LT V Boost DC/DC Converter with APD Current Monitor U DESCRIPTIO FEATURES APPLICATIO S TYPICAL APPLICATIO

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1 FEATRES High Output Voltage: p to 9V Integrated Schottky Diodes 48V, 8mA Internal Switch High Side Current Monitor Adjustable Switching Frequency: 65kHz or.mhz Wide Range:.5V to 6V Surface Mount Components Low Shutdown Current: <µa Soft-Start Internal Compensation Pin Allows Output Adjustment with No Polarity Inversion 3mm 3mm 6-Lead QFN Package APPLICATIO S Bias PIN Diode Bias Optical Receivers and Modules Fiber Optic Network Equipment DESCRIPTIO LT348 9V Boost DC/DC Converter with Current Monitor The LT 348 is a fi xed frequency current mode step-up DC/DC converter with voltage doubler designed to bias avalanche photodiodes (s) in optical receivers. It can provide up to 9V output. The LT348 features high side current monitoring with better than % relative accuracy over the entire temperature range. The integrated power switch, Schottky diodes and current monitor allow a small converter footprint and low solution cost. Constant switching frequency results in predictable output noise that is easy to fi lter. The inductor-based topology ensures an input free from switching noise. An integrated high side current monitor produces a current proportional to current with better than % relative accuracy over four decades of dynamic range in the input range of 5nA to.5ma. This current can be used as a reference to provide a digitally programmed output voltage via the pin. The LT348 is available in the tiny footprint (3mm 3mm) 6-lead QFN package., LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. TYPICAL APPLICATIO 5V µf µh SW.µF PMP Output Voltage Ripple f SET LT348 OFF ON SHDN GND V OT V OT.47µF.47µF.µF M V RIPPLE mv/div MON 85V AT.5mA 4k nf k.µf 348 TAa 5ns/DIV 348 TAb

2 ABSOLTE AXI RATI GS W W W (Note ) Input Voltage ( )...6V V OT, SW Voltage...48V V OT, PMP,, Voltage...9V Voltage...5V SHDN, f SET, Voltage...6V MON Voltage...V Operating Temperature Range (Note )... 4 C to 85 C Maximum Junction Temperature... 5 C Storage Temperature Range C to 5 C PIN CONFIGRATION NC V OT 3 4 TOP VIEW MON fset VOT PMP SW SW SHDN GND 9 GND D PACKAGE 6-LEAD (3mm 3mm) PLASTIC QFN T JMAX = 5 C, θ JA = 68 C/W, θ JC = 4. C/W EXPOSED PAD (PIN 7) IS GND, MST BE SOLDERED TO PCB ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATRE RANGE LT348ED#PBF LT348ID#PBF LT348ED#TRPBF LT348ID#TRPBF LCFG LCFG 6-Lead (3mm 3mm) Plastic QFN 6-Lead (3mm 3mm) Plastic QFN C to 85 C 4 C to 5 C Consult LTC Marketing for parts specifi ed with wider operating temperature ranges. *The temperature grade is identifi ed by a label on the shipping container. For more information on lead free parts, go to: For more information on tape and reel specifi cations, go to: ELECTRICAL CHARACTERISTICS The denotes the specifi cations which apply over the full operating temperature range, otherwise specifi cations are at T A = 5 C. = 3V, V S H D N = 3V unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX NITS Minimum Operating Voltage.5 V Maximum Operating Voltage 6 V Feedback Voltage =.5V V..6 V Feedback Line Regulation.5.7 %/V Pin Bias Current 3 na Supply Current =.3V, Not Switching V S H D N = ma µa Switching Frequency f SET = V f SET = V Maximum Duty Cycle f SET = V 95 % Switch Current Limit ma Switch V CESAT I SW = 5mA 3 mv Switch Leakage Current SW = 5V µa Schottky Forward Voltage I SCHOTTKY = 5mA 88 mv Schottky Reverse Leakage V OT SW = 5V 5 µa SHDN Voltage High.5 V khz MHz

