LT3420

LT3420/LT3420-1 Photoflash Capacitor Chargers with Automatic Refresh DESCRIPTIO The LT®3420/LT3420-1 charge high voltage photoflash capacitors quickly and efficiently. Designed for use in both digital and film cameras, these devices use a flyback topology to achieve efficiencies up to four times better than competing flash modules. A unique adaptive off-time control algorithm* maintains current-limited continuous mode transformer operation throughout the entire charge cycle, eliminating the high inrush current often found in modules. The LT3420/LT3420-1 output voltage sensing scheme* monitors the flyback voltage to indirectly regulate the output voltage, eliminating an output resistor divider or discrete zener diode. This feature allows the capacitor to be held at a fully charged state without excessive power consumption. Automatic refresh (which can be defeated) allows the capacitor to remain charged while consuming an average input current of about 2mA, at a user-defined refresh rate. A logic high on the CHARGE pin initiates charging, while the DONE pin signals that the capacitor is fully charged. , LTC and LT are registered trademarks of Linear Technology Corporation. *U.S. Patent #6,518,733 FEATURES s s s s s s s s s s s s s s Charges 220µF to 320V in 3.7 Seconds from 5V (LT3420) Charges 100µF to 320V in 3.5 Seconds from 5V (LT3420-1) Charges Any Size Photoflash Capacitor Supports Operation from Two AA Cells or Any Supply from 1.8V to 16V Controlled Peak Switch Current: 1.4A (LT3420) 1.0A (LT3420-1) Controlled Input Current: 840mA (LT3420) 450mA (LT3420-1) Uses Standard Transformers Efficient Flyback Operation (>75% Typical) Adjustable Output Automatic Refresh Charge Complete Indicator No High Voltage Zener Diode Required No Output Voltage Divider Required Small 10-Lead MSOP Package APPLICATIO S s s s Digital Camera Flash Unit Film Camera Flash Unit High Voltage Power Supplies TYPICAL APPLICATIO VBAT 1.8V TO 6V C1 4.7µF 5,6 3,4 DANGER HIGH VOLTAGE OPERATION BY HIGH VOLTAGE TRAINED PERSONEL ONLY VCC 2.5V TO 10V 4 C2 4.7µF 9 8 51.1k 2 VBAT VCC LT3420 CHARGE DONE CT 10 0.1µF RREF GND 5 3 RFB 6 SW SEC T1 1:12 8 D1 320V VBAT 1.8V TO 6V C1 4.7µF DANGER HIGH VOLTAGE OPERATION BY HIGH VOLTAGE TRAINED PERSONEL ONLY 1 C3 220µF 7 330V PHOTOFLASH CAPACITOR 1 + VCC 2.5V TO 6V CHARGE DONE CHARGE DONE C2 4.7µF 9 8 2k C1, C2: 4.7µF, X5R or X7R, 10V C3: RUBYCON 220µF PHOTOFLASH CAPACITOR T1: TDK SRW10EPC-U01H003 FLYBACK TRANSFORMER D1: GENERAL SEMICONDUCTOR GSD2004S SOT-23 DUAL DIODE. DIODES CONNECTED IN SERIES 3420 F01 C1, C2: 4.7µF, X5R or X7R, 6.3V C3: RUBYCON 100µF PHOTOFLASH CAPACITOR T1: KIJIMA MUSEN SBL-5.6S-2 D1: GENERAL SEMICONDUCTOR GSD2004S SOT-23 DUAL DIODE. DIODES CONNECTED IN SERIES Figure 1. High Charge Rate LT3420 Photoflash Circuit Figure 2. Small Size LT3420-1 Photoflash Circuit 3420fa U 4 U U T1 1:10 4 5 D1 320V 3 60.4k 2 VBAT VCC LT3420-1 CHARGE DONE CT 10 0.1µF RREF GND 5 1 3 RFB 6 SW SEC 6 7 C3 100µF 330V PHOTOFLASH CAPACITOR + 2k 3420 F02 1 LT3420/LT3420-1 ABSOLUTE (Note 1) AXI U RATI GS PACKAGE/ORDER I FOR ATIO TOP VIEW RREF VBAT RFB VCC GND 1 2 3 4 5 10 9 8 7 6 CT CHARGE DONE SEC SW VCC Voltage .............................................................. 16V VBAT Voltage ............................................................ 16V SW Voltage (Note 2) LT3420 ................................................................ 38V LT3420-1 ............................................................ 50V SEC Current ...................................................... ±200mA RFB Current ........................................................... ±3mA RREF Voltage ........................................................... 2.5V CHARGE Voltage ...................................................... 16V CT Voltage .............................................................. 1.5V DONE Voltage .......................................................... 16V Current into DONE Pin .......................................... ±1mA Maximum Junction Temperature .......................... 125°C Operating Ambient Temperature Range (Note 3) .............................................. – 40°C to 85°C Storage Temperature Range ................. – 65°C to 150°C Lead Temperature (Soldering, 10 sec).................. 