The high powered white LEDs available today for use as camera flash lights require up to 2A through the LED to achieve full light intensity. This requires the battery to provide as much as 3.5A for up to 100mS. A conventional solution would use a boost converter in current control mode to drive 2A through the LED and supply the forward voltage required, which is typically 4.2V but can be up to 5.1V. Assuming the battery voltage = 3.5V, 85% efficiency for the boost, the battery current = 2A x 5.15V/3.5V/85% = 3.4A.
Two alternative solutions use supercapacitors from CAP-XX Inc. Their combined high C (1F or more), and low ESR (< 100mΩ)="" enable="" them="" to="" support="" the="" battery="" and="" provide="" the="" pulse="" power="" required="" by="" the="" led,="" while="" their="" thin,="" flat="" form="" factor="" works="" in="" a="" space-constrained="" design="" like="" a="" camera="" phone.="" solution="" 1="" places="" a="" supercapacitor="" at="" the="" output="" of="" a="" buck-boost="" converter,="" and="" solution="" 2="" uses="" a="" supercapacitor="" in="" series="" with="" the="" battery.="">
Supercapacitor at output of buck-boost converter (Solution 1)
Figure 1 shows a block diagram and Figure 2 shows a circuit implementation. The advantage of this circuit is that the supercapacitor also supports the battery for other high power pulses such as GPRS transmission. There are two modes of operation: Flash mode, where the buck-boost converter charges the supercapacitor to approximately 5.5V, or Torch mode where the supercapacitor is charged to the optimum voltage for the GPRS (typically 3.6V - 3.8V), and drives the LED at a lower continuous current, typically 200mA.
Figure 1: Architecturally, a buck-boost regulator will charge the capacitor, while an amplifier FET does the triggering.
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In Figure 2, we select the buck-boost output voltage for Flash or Torch mode by controlling the FET in series with the 64K resistor in Flash mode, which modifies the ratio of the resistive divider at the FB input to the LTC3442. This FET is on for Flash mode, off for Torch mode. Similarly, we select the flash LED current by controlling the FET in series with the 856W resistor to modify the ratio of the resistive divider across the voltage reference at the +ve input to the ST1851 op amp.
The buck-boost converter must have an input current limit to limit both the charge current to the supercapacitor and the maximum load current. These two cases are considered below:
Inrush current when supercapacitor is discharged:
In a boost converter when power is first applied, there is a low impedance path through the inductor and power diode to the output. Due to its low ESR and high C, the supercapacitor across the output looks like a short circuit at power-up. For example, assume the battery impedance = 120mW, battery voltage = 3.6V, forward voltage on power diode = 0.2V, inductor resistance = 10mW, and supercap ESR = 40mW. At the instant of power-up, the inrush current = (3.6V-0.2V)/170mW = 20A! For the 1.5F supercap in Figure 2, the inrush current will still be about 7A after 255msecs. We chose an LTC3442 because it uses a current mirror to sense current at the input to the inductor and turns off the FET between Vin and the inductor to limit current. Average battery current is limited to 1A by setting RLIM = 63K.
Figure 2: As with any analog circuit, performance may depend on passive component values.
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Excessive battery current under load:
Together, the boost converter and the supercap supply the load current. To a first order approximation, the Boost output current/Supercap current = Supercap ESR/Boost output impedance. To avoid relying on the ratio of Boost output impedance:Supercap ESR to limit battery current, it is safer to limit the boost current as a boost converter with low output impedance could result in excessive battery current.
In Torch mode, the supercap supports the battery to power the GPRS module and other high power consumption circuits through the FET labelled RF PA PWR in Fig 1. The battery supplies the average current and the supercap supplies the peak current. This greatly reduces the voltage droop in the power supply to the GPRS module during transmission, extending talk time. Note: The pulse simulators (Fixed Power and Fixed Current spreadsheets) calculate the supply voltage waveform for a given load, battery voltage, battery impedance and supercap.
