High current LED driver LTC3454

　　In recent years, mobile phones have been continuously updated and have more and more functions. In addition to the main call function, they can also send and receive text messages, take photos, listen to music, and even surf the Internet and watch movies. They are really powerful. In addition, the mobile phone can also be used as a flashlight. Although this function is not novel, it is very practical. What can realize this function is Linear Technology's recently launched high-current white LED driver LTC3454. In addition to being used as a flash when taking pictures on a mobile phone, it can also be used as a dimmable flashlight when the current is reduced.

       The LTC3454 can be powered by a lithium-ion battery (2.7~4.2V) and can drive a white LED as a flash with a current of 1A. When the battery voltage VBAT is greater than the LED forward voltage drop VF, it works in buck mode; if the battery voltage drops, VBAT is less thanVF, it automatically works in boost mode and has high efficiency to extend battery life.

       LTC3454 Features

       Inside the LTC3454 is a switching boost/buck DC/DC converter. The main features of this device: the input voltage VIN can work under the conditions of greater than, less than or equal to the forward voltage drop VF of the LED, extending the working time of the battery between two charges; using synchronous rectification boost and synchronous rectification buck technology , improves the conversion efficiency: in the flashlight working mode, its efficiency is greater than 90%; in the flashlight mode, its efficiency is greater than 80%; the input voltage range is wide, 2.7~5.5V; the output current is large, and the continuous output current can reach 1A; The current that drives the power LED is programmable and can be adjusted externally to achieve dimming; the programmed current accuracy can reach 3.5%; there is an internal soft start, LED open circuit and short circuit protection; a fixed 1MHz switching frequency; there is a shutdown driver control, The power consumption is almost zero when in the off state; it has overheating protection and input low voltage latch function; small size heat dissipation enhanced 10-pin DFN package (3mm×3mm); operating temperature -40~+85℃.

       The main application areas are mobile phones, digital cameras, PDAs, etc. It can also be used for miner's lamps, emergency lights and strong flashlights.

       Pinout and functions

       The pin arrangement of the LTC3454 is shown in Figure 1, and the functions of each pin are shown in Table 1.

                             
                                      Figure 1 Pin layout of LTC3454

       Table 1 Detailed explanation of LTC3454 pin functions

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       The main parameters

       The main parameters of LTC3454: input voltage VIN=2.7~5.5V; operating current typical value is 825μA; power consumption in off state is less than 1μA; power consumption in low-voltage latch is 5μA (input low-voltage latch threshold voltage is about 2V); VEN1, VEN2 The high-level threshold is 0.68~1.2V, and the low-level thresholds of VEN1 and VEN2 are 0.2~0.68V; the adjusted maximum output voltage VOUT=5.15V (typical value); the oscillator frequency fsw=1MHz; the soft-start time is typical The value is 200μs.

       Introduction to working principle

       The internal structure of LTC3454 can be divided into two parts: the step-up/step-down DC/DC converter part and the LED current setting circuit part.

       Step-up/step-down DC/DC converter section

       The structural block diagram of the up/down DC/DC converter part is shown in Figure 2. It mainly consists of 4 power MOSFETs, A, B, C, and D switches (A and D are P-MOSFETs, C and D are N-MOSFETs), a control circuit, a gate drive circuit, and an error amplifier (its inverting terminal The input voltage is the LED current ILED×the value of the current detection resistor R; the input voltage at the non-inverting terminal is the LED set current ISET×the value of the current detection resistor R).

　　

                                Figure 2 Specific structure of DC/DC part

       The voltage VC output by the error amplifier is related to the working status of the DC/DC converter: when VIN>VF, ILED×R>ISET×R, so that the error amplifier output voltage VC<1.55V, the DC/DC converter works in buck mode. mode, as shown in Figure 3. At this time, switch D is turned on and switch C is turned off; the PWM signal controlled by VC causes switches A and B to turn on in turn. In this case, the circuit can be simplified into a buck circuit as shown in Figure 4. A is the switching tube and B is the synchronous rectifier tube.

                                          Figure 3 DC/DC converter operating mode

                                            Figure 4 Buck mode DC/DC simplified circuit

       When VIN1.65V, the DC/DC converter works in boost mode, as shown in Figure 3. At this time, switch A is turned on and switch B is turned off; the PWM signal controlled by VC makes switches C and D turn on in turn. In this case, the circuit can be simplified into a boost circuit as shown in Figure 5. C is the switching tube, and D is the synchronous rectifier tube.

                                      Figure 5 Boost mode DC/DC simplified circuit

       When VIN≈VF, the error amplifier output voltage VC is in the range of 1.55~1.65V, and it is in the boost/buck mode, that is, it may be a boost mode or a buck mode.

       As can be seen from Figure 3, at VINWhen VF, the converter is in boost mode or buck mode, and the output voltage VC of the error amplifier changes the duty cycle (D) of the PWM, so that the current ILED flowing through the LED is connected to the set LED current ISET.