300°C ORDER PART NUMBER LT3420EMS LT3420EMS-1 MS PART MARKING LTYH LTAJG MS10 PACKAGE 10-LEAD PLASTIC MSOP TJMAX = 125°C, θJA = 100°C/W, θJC = 45°C/W (4-LAYER BOARD) Consult LTC Marketing for parts specified with wider operating temperature ranges. The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VCC = VBAT = 3.3V, VCHARGE = VCC unless otherwise noted. (Note 3) PARAMETER Minimum Operating Voltage, VCC Maximum Operating Voltage, VCC VCC UVLO Hysteresis Minimum VBAT Voltage Maximum VBAT Voltage VBAT UVLO Hysteresis RREF Threshold Voltage q ELECTRICAL CHARACTERISTICS CONDITIONS q MIN TYP 2.2 40 1.6 275 MAX 2.5 16 1.8 16 UNITS V V mV V V mV V V µA µA µA A A mA mA ns ns 0.98 0.975 1.00 2 90 0.01 1.02 1.025 4 130 1 1.60 1.05 50 25 RREF Pin Bias Current Quiescent Current Quiescent Current in Shutdown Primary Side Current Limit Secondary Side Current Limit Leakage Blanking Pulse Width Refresh Timer Charge/Discharge Current Refresh Timer Upper Threshold Refresh Timer Lower Threshold VRREF = 0V, Switching VRFB = VBAT – 0.2V (Note 4) VRREF = 1.1V, Not Switching VCHARGE = 0V, VIN = 3.3V LT3420 (Note 5) LT3420-1 (Note 5) LT3420 (Note 5) LT3420-1 (Note 5) LT3420 LT3420-1 VCT = 0.75V 1.5 0.9 0.45 1.20 0.75 20 5 1.4 0.9 40 15 200 0 2.5 1.0 0.5 3.5 1.1 0.55 3420fa 2 U µA V V W U U WW W LT3420/LT3420-1 The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VCC = VBAT = 3.3V, VCHARGE = VCC unless otherwise noted. (Note 3) PARAMETER Switch VCESAT Switch Leakage Current CHARGE Input Voltage High CHARGE Input Voltage Low CHARGE Pin Bias Current DONE Output Signal High DONE Output Signal Low VCHARGE = 3V VCHARGE = 0V 100k from VCC to DONE 33µA into DONE Pin 4.5 0.01 3.3 100 200 CONDITIONS LT3420, SW = 1A LT3420-1, SW = 0.5A VSW = 38V (LT3420), VSW = 50V (LT3420-1) 1.5 0.2 15 0.1 MIN TYP 220 130 0.01 MAX 340 230 1 UNITS mV mV µA V V µA µA V mV ELECTRICAL CHARACTERISTICS Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: Rated breakdown with LT3420 in power delivery mode and power switch off. Note 3: The LT3420/LT3420-1 are guaranteed to meet performance specifications from 0°C to 70°C. Specifications over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. Note 4: Bias current flows out of RFB pin. Note 5: Current limit guaranteed by design and/or correlation to static test. TYPICAL PERFOR A CE CHARACTERISTICS Graphs apply to both the LT3420 and LT34201-1 unless otherwise noted. Output Voltage in Refresh Mode, LT3420 335 330 325 320 VOUT (V) VOUT (V) TIME (s) 315 310 305 300 295 –50 –25 0 25 FIGURE 1 CIRCUIT VCC = 3.3V VBAT = 3.3V 50 75 100 125 TEMPERATURE (°C) 3420 G01 UW Output Voltage in Refresh Mode, LT3420 335 330 8 325 320 315 310 305 300 295 2.5 3.0 3.5 4.0 FIGURE 1 CIRCUIT VCC = VIN VBAT = VIN TA = 25°C 4.5 5.0 5.5 6.0 VIN (V) 3420 G02 Charge Time, LT3420 10 FIGURE 1 CIRCUIT UNLESS OTHERWISE NOTED. VOUT CHARGED FROM 50V TO 320V TA = 25°C 6 COUT = 220µF 4 COUT = 100µF 2 0 2 3 4 VBAT (V) 5 6 3420 G03 3420fa 3 LT3420/LT3420-1 TYPICAL PERFOR A CE CHARACTERISTICS Graphs apply to both the LT3420 and LT34201-1 unless otherwise noted. Output Voltage in Refresh Mode, LT3420-1 335 330 325 320 VOUT (V) VOUT (V) TIME (s) 315 310 305 300 295 –50 –25 0 25 FIGURE 2 CIRCUIT VCC = 3.3V VBAT = 3.3V 50 75 100 125 TEMPERATURE (°C) 3420 G04 Charge Pin Input Current 10 TA = 25°C 8 CURRENT (µA) CURRENT (mA) CURRENT (A) 6 4 2 25 0 2 3 6 5 7 4 8 CHARGE PIN VOLTAGE (V) 9 10 Efficiency of Figure 1 Circuit, LT3420 90 TA = 25°C VIN = 5V 1.2 80 EFFICIENCY (%) 70 1.0 CURRENT (mA) VIN = 3.3V CURRENT (A) 60 50 VCC = VBAT = VIN 40 50 100 150 250 200 VOUT (V) 300 350 3420 G10 4 UW 3420 G07 Output Voltage in Refresh Mode, LT3420-1 335 330 325 320 315 310 305 300 295 2.5 3.0 3.5 4.0 FIGURE 2 CIRCUIT VCC = VIN VBAT = VIN TA = 25°C 4.5 5.0 5.5 6.0 VIN (V) 3420 G05 Charge Time, LT3420-1 10 FIGURE 2 CIRCUIT VOUT CHARGED FROM 50V TO 320V TA = 25°C 8 6 COUT = 100µF 4 COUT = 40µF 2 0 2 3 4 VBAT (V) 3420 G06 5 6 Primary Current Limit, LT3420 1.7 60 55 Secondary Current Limit, LT3420 1.5 50 45 40 35 30 1.3 1.1 0.9 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 20 –50 –25 0 25 50 75 100 125 TEMPERATURE (°C) 3420 G08 3420 G09 Primary Current Limit, LT3420-1 35 30 1.1 Secondary Current Limit, LT3420-1 25 20 15 10 0.9 0.8 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 5 –50 –25 0 25 50 75 100 125 TEMPERATURE (°C) 3420 G11 3420 G12 3420fa LT3420/LT3420-1 TYPICAL PERFOR A CE CHARACTERISTICS Graphs apply to both the LT3420 and LT34201-1 unless otherwise noted. Efficiency for Figure 2 Circuit, LT3420-1 90 TA = 25°C AVERAGE INPUT CURRENT (mA) 80 VIN = 5V VIN = 3.3V 900 AVERAGE INPUT CURRENT (mA) EFFICIENCY (%) 70 60 50 VCC = VBAT = VIN 100 150 250 200 VOUT (V) 300 350 3420 G13 40 50 Quiescent Current in Refresh Mode 140 TA = 25°C QUIESCENT CURRENT (µA) VBAT PIN VOLTAGE (V) VCC PIN VOLTAGE (V) 120 100 80 60 2.5 4.0 5.5 7.0 VCC (V) 3420 G16 UW 8.5 10 Input Current, LT3420 1000 FIGURE 1 CIRCUIT VCC = VBAT = 3.3V TA = 25°C 600 550 500 450 400 350 300 Input Current, LT3420-1 FIGURE 2 CIRCUIT VCC = VBAT = 3.3V TA = 25°C 800 700 600 500 50 100 150 250 200 VOUT (V) 300 350 3420 G14 50 100 150 200 250 VOUT (V) 300 350 3420 G15 VCC Minimum Operating Voltage 2.6 2.5 2.4 2.3 2.2 2.1 2.0 1.9 1.8 –50 –25 0 25 50 75 100 125 V– V+ ENABLE VOLTAGE IS HYSTERETIC 2.0 VBAT Minimum Operating Voltage ENABLE VOLTAGE IS HYSTERETIC V+ 1.6 V– 1.8 1.4 1.2 1.0 –50 –25 0 25 50 75 100 125 TEMPERATURE (°C) 3420 G17 TEMPERATURE (°C) 3420 G18 3420fa 5 LT3420/LT3420-1 PI FU CTIO S RREF (Pin 1): Reference Resistor Pin. Place a resistor (R2) from the RREF pin to GND. 2k is recommended. VBAT (Pin 2): Battery Voltage Input. This pin should be connected to the power supply or battery, which supplies power to transformer T1. Must be locally bypassed. RFB (Pin 3): Feedback Resistor Pin. Place a resistor (R1) from the SW pin to the RFB pin. Set R1 according to the following formula: R1 = [(1.4 • R N R2 2 SEC ) + N(VOUT R1 = R2 [(RSEC ) + N(VOUT + 2VD )] (LT3420-1) N2 VOUT : Desired Output Voltage N: Transformer Turns Ratio RSEC: Transformer Secondary Resistance VD: Diode Forward Voltage Drop R2: Resistor from the RREF Pin to GND. 2k is a Typical Choice VCC (Pin 4): Input Supply Pin. Must be locally bypassed with a 4.7µF or larger ceramic capacitor. 6 U U U GND (Pin 5): Ground. Tie directly to local ground plane. SW (Pin 6): Switch Pin. This is the collector of the internal NPN power switch. Minimize the metal trace area connected to this pin to minimize EMI. SEC (Pin 7): Transformer Secondary Pin. Tie one end of the transformer secondary to this pin. Take care to use the correct phasing of the transformer (Refer to Figures 1 and 2). DONE (Pin 8): Done Output Pin. Open collector NPN output. DONE is pulled low whenever the chip is delivering power to the output and goes high when power delivery stops. CHARGE (Pin 9): Charge Pin. Drive CHARGE high (1.5V or more) to commence charging of the output capacitor. Drive to 0.2V or less to put the part in shutdown mode. CT (Pin 10): Refresh Timer Capacitor Pin. Place a capacitor from the CT pin to GND to set the refresh timer sample rate according to the following formula: CT = 2.5 • 10 –6 • tREFRESH tREFRESH: Desired Refresh Period in Seconds. + 2VD ) (LT3420) ] 3420fa LT3420/LT3420-1 BLOCK DIAGRA S VBAT C1 PRIMARY R1 R2 DONE 8 2 VBAT 3 RFB 1 RREF 6 C4 PHOTOFLASH CAPACITOR R Q3 S Q DRIVER ONESHOT Q1 SW T1 1:12 D1 SECONDARY VOUT CT C3 10 REFRESH TIMER Q S R A3 A2 CHARGE 9 ONESHOT VCC CHIP ENABLE POWER DELIVERY BLOCK VCC C2 4 Figure 3. Block Diagram, LT3420 VBAT C1 PRIMARY R1 R2 DONE 8 2 VBAT 3 RFB 1 RREF 6 C4 PHOTOFLASH CAPACITOR R Q3 S Q DRIVER ONESHOT Q1 SW T1 1:10 D1 SECONDARY VOUT D3 CT C3 ENABLE 10 REFRESH TIMER Q5 Q S R A3 A2 CHARGE 9 ONESHOT VCC CHIP ENABLE POWER DELIVERY BLOCK VCC C2 4 Figure 4. Block Diagram, LT3420-1 3420fa + 1V REFERENCE – + MASTER LATCH Q Q4 – BLOCK ENABLE A1 + 1V REFERENCE – + + MASTER LATCH Q Q4 – BLOCK ENABLE Q2 A1 + – – W + D3 Q5 ENABLE 20mV 0.014Ω +– 5 GND 10mV 0.25Ω 7 SEC +– LT3420 3420 F03 + 20mV 0.02Ω +– 5 GND 10mV 0.66Ω 7 SEC +– LT3420-1 3420 F04 7 LT3420/LT3420-1 OPERATIO Overview The following text focuses on the operation of the LT3420. The operation of the LT3420-1 is nearly identical with the differences discussed at the end of this section. The LT3420 uses an adaptive on-time/off-time control scheme to provide excellent efficiency and precise control of switching currents. Please refer to Figure 3 for the following overview of the part’s operation. At any given instant, the master latch determines which mode the LT3420 is in: “charging” or “refresh”. In charging mode, the circuitry enclosed by the smaller dashed box is enabled, providing power to charge photoflash capacitor C1. The output voltage is monitored via the flyback pulse on the primary of the transformer. When the target output voltage is reached, the charging mode is terminated and the part enters the refresh mode. In refresh mode, the power delivery block is disabled, reducing quiescent current, while the refresh timer is enabled. The refresh timer simply generates a user programmable delay, after which the part reenters the charging mode. Once in the charging mode, the LT3420 will again provide power to the output until the target voltage is reached. Figure 5 is an oscillograph photo showing both the initial charging of the photoflash capacitor and the subsequent refresh action. The upper waveform is the output voltage. The middle waveform is the voltage on the CT pin. The lower waveform shows the input current. The mode of the part is indicated below the photo. The user can defeat the refresh timer and force the part into charging mode by toggling the CHARGE pin VOUT 100V/DIV VCT 1V/DIV IIN 1A/DIV MODE SHUTDOWN Figure 5. Demonstrating 3 Operating Modes of LT3420: Shutdown, Charging and Refresh of Photoflash Capacitor 8 U (high→low→high). The low to high transition on the CHARGE pin fires a one-shot that sets the master latch, putting the part in charging mode. Bringing CHARGE low puts the part in