In Flash mode, the buck- boost converter has a current limit selected so that the max battery current is within specification, but the supercapacitor is charged from the Torch mode voltage (in the examples of Figs 1 & 2, 3.6V) to 5.5V. When in flash mode, the supercapacitor is charged from 5V to 5.5V in less time than the thermal recovery time of the LED. In Fig 2, the LTC3442 has a minimum input current limit specified at 1A with typical value of 2A. The time required to charge a 1.5F supercap from 3.6V to 5.5V at 1A = 1.5F x (5.5V " 3.6V)/1A = 2.8secs, or 1.4secs at 2A. The user can then take a flash photo 1.5 " 3 secs after selecting Flash mode. Once in Flash mode, the maximum time required to recharge the supercap between flashes = 1.5F x (5.5V " 5.0V)/1A = 0.75s. A typical flash LED thermal recovery time between flashes = 2.5s1. The flash pulse duration is limited by thermal constraints for the flash LED. For the LED chosen the max pulsewidth at 2A = 100mS.
Since the supercap is charged to 5.5V in Flash mode, the RF PA PWR FET is turned off to prevent over voltage damaging the GPRS module or any other high current consumption circuits. Hence, with this architecture it is not possible to connect to the network while using the flash. However, people are not usually making phone calls while taking pictures so this should not be a problem.
When the phone is switched from Flash mode to Torch mode, the supercapacitor can be quickly discharged from 5.5V to the Torch mode voltage (in the example of Figure 1, 3.6V), by enabling the flash LED (in the examples of Figures 1 and 2, this draws 200mA from the supercap). The RF PA PWR FET in Fig 1 would only be turned on once the supercap had been discharged to the correct voltage. Note that a CAP-XX supercap rated for 5.5V maximum must consist of 2 cells in series.
A balancing circuit is also required to ensure the voltage across each cell is roughly equal. In Figures 1 and 2, this is simply a pair of balancing resistors. The balancing resistor value will depend on the supercap operating temperature and charge/discharge profile " a typical value = 39KW. If the current drain of the balancing resistors significantly impacts battery life, designers can use a very low current active balance circuit.
The supercap C & ESR requirements for this solution follow (referring to Figure 2):
- Set final voltage for the supercap at the end of the flash pulse. From the Luxeon Flash LXCL-PWF1 Technical Datasheet and Reference Design the typical forward voltage @ 2A = 4.2V. The RDSON for the ZXM64N02X current control FET = 50mΩ and the current control sense resistor = 47mΩ, therefore at 2A, the voltage drop across the current control circuit appoximately 200mV. Therefore, for a minimum supercap voltage = 5V, the current control circuit will work for all LEDs with a forward voltage about 4.8V.
- Set the charge voltage for the supercap (boost output voltage). The maximum voltage for a 4.5V rated CAP-XX supercap is 5V. However, this is the minimum final voltage at the end of the flash pulse. Since the supercap will only be at the boost voltage while the unit is in flash mode, set the buck-boost output = 5.25V. This will not significantly affect supercap life if it remains at this voltage for only a small percentage of its operating life. The camera phone logic should have a timeout so the user cannot inadvertently leave the unit in flash mode. (CAP-XX anticipates having supercapacitors rated to a maximum 5.5V by end 2005.)
- Determine the supercap C & ESR. The voltage drop during the Flash LED pulse will consist of an IR drop plus capacitor discharge. Assuming a 1A current limit for the buck-boost, in the example of Fig 1, the IR drop approximately (2A " 1A) x 40mW = 40mV. This leaves 5.25V " 5.0V - 40mV = 210mV for supercapacitor discharge to achieve a final voltage 5.0V. From I = CdV/dt, for I = constant, we have C = (2A-1A)x100mS/210mV = 0.48F. The CAP-XX GS216 supercap we chose in Figure 1 has ample headroom. Note: For solved equations, refer to the Fixed Current spreadsheet available at www.cap-xx.com.
Supercapacitor in Series with the Battery (Solution 2)
A block diagram is shown in Figure 3, a circuit implementation in Figure 4 and test waveforms in Fig 5. In this solution, the supercap is in series with the battery. The Flash LED draws about 10W @ 2A, but the battery current = flash LED current so battery power is considerably less than flash LED power. If the battery voltage = 3.5V and the Flash LED current = 2A this is only 7W. The supercap supplies the other 3W. Compare this to the example given in the beginning of this article where the battery had to supply up to 12.25W for a 2A Flash LED current.
