

In the flash state (FLASH to VIN), the feedback foot (FB) voltage is set by the resistor RSET connected between the RSET foot and the SGND foot, with a value of

VFB= (1.26V/RSET) × 11.2 K

The internal reference voltage is 1.26V, and the 11.2k resistor is used to set the internal resistance of the RSET current. In flash state, VFB should be between 150 mV and 30 mV, and RSET resistance should be between 100 K and 500 K. The output current of SP6685 can be calculated by the following formula in the flashing or intermittent state:

IOUT=VFB/RSENSE

3.2 Selection of Application Circuits and External Components

The typical application circuit of SP6685 is shown in Figure 3. As can be seen from the diagram, SP6685 requires three additional capacitors: input filter capacitor, output filter capacitor and floating capacitor. Output filter capacitor and floating capacitor should have a capacity of 1 μ F, input filter capacitor capacity should be 2.2 μ F or 4.7 μ F. Increasing the capacity of the input capacitor can reduce the ripple of the input voltage. It meets the requirements of digital cameras and camera phones with higher battery voltage ripple requirements. In order to reduce the equivalent series resistance ESR, all capacitors should use ceramic capacitors. Reducing the ESR improves the bypass effect of the output and input capacitors, and also improves the output voltage drive by reducing the output resistance. In order to improve bypass effect, input and output capacitors should be as close as possible to the VIN foot and VOUT foot. Floating capacitor should be as close as possible to C1 foot and C2 foot.

In intermittent illumination, the current sampling resistance RSENSE is determined by the desired output current:

RSENSE=VFB/IOUT (intermittent lighting)

In the formula, VFB = 50mV, when RSENSE is selected, the feedback voltage VFB in flash state can be calculated by using the following formula:

VFB=IOUT × RSENSE

IOUT is the output current in flash state. In the flash state, RSET resistance can be calculated by using the following formula:

RSET=(1.26V/VFB) × 11.2 K (Flash state)

For example, in intermittent illumination and flash state, when the output current is required to be 200mA and 600mA respectively, the following formulas can be calculated: RSENSE=0.25, VFB=150mV (flash state), RSET=88.7k. In this way, in the flash state, the power required for the feedback resistance is:

PFLASH=VFBIOUT=90mW

The common 0603 meter-mounted resistor has a rated power of 1/10W (continuous power) and 1/5W (pulse power), so it can completely meet the requirements of this design.