3 ELECTRICAL CHARACTERISTICS The denotes the specifi cations which apply over the full operating temperature range, otherwise specifi cations are at T A = 5 C. = 3V, V S H D N = 3V unless otherwise noted. LT348 PARAMETER CONDITIONS MIN TYP MAX NITS SHDN Voltage Low.4 V SHDN Pin Bias Current 35 5 µa f SET Voltage High.5 V f SET Voltage Low.4 V f SET Bias Current f SET = V 4 µa to Offset =.5V 5 mv 5 mv Current Monitor Gain I = 5nA, V 9V I =.5mA, V 9V Monitor Output Voltage Clamp.5 V Monitor Voltage Drop at I = ma, = 9V 5 V Pin Current Limit = V, = 4V 5 ma Note : Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note : The LT348E is guaranteed to meet specifi ed performance from C to 85 C. Specifi cations over the 4 C to 85 C operating temperature range are assured by design, characterization and correlation with statistical process controls. The LT348I is guaranteed to meet performance specifi cations over the 4 C to 5 C operating temperature range. 3

4 TYPICAL PERFOR A CE CHARACTERISTICS W (T A = 5 C unless otherwise specifi ed) Oscillator Frequency vs Temperature 4 Switch Current Limit vs Duty Cycle 36 Switch Current Limit vs Temperature FREQENCY (khz) f SET = V f SET = V SWITCH CRRENT LIMIT (ma) SWITCH CRRENT LIMIT (ma) TEMPERATRE ( C) DTY CYCLE (%) TEMPERATRE ( C) 348 G 348 G 348 G3 I MON (µa) 9 8 Current Monitor Output vs I = µa (V) 348 G4 I MON (A) Current Monitor Accuracy.E- = 9V.E-3.E-4.E-5.E-6.E-7.E-8.E-9.E-8.E-6.E-4.E- I (A) 348 G5 ERROR (%) Current Monitor Accuracy vs Temperature = 9V I =.5mA I = µa I = 5nA TEMPERATRE ( C) 348 G6 7 Current Monitor Voltage Drop vs Reference Current 3 Switch Saturation Voltage (V CESAT ).5 Pin Voltage vs Temperature 6 5 (V) V CESAT (mv) 5 PIN VOLTAGE (V).4.3 = 6V = 3V 5.E-7.E-5.E-3 REFERENCE CRRENT (A) SWITCH CRRENT (ma) TEMPERATRE ( C)!"& /% 348 G8 348 G9 4

5 PI F CTIO S (Pin ): Connect cathode to this pin. (Pin 3): Current Monitor Power Supply Pin. An external lowpass filter can be included here to further reduce supply voltage ripple. V OT (Pin 4): Voltage Doubler Output Pin. Put a 5V rated capacitor between this pin and V OT. Tie a resistor divider to the pin and GND. V OT (Pin 5): Boost Output Pin. Put a capacitor between this pin and the GND plane. Minimize the length of the trace to the capacitor. PMP (Pin 6): Charge Pump Pin. Put a 5V rating bypass capacitor between SW and PMP to form a complete voltage doubler with the internal integrated Schottky diodes. Minimize trace length to the capacitor. SW (Pins 7, 8): Switch Pin. Minimize the trace length on this pin to reduce EMI. GND (Pins 9, ): Ground. Pins connected internally. For best performance, connect both pins to board ground. (Pin ): Input Supply Pin. This pin must be locally bypassed. LT348 SHDN (Pin ): Shutdown Pin. Tie to.5v or higher to enable device;.4v or less to disable device. This pin also functions as soft-start between.5v and V. (Pin 3): Internal Reference Override Pin. This allows the voltage to be externally set between V and.v. Tie this pin higher than.5v to use the internal reference of.35v. (Pin 4): Feedback Pin. Connect the output resistor divider tap here. f SET (Pin 5): Oscillator Frequency Selection Pin. Tie this pin to above.5v or higher to select the higher switching frequency of.mhz. For lower switching frequency, tie to GND. MON (Pin 6): Current Monitor Output Pin. It sources a current equal to % of the current and converts to a reference voltage through an external resistor. Exposed Pad (Pin 7): GND. This pin must be soldered to the PCB. 5