Figure 3: The supercap can be charged off the battery. Here, a boost converter regulates the charging.
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Figure 4: A boost regulator simplifies the typology, but cannot be used for powering RF sections. The trigger circuit remains the same..
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In Flash mode, the boost is enabled, and the current reference is set for full intensity. The supercap is charged to about 5.5V. The negative terminal of the supercap is at the battery voltage (3.3V " 4.2V), so the supercap will be charged to have anywhere from 2.3V to 1.1V across it. This enables designers to use a much thinner supercapacitor compared to the one used in solution 1. As in solution 1, the boost converter has a current limit which is set so that the supercapacitor is charged to 5.5V in reasonable time, and while in flash mode, the supercapacitor is charged from 5V to 5.5V in less time than the rest time the LED requires between flashes.
Unlike solution 1, there is no inrush current problem at power-up since the boost converter o/p voltage is always input voltage. We chose the Zetex ZXSC100 as the boost controller IC for its low cost and accuracy in setting the maximum current. The ZXSC100 has an internal reference = 25mV for the Isense input, so the 47mW current sense resistor sets the max current at approx 0.5A. If the battery voltage = 3.5V, then it will take 1.2F x (5.5V " 3.5V)/0.5A = 4.8s to charge the CAP-XX GW118 supercap in Fig 4. Once charged to 5.5V, the supercap will take 1.2F x (5.5V " 5.0V)/0.5A = 1.2s to recharge between flashes which is much less than the thermal recovery time required by the flash LED. The boost converter is turned off during the Flash pulse.
The supercap C & ESR requirements for this solution follow:
- The final voltage for the supercap at the end of the flash pulse = 5V, like solution 1.
- Set the charge voltage for the supercap (boost output voltage). The working voltage for a single cell CAP-XX supercapacitor = 2.3V. If the minimum battery voltage at which the flash LED is expected to work = 3.3V, then set the boost output voltage = 3.3V + 2.3V = 5.6V.
- Now determine the supercap C & ESR. The IR drop at the leading edge of Flash LED pulse = LED current x (ESR of supercap + battery impedance). In Fig 4, this = 2A x (30mΩ + 125mΩ) = 310mV. This leaves 5.6V " 5.0V - 310mV = 290mV for supercapacitor discharge to achieve a final voltage 5.0V. This gives C = 2Ax100mS/290mV = 0.69F. Therefore, for a supercapacitor with 30m*937; ESR and battery impedance = 125mW, we need a supercapacitor with C > 0.69F for the voltage at the end of the flash pulse to be 5.0V. The CAP-XX GW118 has ample headroom.
Note: For solved equations, refer to the VoltageDecay Simulator (Fixed Current sheet).
Figure 5 shows voltage and current waveforms for the circuit of Figure 4, except we replaced the CAP-XX GW118 with a GW101 (600mF, 80mΩ) and left the boost converter on during the flash pulse. The battery then provides 200mA charge current to the supercap plus 2A to the flash LED, for a total of 2.2A. For each flash pulse the LED draws approximately 10W: the battery supplies about 7.7W while the supercapacitor provides the difference.
Figure 5: Test waveforms show very little tax on the battery voltage despite current spikes through the flash.
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Figure 6 compares battery current for the different power architectures discussed. The "No supercap" case has the battery at the input of a boost converter supplying the LED. The typical and maximum battery current curves for this case correspond to the forward voltage range of the LED. For solution 1, the battery current is constant and set by the buck-boost converter input current limit. For solution 2, battery current = LED current, but LED power is far greater than battery power and the supercap provides the difference.
Figure 6: Battery current for a variety of architectures show minimal power drain where a supercap is used.
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Figure 1: Two solutions compared.
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A CAP-XX supercapacitor has the high C, low ESR, and thin prismatic form factor that offers a practical solution for providing the pulse power required for flash photography using the high intensity white LEDs available today.
Pierre Mars is the applications engineering leader for CAP-XX Inc. Headquartered in Sydney, Australia, with offices also in South Carolina, Texas and Taipei, Taiwan, the company can be reached at (214) 368-3172, www.cap-xx.com or firstname.lastname@example.org.