       LED current setting part

　　 The LED current is set by setting a RISET1 at the ISET1 end and a RISET2 at the SET2 end. The circuit block diagram of this part is shown in Figure 6. It consists of LED current setting amplifier 1, LED current setting amplifier 2, reference voltage source (0.8V), two N-MOSFETs and current mirror circuits.

       The ratio of the current mirror is 1:3850, the current flowing out of one channel is I, and the other channel is 3850I. The current I is divided into two paths: IISET1 and IISET2, and there is a relationship of I=IISET1+IISET2. IISET1 flows into ground through N-MOSEFT (Q1) and RISET1, and IISET2 flows into ground through Q2 and RISET2. The relationship between IISET1 and RISET1 is: IISET1=0.8V/RISET1

       Similarly, the relationship between IISET2 and RISET2 is: IISET2=0.8V/RISET2

       Then I is: I=0.8V(1/RISET1+1/RISET2)

       It can be seen from Figure 6 that when I flows into RISET1 and RISET2, 3850I flows into R, and the voltage at the non-inverting terminal of the error amplifier is equal to 3850I×R. The voltage at the inverting terminal of the error amplifier is equal to ILED×R, based on the principle that the voltage at the non-inverting terminal is equal to the voltage at the inverting terminal.

                                                              Figure 6 LED control circuit

       ILED×R=3850I×R

       ILED=3850I=3850×0.8V(1/RISET1+1/RISET2)

       When a certain ILED is required, appropriate RISET1 and RISET2 can be used to meet the requirements. If the required ILED is <500mA, just use 1 RISET1 or RISET2. If RISET1 is selected, ISET2 can be left floating and EN2 can be grounded. but

       ILED=3850×0.8V/RISET1

       application circuit

      White LED driver circuit with flash and flashlight functions

       Figure 7 is a white light LED drive circuit with flash and flashlight functions. This circuit is powered by a lithium-ion battery. Assume RISET1=20.5kΩ and RISET2=3.65kΩ, then different levels are applied to EN1 and EN2, and the LED has off and three different currents ILED, as shown in Table 2.

                         Figure 7 White LED drive circuit with flash and flashlight functions

      Table 2 LED current values ​​in three modes

        
       150mA can be used as ILED for flashlight, and 850mA can be used as current for flash. If the flash lamp is required to have a larger current,

       EN1=EN2=1, then ILED=1A.

       When EN1=0, EN2=1,

       ILED=3850×0.8V(1/20.5k)=150mA

　　when EN1=1, EN2=0,

       ILED=3850×0.8V(1/3.65k) = 843.8mA

       When EN1=1=EN2=1,

       ILED=150mA+843.8mA≈1000mA

       In Figure 7, the LED used is the LUMILEDS company model LXL-PWF1 LED, and the inductor L1 is the SUMIDA company model CDRH6D28-5RONC.

       Circuit driven by 3 NiMH batteries with ILED=500mA

       A circuit that uses 3 nickel-metal hydride batteries to drive a white LED with a current of ILED=500mA is shown in Figure 8. In Figure 8, a 619kΩ resistor is set at the ISET1 end, and EN1 controls its on and off. ISET2 is left floating and EN2 is grounded. The LED used is a product of LUMILED Company, model number is LXCLLW3C; the inductor L1 is A997AS-4R7M of TOKO Company.

                         Figure 8 White LED circuit driven by 3-cell NiMH batteries

       If different ILEDs are required, just change the resistance of RISET1.

       LED dimming

       From the introduction of the application circuit above, it is known that changing the resistor at the ISETX terminal can change the current ILED of the LED, and then the brightness of the LED can be changed to achieve the purpose of dimming. There are four methods to achieve LED dimming, as shown in Figure 9.

                                             Figure 9 LED dimming circuit

       Figure 9-a shows the relationship between ILED and VDAC using a voltage-type DAC to achieve dimming:

       ILED=3850(0.8V－VDAC)/RSET

       RSET≥Rmin (Rmin does not make ILED>1A)

       Figure 9-b shows the use of current-type DAC to achieve dimming. The relationship between ILED and IDAC:

       ILED=3850×IDAC

       IDAC≤0.8V/Rmin

       Figure 9-c shows the relationship between ILED and the potentiometer resistance RPOT when using a potentiometer to dim the light: ILED=3850×0.8V/(Rmin+RPOT)

       Figure 9-d shows the use of PW signal for dimming. The frequency of PWM is ≥10kHz. The relationship between ILED and PWM duty cycle D and amplitude voltage DVCC:

       ILED=[0.8V-(D%×VDVCC)]/RSET

       Users can choose according to product requirements and usage conditions. In Figure 9-d, the original data does not provide the capacity of the capacitor, which can be determined experimentally by adding different capacities.