6 F CTIO AL DIAGRA W L C FLY GND SW SW PMP D3 V OT 4 GND D D V OT 5 C C R R C PL.35V REFERENCE + + A EAMP R C C C Σ + RAMP GENERATOR A PWM COMPARATOR R S Q CRRENT SENSE AMPLIFIER DRIVER + Q CRRENT MIRROR 3 C3 R S C S SHDN 65kHz/.MHz OSCILLATOR 3 4 f SET 5 6 MON NC 348 BD CONTROL BLOCK R3 C4 OPERATIO The LT384 boost converter uses a constant frequency current mode control scheme to provide excellent line and load regulation. Operation can be best understood by referring to the Functional Diagram. At the start of each oscillator cycle, the SR latch is set, which turns on the power switch, Q. A voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator, A. When this voltage exceeds the level at the negative input of A, the SR latch is reset turning off the power switch. The level at the negative input of A is set by the error amplifier A, and is simply an amplified version of the difference between the feedback voltage and the reference voltage of.35v, or externally provided voltage. In this manner, the error amplifi er sets the correct peak current level to keep the output in regulation. If the error amplifier s output increases, more current is delivered to the output; if it decreases, less current is delivered. The LT348 has an integrated high side current monitor with a 5: ratio. The pin can accept a supply voltage up to 9V, which is suitable for photodiode applications. The MON pin has an open-circuit protection feature and is internally clamped to.5v. If an is tied to the pin, the current will be mirrored to the MON pin and converted to a voltage signal by the resistor R3. This voltage signal can be used to drive an external control block to adjust the voltage by adjusting the feedback threshold of EAMP A through the input. 6

7 APPLICATIO S I FOR ATIO Switching Frequency W The LT348 can operate at either 65kHz nominal or.mhz nominal; the voltage at the f SET pin selects which frequency is used. At.MHz, a physically smaller inductor and capacitor can be used in a given application, but higher frequencies will slightly decrease efficiency and maximum duty cycle. Generally if efficiency and maximum duty cycle are crucial, the lower switching frequency should be selected by connecting f SET to GND. If application size and cost are more important, connect f SET to to select the higher switching frequency. Inrush Current The LT348 has built-in Schottky diodes for the boost and charge pump. When supply voltage is applied to the pin, the voltage difference between and V OT generates inrush current flowing from input through the inductor and the Schottky diode (D in the Functional Diagram) to charge the output capacitor. The selection of inductor and capacitor value should ensure the peak of the inrush current to be below A. In addition, the LT348 turn-on should be delayed until the inrush current is less than the maximum current limit. The peak inrush current can be estimated as follows: I P V = IN 6. π exp L C L C where L is the inductance and C is the output capacitance. Table gives inrush peak currents for some component selections. Table. Inrush Peak Current (V) L (µh) C (µf) I P (A) Setting Output Voltage The LT348 is equipped with both an internal.35v reference and an auxiliary reference input (the pin). This allows users to select between using the built-in reference and supplying an external reference voltage. The voltage at the pin can be adjusted while the chip is operating to alter the output voltage of LT348 for purposes such as s bias voltage adjustment. To use the internal.35v reference, the pin should be held higher than.5v, which can be done by tying it to. When the pin is between V and.v, the LT348 will regulate the output such that the pin voltage is equal to the pin voltage. To set the output voltage, select the values of R and R (see Figure ) according to the following equation: R R V OT = V REF where V REF =.35V if the internal reference is used or V REF = if is between V and.v. R can be selected to load the output to maintain a constant switching frequency when the load is very low. Preventing entry into pulse skipping mode is an important consideration for post fi ltering the regulator output. V OT 4 3 LT348 4 R R 348 F Figure. Output Voltage Feedback Connection 7

