High Current LED Driver LTC3454

In recent years, mobile phones have been constantly updated, and their functions are more and more. In addition to the main call functions, they can also send and receive text messages, take photos, listen to music, and even access the Internet and watch movies. In addition, the phone can also be used as a flashlight, although this feature is nothing new, but very practical. What can be achieved is Linear Technology's recently introduced high-current white LED Driver, the LTC3454, which acts as a dimmable flashlight when reducing current, in addition to being used as a flash for cell phone photography.

The LTC3454 can be powered by a 1-cell lithium-ionbattery(2.7 to 4.2V) and can drive a white LED as a flash with a current of 1A. When the voltage of the battery VBAT is greater than the forward voltage drop VF of the LED, it operates in the buck mode; if the battery voltage drops, VBAT< P> < /p>

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LTC3454 features

The inside of the LTC3454 is a switching type step-up/step-downDC/DCconverter. The main features of the device: the input voltage VIN can work under the condition of greater than, less than or equal to the forward voltage drop VF of the LED, extending

The working time of the battery between the two charging; the synchronous rectification boosting and synchronous rectification step-down technology is adopted to improve the conversion efficiency: the efficiency is greater than 90% in the flashlight working mode; the efficiency is greater than 80 in the flash mode. %; wide input voltage range, 2.7 ~ 5.5V; large output current, continuous output current up to 1A; drive power LED current programmable, and can be adjusted externally to achieve dimming; programming current accuracy up to 3.5% Internal soft start, LED open circuit and short circuit protection; fixed 1MHz switching frequency; closed drive control, almost zero power consumption in the off state; overheat protection and input low voltage latching function; small size heat dissipation enhanced 10 Pin DFN package (3mm × 3mm); operating temperature -40 ~ +85 ° C.

Applications are mainly mobile phones, digital cameras, PDAs, etc., and can also be used for miner's lamps, emergency lights and flashlights.

Pinout and function

The pinout of the LTC3454 is shown in Figure 1. The function of each pin is shown in Table 1.

Figure 1 LTC3454 pinout

Table 1 LTC3454 pin function details

Click to see the original picture

The main parameters

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

Introduction to working principle

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

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

The block diagram of the rise/fallDC/DCconvertersection is shown in Figure 2. It consists of four power MOSFETs consisting of four switches A, B, C, and D (A and D are P-MOSFETs, C and D are N-MOSFETs), control circuit, gate drive circuit, and erroramplifier(the inverting terminal) The input voltage is the value of the current ILED of the LED × the current detecting resistor R; the voltage input to the non-inverting terminal is the set current ISET of the LED × the value of the current detecting resistor R).

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Sp; Figure 2 DC / DC part of the specific structure

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

Figure 3 DC / DC converter working mode

Figure 4 Buck Mode DC/DC Simplified Circuit

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

Figure 5 Boost Mode DC/DC Simplified Circuit

When VIN ≈ VF, the erroramplifieroutput voltage VC is in the range of 1.55 to 1.65V, which is in the boost/buck mode, which may be the boost mode or the buck mode. < /p>

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

LED current setting section

LED current is passed in IS

One RISET1 is set on the ET1 side and one RISET2 is set on the SET2 side. 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 circuit.

The current mirror ratio is 1:3850, with one current flowing out of I and the other being 3850I. I this current is divided into two ways: IISET1 and IISET2, and there is a relationship of I=IISET1+IISET2. IISET1 passes through N-MOSEFT (Q1), flows into the ground via RISET1, IISET2 passes through Q2, and flows into the ground via 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 Fig. 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, according to the principle that the voltage at the same phase is equal to the phase 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, the appropriate RISET1 and RISET2 can be used to satisfy. If the required ILED is <500 mA, then only one RISET1 or RISET2 can be used. If RISET1 is selected, ISET2 can be left floating and EN2 can be grounded. then

ILED="3850"×0.8V/RISET1

Application circuit

       White LED driver circuit with flash and flashlight function

Figure 7 is a white LED driving circuit with flash and flashlight functions. The circuit is powered by a 1-cell lithium-ionbattery. With RISET1=20.5kΩ and RISET2=3.65kΩ, different levels are applied to EN1 and EN2. The LEDs are broken and three different current ILEDs are shown in Table 2.

Figure 7 White LED driver circuit with flash and flashlight function

Table 2 LED current values in three modes

150mA can be used as an ILED for flashlights, and 850mA can be used as a flash current. If the flash 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 LEDs are LEDs of the LUMILEDS company model LXL-PWF1, and the inductor L1 is the SUMIDA company model CDRH6D28-5RONC.

A circuit with ILED=500mA driven by three NiMH batteries

A circuit in which a white LED is driven by a three-cell nickel-hydrogen battery, and a current of ILED=500 mA is shown in FIG. In Figure 8, a 619kΩ resistor is placed at the ISET1 terminal, which is controlled by EN1 to turn on and off. ISET2 is left floating and EN2 is grounded. LEDs are manufactured by LUMILED, model number LXCLLW3C; inductor L1 is TOKO's A997AS-4R7M.

Figure 8: White LED circuit driven by 3-cell NiMH battery

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

LED dimming

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

Figure 9 LED dimming circuit

Figure 9-a shows the dimming, ILED and VDAC with a voltageDAC:

ILED="3850" (0.8V-VDAC)/RSET

RSET≥Rmin (Rmin is not to make ILED>1A)

Figure 9-b shows the relationship between ILED and IDAC using a current-mode DAC:

ILED="3850"×

IDAC

IDAC≤0.8V/Rmin

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

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

ILED=[0.8V-(D%×VDVCC)]/RSET

Users can choose according to the requirements of the product and the conditions of use. In Figure 9-d, the capacity of the capacitor is not given in the original data, and can be experimentally determined by adding different capacities.