shutdown. The refresh timer can be programmed to wait indefinitely by simply grounding the CT pin. In this configuration, the LT3420 will only reenter the charging mode by toggling the CHARGE pin. Power Delivery Block The power delivery block consists of all circuitry enclosed by the smaller dashed box in Figure 3. This circuit block contains all elements needed for charging and output voltage detection. To better understand the circuit operation, follow the subsequent description of one cycle of operation and refer to Figure 6. Assume that initially there is no current in the primary or secondary of the transformer, so the output of comparator A1 is low, while that of A2 is high (note the small offset voltages at the inputs of A1 and A2). The SR latch is thus set and the power NPN switch, Q1, is turned on. Current increases linearly in the primary of the transformer at a rate determined by the VBAT voltage and the primary inductance of the transformer. As the current builds up, the voltage across the 14mΩ resistor increases. When this voltage exceeds the 20mV offset voltage of A1, the output of A1 goes high, resetting the SR latch and turning off Q1. The current needed to reset the latch is approximately 1.4A (~20mV/14mΩ). When Q1 turns off, the secondary side current quickly jumps from zero current to the primary side current divided by N (the turns ratio of transformer T1). In this example, the peak secondary current is 116mA (1.4A/12). Diode D1 now conducts, providing power to the output. Since a positive voltage exists across the secondary winding of the transformer, the secondary current decreases linearly at a rate determined by the secondary inductance and the output voltage (neglecting the diode voltage drop). When the secondary side current drops below 40mA (10mV/0.25Ω), the output of A2 goes high, setting the SR latch and turning on Q1. The initial primary current is simply the minimum secondary current times N, in this case 0.48A (40mA • 12) . Q1 will now remain on until the primary current again reaches 1.4A. This cycle of operation repeats itself, automatically adjusting the On and Off times 3420fa CHARGING 1s/DIV REFRESH 3420 F05 LT3420/LT3420-1 OPERATIO ISW 1A/DIV ISEC 200mA/DIV VSW 20V/DIV 2µs/DIV 3420 F06a Figure 6a. Switching Waveforms with VOUT = 100V, VCC = VBAT = 3.3V of Q1 so that the peak current of Q1 is 1.4A and the minimum secondary current is 40mA (typical values). The previously described charging cycle must be halted when the output voltage reaches the desired value. The LT3420 monitors the output voltage via the flyback pulse on the SW pin. When Q1 turns off, the secondary side conducts current turning on diode D1. Since the diode is conducting and the SEC pin is at nearly ground, the voltage across the secondary is nearly equal to VOUT. The voltage across the primary is therefore close to VOUT/N. A current proportional to VOUT/N flows through R1 and into the RFB pin. The current flows out of the RREF pin through a resistor creating a ground referred voltage. When this voltage exceeds an internal 1V reference voltage, the output of comparator A3 goes high which resets the master latch. The Q output of the master latch goes low, disabling the entire power delivery block and enabling the refresh timer. Leakage Spike Blanking Another function of the LT3420 is leakage spike blanking when the power switch, Q1, turns off. Right after Q1 turns off, a one-shot turns on Q2 for 200ns (typ). With Q2 on, comparator A3 is disabled. This function may prevent A3 from false tripping on the leakage inductance spike on the SW pin. In practice, the PNP transistor Q3 filters out the leakage spike. Refresh Timer When the refresh timer is enabled, a 2.5µA current source is switched on, charging up the external timing capacitor, U ISW 1A/DIV ISEC 200mA/DIV VSW 20V/DIV 2µs/DIV 3420 F06b Figure 6b. Switching Waveforms with VOUT = 300V, VCC = VBAT = 3.3V C3, from its initial voltage towards 1V. When the voltage on C3 reaches 1V, the polarity of the current source changes and 2.5µA discharges C3. When the voltage on C3 reaches 0.5V, the refresh timer sends a set pulse to the master latch, which puts the LT3420 into the charging mode. Interface/Control The CHARGE pin serves two functions. The first is to enable or shutdown the part depending on the level of the pin (high = enable, low = shutdown). The second is to force the part into the charging mode (low→high transition). The LT3420 also has a DONE pin, which signals whether or not the part is done charging the photoflash capacitor. The DONE pin is an open collector NPN switch (Q5) so an external pull-up resistor is needed. Whenever the part is in charging mode, DONE will be low. DONE will go high when the charging mode is complete. Both the CHARGE and DONE pins can be easily interfaced to a microprocessor in a digital or film camera. LT3420-1 Differences The LT3420-1 has different primary and secondary current limit levels. The primary current limit level of the LT3420-1 is 1A (typ) and the secondary current limit is 15mA (typ). The LT3420-1 has no leakage spike blanking which causes no problems since the PNP transistor, Q3, provides adequate filtering. Finally, the breakdown voltage of the SW pin of the LT3420-1 is higher at 50V. 3420fa 9 LT3420/LT3420-1 APPLICATIO S I FOR ATIO COMPONENT SELECTION Choosing the Right Transformer The flyback transformer plays a key role in any LT3420/ LT3420-1 application. A poorly designed transformer can result in inefficient operation. Linear Technology Corporation has worked with a number of transformer manufacturers to develop specific transformers for use with the LT3420/LT3420-1. These predesigned transformers are sufficient for a large majority of the applications that may be encountered. In some cases, the reader may choose to design his own transformer or may simply be curious about the issues involved in designing the transformer. The following is a brief discussion of the issues relating to transformer design. Transformer Turns Ratio The turns ratio for the transformer, N, should be high enough so that the absolute maximum voltage rating for the NPN power switch is not exceeded. When the power switch turns off, the voltage on the collector of the switch (SW Pin) will “fly” up to the output voltage divided by N plus the battery voltage (neglecting the voltage drop across the rectifying diodes). This voltage should not exceed the 38V (LT3420) or 50V (LT3420-1) breakdown rating of the power switch. Choose the minimum N by the following formula. VOUT (LT3420) 38 – VBAT VOUT NMIN ≥ (LT3420 − 1) 50 – VBAT NMIN ≥ For an LT3420 design, a 5V battery voltage and a 330V output results in a NMIN of 10 so a turns ratio of 10 or greater should be used. 10 U Transformer Primary Inductance A flyback transformer needs to store substantial amounts of energy in the core during each switching cycle. The transformer, therefore, will generally require an air gap. The use of an air gap in the core makes the energy storage ability, or inductance, much more stable with temperature and variations in the core material. Most core manufacturers will supply standard sizes of air gaps with a given type of core, resulting in different AL values. AL is the inductance of a particular core per square turns of winding. To get a certain inductance, simply divide the desired inductance by the AL value and take the square root of the result to find the number of turns needed on the primary of the transformer. The LT3420/LT3420-1 detect the output voltage via the flyback pulse on the SW pin. Since this can only occur while the power switch is off, an important criteria is that the value of the primary inductance of the transformer be larger than a certain minimum value. The switch off time should be 500ns or larger for the LT3420 and 350ns or larger for the LT3420-1. The minimum inductance can be calculated with the following formula: LPRI ≥ LPRI ≥ 500 • 10 –9 • VOUT N • (1.4 – 0.04N) (LT3420) 350 • 10 –9 • VOUT (LT3420 − 1) N • (1.0 – 0.015N) W UU VOUT: Target Output Voltage N: Transformer Turns Ratio Transformer Leakage Inductance The leakage inductance of the transformer must be carefully minimized for both proper and efficient operation of the part. The DC voltage rating of the SW pin on the LT3420 is 38V while on the LT3420-1 it is 50V. These ratings are for DC blocking voltages only and additional precautions 3420fa LT3420/LT3420-1 APPLICATIO S I FOR ATIO must be taken into account for the dynamic blocking voltage capabilities of the LT3420/LT3420-1. The dynamic blocking voltage capability of both parts is 38V. Table 1 summarizes the various breakdown voltages of the SW pin for both parts. Table 1. SW Pin Voltage Ratings PART LT3420 LT3420-1 SW PIN DC RATING 38V 50V SW PIN DYNAMIC RATING 38V 38V Figure 7 shows what to examine in a new transformer design to determine if the specifications for the SW pin are met. The first leakage inductance spike labeled “A” must not exceed the dynamic rating of the SW pin. If it does exceed the rating, then the transformer leakage inductance must be lowered. The flyback waveform after the initial spike labeled “B” must not exceed the DC rating of the SW pin. If it does exceed the rating, then the turns ratio of the transformer must be lowered. In measuring the voltage on the SW pin, care must be taken in minimizing the ground loop of the voltage probe. Careless probing will result in inaccurate readings. IPRI 0A VSW 0V 3420 F07 Figure 7. New Transformer Design Check (Not to Scale) 3420fa U Note also the magnitude of the initial current spike in the primary of the transformer labeled “C” when the power switch turns on. If the leakage inductance is lowered to a very low level, the internal capacitances of the transformer will be high. This will result in the initial spike of current in the primary becoming excessively high. The level of “C” should be kept to 4A or less in a typical design for both the LT3420 and LT3420-1. Please note that by inserting a loop of wire in the primary to measure the primary current, the leakage inductance of the primary will be made artificially high. This may result in erroneous voltage measurements on the SW pin. The measurements shown in Figure 7 should be made with both VOUT and VBAT at the maximum levels for the given application. This results in the highest voltage and current stress on the SW pin. Transformer Secondary Capacitance The total capacitance of the secondary should be minimized for both efficient and proper operation of the LT3420/ LT3420-1. Since the secondary of the transformer undergoes large voltage swings (approaching 600VP-P), any capacitance on the secondary can severely affect the MUST BE LESS THAN 4A FOR BOTH THE LT3420 AND LT3420-1 “C” “B” “A” MUST BE LESS THAN 38V FOR BOTH THE LT3420 AND LT3420-1 MUST BE LESS THAN 38V FOR THE LT3420 MUST BE LESS THAN 50V FOR THE LT3420-1 W UU 11 LT3420/LT3420-1 APPLICATIO S I FOR ATIO efficiency of the circuit. In addition, the effective capacitance on the primary is largely dominated by the actual secondary capacitance. This is simply a result of any secondary capacitance being multiplied by N2 when reflected to the primary. Since N is generally 10 or higher, a small capacitance of 10pF on the secondary is 100 times larger, or 1.0nF, on the primary. This capacitance forms a resonant circuit with the primary leakage inductance of the transformer. As such, both the primary leakage inductance and secondary side capacitance should be minimized. Table 2 shows various predesigned transformers along with relevant parameters. Contact the individual transformer manufacturer for additional information or customization. Table 2a. Predesigned Transformers, LT3420 PART SRW10EPC -U01H003 6375-T108 TURNS L SIZE RATIO (µH) LxWxH (mm) VENDOR 1:12 24 10.9x10.8x5.2 TDK (847) 803-6100 www.components.tdk.com 1:12 15 10.8x9.5x3.6 Sumida (847) 956-0666 www.sumida.com 1:12 24 9.1x9.1x5.1 Pulse (858) 674-8100 www.pulseeng.com 1:12 17.5 10.3x6.4x5.2 Kijima Musen 852-2489-8266 kijimahk@netvigator.com PA0367 SBL-6.4 Table 2b. Predesigned Transformers, LT3420-1 PART SBL-5.6S-2 TURNS L RATIO (µH) 1:10 15 SIZE LxWxH (mm) VENDOR 5.6x8.5x3.0 Kijima Musen 852-2489-8266 kijimahk@netvigator.com 5.8x5.8x3.0 TDK (847) 803-6100 www.components.tdk.com LDT565630T 1:10.2 14.5 -002 12 U DIODE SELECTION The rectifying diode(s) should be low capacitance type with sufficient reverse voltage and forward current ratings. The peak reverse voltage that the diode(s) will see is approximately: VPK-R ≈ ( VOUT + (N • VBAT )) • 1.65 The peak current of the diode is simply: 1.4A (LT3420) N 1.0 A IPK-SEC = (LT3420 − 1) N IPK-SEC = For the circuit of Figure 1 with VBAT of 3.3V, VPK-R is 590V and IPK-SEC is 116mA. Table 3 shows various diodes that can work with the LT3420/LT3420-1. These are chosen for low capacitance and high reverse blocking voltage. Use the appropriate number of diodes to achieve the necessary reverse breakdown voltage. Table 3 MAX REVERSE CAPACITANCE VOLTAGE (V) (pF) VENDOR 2x300 5 Vishay (402) 563-6866 www.vishay.com BAS21 250 1.5 Philips Semiconductor (Single diode) (800) 234-7381 www.philips.com ISS306 2x250 3 Toshiba (Dual Diode) (949) 455-2000 www.semicon. toshiba.co.jp PART GSD2004S (Dual diode) 3420fa W U U LT3420/LT3420-1 APPLICATIO S I FOR ATIO CAPACITOR SELECTION The VBAT and VCC decoupling capacitors should be multilayer ceramic type with X5R or X7R dielectric. This insures adequate decoupling across wide ambient temperature ranges. A good quality ceramic capacitor is also recommended for the timing capacitor on the CT pin. Avoid Y5V or Z5U dielectrics. Selectively Disabling the LT3420/LT3420-1 The LT3420/LT3420-1 can be disabled at any time, even during the charge phase. This may be useful when a digital camera enters a sensitive data acquisition phase. Figure 8 illustrates this feature. Midway through the charge cycle, the CHARGE pin is brought low, which disables the part. After the sensitive data operation is complete, the CHARGE pin is brought high and the charging operation continues. Measuring Efficiency Measuring the efficiency of a circuit designed to charge large capacitive loads is a difficult issue, particularly with photoflash capacitors. The ideal way to measure the efficiency of a capacitor charging circuit would be to find the energy delivered to the output capacitor (0.5 • C • V 2) and divide it by the total input energy. This method does not work well here because photoflash capacitors are far from ideal. Among other things, they have relatively high leakage currents, large amounts of dielectric absorption, VOUT 50V/DIV VCHARGE CHARGE NO CHARGE Figure 8. Halting the Charge Cycle at Any Time U and significant voltage coefficients. A much more accurate, and easier, method is to measure the efficiency as a function of the output voltage. In place of the photoflash capacitor, use a smaller, high quality capacitor, reducing errors associated with the non-ideal photoflash capacitor. Using an adjustable load, the output voltage can be set anywhere between ground and the maximum output voltage. The efficiency is measured as the output power (VOUT • IOUT) divided by the input power (VIN • IIN). This method also provides a good means to compare various charging circuits since it removes the variability of the photoflash capacitor from the measurement. The total efficiency of the circuit, charging an ideal capacitor, would be the time average of the given efficiency curve, over time as VOUT changes. Adjustable Input Current With many types of modern batteries, the maximum allowable current that can be drawn from the battery is limited. This is generally accomplished by active circuitry or a polyfuse. Different parts of a digital camera may require high currents during certain phases of operation and very little at other times. A photoflash charging circuit should be able to adapt to these varying currents by drawing more current when the rest of the camera is drawing less, and vice-versa. This helps to reduce the charge time of the photoflash capacitor, while avoiding the 5V/ DIV 0.5s/DIV 3420 F08 W UU 3420fa 13 LT3420/LT3420-1 APPLICATIO S I FOR ATIO risk of drawing too much current from the battery. The input current to the LT3420/LT3420-1 circuit can be adjusted by driving the CHARGE pin with a PWM (pulse width modulation) signal. The microprocessor can adjust the duty cycle of the PWM signal to achieve the desired level of input current. Many schemes exist to achieve this function. Once the target output voltage is reached, the PWM signal should be halted to avoid overcharging the photoflash capacitor, since the signal at the CHARGE pin overrides the refresh timer. A simple method to achieve adjustable input current is shown in Figure 9. The PWM signal has a frequency of 1kHz PWM SIGNAL A2 ON A3 Figure 9. Simple Logic for Adjustable Input Current 800 INPUT CURRENT (mA) 600 400 LT3420-1 200 0 10 30 50 70 DUTY CYCLE (%) 90 3420 F10 Figure 10. Input Current as Duty Cycle is Varied 14 U 1kHz. When ON is logic high, the circuit is enabled and the CHARGE pin is driven by the PWM signal. When the target output voltage is reached, DONE goes high while CHARGE is also high. The output of A1 goes high, which forces CHARGE high regardless of the PWM signal. The part is now in the Refresh mode. Once the refresh period is over, the DONE pin goes low, allowing the PWM signal to drive the CHARGE pin once again. This function can be easily implemented in a microcontroller. Figure 10 shows the input current for the LT3420 and LT3420-1 as the duty cycle of the PWM signal is varied. A1 DONE CHARGE TO LT3420 CIRCUIT 3420 F09 W UU LT3420 3420fa LT3420/LT3420-1 APPLICATIO S I FOR ATIO BOARD LAYOUT The high voltage operation of the LT3420/LT3420-1 demands careful attention to board layout. You will not get advertised performance with careless layout. Figure 11 shows the recommended component placement. Keep the area for the high voltage end of the secondary as small as possible. Note the larger than minimum spacing for all high voltage nodes. This is necessary to R2 C3 PHOTOFLASH CAPACITOR VCC C2 GND R1 C1 VOUT T1 VBAT D1A 3420 F11 Figure 11. Suggested Layout + – U meet the breakdown specifications for the circuit board. If the Photoflash capacitor is placed far from the LT3420/ LT3420-1 circuit, place a small (20nF-50nF) ceramic capacitor with sufficient voltage rating close to the part. This insures adequate bypassing. Remember that LETHAL VOLTAGES ARE PRESENT in this circuit. Use caution when working with the circuit. CHARGE DONE D1B 3420fa W UU 15 LT3420/LT3420-1 TYPICAL APPLICATIO S Professional Charger uses Multiple LT3420 Circuits in Parallel to Charge Large Photoflash Capacitors Quickly VBAT 1.8V TO 6V C1 4.7µF DANGER HIGH VOLTAGE OPERATION BY HIGH VOLTAGE TRAINED PERSONEL ONLY VCC 2.5V TO 10V CHARGE D1 3 4 T1 1:12 1 320V C1, C2, C4, C5, C6, C7: 4.7µF, X5R or X7R, 10V T1-T3: PULSE PA0367 FLYBACK TRANSFORMER D1-D3: GENERAL SEMICONDUCTOR GSD2004S SOT-23 DUAL DIODE. DIODES CONNECTED IN SERIES Q1: 2N3904 OR EQUIVALENT * CAN CHARGE ANY SIZE PHOTOFLASH CAPACITOR ** USE AS MANY SLAVE CHARGERS AS NEEDED. 16 U + 2 R1 52.3k 3 RFB 650µF* 350V PHOTOFLASH CAPACITOR 2 VBAT 6 SW 4 VCC SEC 7 C2 MASTER 4.7µF LT3420 CHARGER 9 CHARGE 1 8 RREF DONE GND CT R2 2k 10 5 C3 0.1µF D2 VBAT C4 4.7µF 2 VBAT VCC R3 100k R4 100k 3 RFB 3 2 6 SW 4 T2 1:12 1 4 VCC SEC 7 C5 4.7µF SLAVE** LT3420 CHARGER 9 CHARGE 1 8 RREF DONE GND CT 10 5 D3 VBAT C6 4.7µF 2 VBAT VCC 3 RFB 3 2 6 SW 4 T3 1:12 1 Q1 2N3904 4 VCC SEC 7 C7 SLAVE** 4.7µF LT3420 CHARGER 9 CHARGE 1 8 RREF DONE GND CT 10 5 3420 TA01 3420fa LT3420/LT3420-1 TYPICAL APPLICATIO S LT3420 Photoflash Charging Circuit uses Low Cost Toroid Transformer DANGER HIGH VOLTAGE OPERATION BY HIGH VOLTAGE TRAINED PERSONEL ONLY VBAT 1.8V TO 6V C1 4.7µF T1 1:12 3 4 D1 90 2 R1 52.3k 3 RFB VCC 2.5V TO 10V 4 C2 4.7µF 9 8 VCC LT3420 CHARGE DONE CT 10 C3 0.1µF SEC C4 100µF 7 330V PHOTOFLASH CAPACITOR 1 + EFFICIENCY (%) 2 VBAT 6 SW CHARGE DONE RREF GND 5 C1, C2: 4.7µF, X5R or X7R, 10V C4: RUBYCON 100µF PHOTOFLASH CAPACITOR T1: PULSE PA0367 FLYBACK TRANSFORMER D1: GENERAL SEMICONDUCTOR GSD2004S SOT-23 DUAL DIODE. DIODES CONNECTED IN SERIES Charging Waveform 10 CHARGE TIME (s) VOUT 50V/DIV VCHARGE 5V/DIV VBAT = 3.3V 0.5s/DIV 3420 TA02c U 320V Efficiency VIN = 5V VIN = 3.3V 70 1 80 60 50 R2 2k 40 50 3420 TA02a VCC = VBAT = VIN 100 150 200 VOUT (V) 3420 TA02b 250 300 350 Charge Time VOUT CHARGED FROM 50V TO 320V 8 6 COUT = 220µF 4 COUT = 100µF 2 0 2 4 6 VBAT (V) 3420 TA02d 8 10 3420fa 17 LT3420/LT3420-1 TYPICAL APPLICATIO S LT3420 Photoflash Charging Circuit Uses Small Transformer DANGER HIGH VOLTAGE OPERATION BY HIGH VOLTAGE TRAINED PERSONEL ONLY VBAT 1.8V TO 5V C1 4.7µF VCC 2.5V TO 10V C1: 4.7µF, X5R or X7R, 6.3V C2: 4.7µF, X5R or X7R, 10V C4: RUBYCON 220µF PHOTOFLASH CAPACITOR 3420 TA03 D1: GENERAL SEMICONDUCTOR GSD2004S SOT-23 DUAL DIODE. DIODES CONNECTED IN SERIES T1: KIJIMA MUSEN SBL-6.4 * MAXIMUM AMBIENT TEMPERATURE OF 60°C DICTATED BY TRANSFORMER EFFICIENCY (%) 18 U T1* 1:12 5,6 8 D1 300V 3,4 R1 47.5k 3 RFB 1 4 C2 4.7µF 9 8 2 VBAT VCC 6 SW SEC 7 LT3420 CHARGE DONE CT 10 C3 0.1µF RREF GND 5 1 C4 220µF 330V PHOTOFLASH CAPACITOR + CHARGE DONE R2 2k Efficiency 90 VCC = VBAT = VIN 80 VIN = 5V 70 VIN = 3.3V 60 50 40 50 100 150 200 250 VOUT (V) 300 350 3420 TA04 3420fa LT3420/LT3420-1 PACKAGE DESCRIPTIO 5.23 (.206) MIN 0.50 0.305 ± 0.038 (.0197) (.0120 ± .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT 0.254 (.010) GAUGE PLANE 0.18 (.007) SEATING PLANE 0.17 – 0.27 (.007 – .011) TYP 0.127 ± 0.076 (.005 ± .003) MSOP (MS) 0603 NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. U MS Package 10-Lead Plastic MSOP (Reference LTC DWG # 05-08-1661) 0.889 ± 0.127 (.035 ± .005) 3.20 – 3.45 (.126 – .136) 3.00 ± 0.102 (.118 ± .004) (NOTE 3) 10 9 8 7 6 0.497 ± 0.076 (.0196 ± .003) REF DETAIL “A” 0° – 6° TYP 12345 0.53 ± 0.152 (.021 ± .006) DETAIL “A” 1.10 (.043) MAX 0.86 (.034) REF 4.90 ± 0.152 (.193 ± .006) 3.00 ± 0.102 (.118 ± .004) (NOTE 4) 0.50 (.0197) BSC 3420fa 19 LT3420/LT3420-1 TYPICAL APPLICATIO VBAT 1.8V TO 6V C1 4.7µF LT3420-1 Photoflash Circuit Uses Tiny (3mm Tall) Transformer T1 1:10.2 5 4 D1 8 DANGER HIGH VOLTAGE OPERATION BY HIGH VOLTAGE TRAINED PERSONEL ONLY VCC 2.5V TO 6V 4 C2 4.7µF 9 8 60.4k 2 VBAT VCC LT3420-1 CHARGE DONE CT 10 0.1µF C1, C2: 4.7µF, X5R or X7R, 6.3V C3: RUBYCON 100µF PHOTOFLASH CAPACITOR T1: TDK LDT565630T-002 FLYBACK TRANSFORMER D1: GENERAL SEMICONDUCTOR GSD2004S SOT-23 DUAL DIODE. DIODES CONNECTED IN SERIES RREF GND 5 1 3 RFB 6 SW SEC TIME (s) CHARGE DONE RELATED PARTS PART NUMBER LTC 3400/LTC3400B LTC3401/LTC3402 LTC3405/LTC3405A LTC3406/LTC3406B LTC3407 LTC3411 LTC3425 LTC3440/LTC3441 LT3464 ® DESCRIPTION 600mA (ISW), 1.2MHz, Synchronous Step-Up DC/DC Converters 1A/2A (ISW), 3MHz, Synchronous Step-Up DC/DC Converters 300mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converters 600mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converters Dual 600mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter 1.25A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 5A (ISW), 8MHz, Multiphase Synchronous Step-Up DC/DC Converter 600mA/1A (IOUT), 2MHz/1MHz, Synchronous Buck-Boost DC/DC Converters 85mA (ISW), Constant Off-Time, High Efficiency Step-Up DC/DC Converter with Integrated Schottky and Output Disconnect 1.1A (ISW), 1.3MHz, High Efficiency Step-Up DC/DC Converter LTC3467 ThinsSOT is a trademark of Linear Technology Corporation. 3420fa 20 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 q FAX: (408) 434-0507 q www.linear.com U 320V Charge Time 10 VOUT CHARGED FROM 50V TO 320V 1 8 C3 100µF 7 330V PHOTOFLASH CAPACITOR + 6 COUT = 100µF 4 COUT = 40µF 2 2k 0 2 3420 TA05 3 4 VBAT (V) 5 6 3420 TA06 COMMENTS VIN = 0.85V to 5V, VOUT(MAX) = 5V, IQ = 19µA/300µA, ISD =