8 APPLICATIO S I FOR ATIO Inductor Selection The inductors used with the LT348 should have a saturation current rating of.3a or greater. If the device is used in an application where the input supply will be hot-plugged, the saturation current rating should be equal to or greater than the peak inrush current. For best loop stability, the inductor value selected should provide a ripple current of 6mA or more. For a given and V OT, the inductor value to use in continuous conduction mode (CCM) is estimated by the formula: 8 D V L = IN ƒ 6mA where: D= V OT+ V V + OT IN and f is the switching frequency. To achieve low output voltage ripple, a small value inductor should be selected to force the LT348 work in discontinuous conduction mode (DCM). The inequality is true when the LT348 is operating in discontinuous condition mode. D V L < ƒ I IN LIMIT W where I LIMIT is the switch current limit. Operating in DCM reduces the maximum load current and the conversion efficiency. Capacitor Selection Low ESR capacitors should be used at the output to minimize the output voltage ripple. se only X5R and X7R types because they retain their capacitance over wider voltage and temperature ranges than other types. High output voltages typically require less capacitance for loop stability. For applications with out voltage less than 45V, intermediate output pin V OT can directly serve as the output pin. Typically use a µf capacitor for output voltage less than 5V and µf capacitor for output voltage between 5V and 45V. When output voltage goes beyond 45V, a charge pump must be formed with cascaded.47µf capacitors C and C at the output nodes. A typical.µf capacitor is used as the fl ying capacitor C FLY to form the charge pump. Always use a capacitor with sufficient voltage rating. Either ceramic or solid tantalum capacitors may be used for the input decoupling capacitor, which should be placed as close as possible to the LT348. A µf capacitor is suffi cient for most applications. Phase Lead Capacitor A small value capacitor (i.e., pf to pf) can be added in parallel with the resistor between the output and the pin to reduce output perturbation due to a load step and to improve transient response. This phase lead capacitor introduces a pole-zero pair to the feedback that boosts phase margin near the crossover frequency. The is very sensitive to a noisy bias supply. To lowpass fi lter noise from the internal reference and error amplifier, a.µf phase lead capacitor can be used. The corner frequency of the noise fi lter is R C PL. Current Monitor The power supply switching noise associated with a switching power supply can interfere with the photodiode DC measurement. To suppress this noise, a.µf capacitor is recommended at pin. An additional output lowpass fi lter, a k resistor and a nf capacitor in parallel at MON pin, can further reduce the power supply noise and other wide band noise, which might limit the measurement accuracy of low level signals. For applications requiring fast current monitor response time, a RC lowpass fi lter at pin is used to replace the.µf capacitor at pin, as illustrated in Figure. LT348 V OT V OT Figure R MON C MON 348 F C C

9 APPLICATIO S I FOR ATIO W In some applications, a long cable or wire is used to connect the LT348 to. When is shorted to GND, pin voltage might ring below ground and damage the internal circuitry. To prevent damage during short-circuit event, a Ω resistor must be added in series with the. Layout Hints The high speed operation of the LT384 demands careful attention to board layout. You will not get advertised performance with careless layout. Figure 3 shows the recommended component placement. MON f SET R R C PL C MON (OPT) L C IN GND R MON (OPT) C C FLY 8 C 348 F3 Figure 3. Suggested Layout 9

10 TYPICAL APPLICATIO S 5V to 85V Bias Power Supply Effi ciency 5V OFF ON C µf L µh C.µF SW PMP f SET LT348 SHDN V OT GND MON C7 nf R3 k V OT 85V AT.5mA C3.µF C: MRATA X7R GRMBR7C5KAB C: AVX 635C4KATA C3, C6: AVX 85C4KATA C4, C5: MRATA X7R GRM3MR7H474KAB C7: MRATA GRM67H3JAB L: COILCRAFT ME3-3KL OR EQIVALENT C4.47µF C5.47µF C6.µF R M R 4k 348 TAa EFFICIENCY (%) I (ma) 348 TAb 3.3V to 7V Bias Power Supply with Fast Current Monitor Response Effi ciency 3.3V OFF ON C µf L 6.8µH C.µF SW PMP f SET LT348 SHDN V OT GND MON R4 k V OT 7V AT ma C: MRATA X7R GRMBR7C5KAB C: AVX 635C4KATA C3, C6: AVX 85C4KATA C4, C5: MRATA X7R GRM3MR7H474KAB L: COILCRAFT ME3-68ML OR EQIVALENT C4.47µF C5.47µF C6.µF R Ω R M R3 6.5k C3.µF 348 TA3a Input Load for Current Monitor Step Response Measurement EFFICIENCY (%) I (ma) 348 TA3b Current Monitor Step Response NODE A k 5V V MON V/DIV REF GND k V ma ma V NODE A V/DIV REF GND 5ns/DIV 348 TA3c

11 PACKAGE DESCRIPTIO D Package 6-Lead Plastic QFN (3mm 3mm) (Reference LTC DWG # ).7 ± ±.5. ±.5.45 ±.5 (4 SIDES) PACKAGE OTLINE.5 ±.5.5 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3. ±. (4 SIDES) PIN TOP MARK (NOTE 6).75 ±.5.45 ±. (4-SIDES) BOTTOM VIEW EXPOSED PAD R =.5 TYP 5 6 PIN NOTCH R =. TYP OR.5 45 CHAMFER.4 ±. (D6) QFN 94. REF..5 NOTE:. DRAWING CONFORMS TO JEDEC PACKAGE OTLINE MO- VARIATION (WEED-). DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED.5mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN LOCATION ON THE TOP AND BOTTOM OF PACKAGE.5 ±.5.5 BSC 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.

12 TYPICAL APPLICATIO 3.3V to 4V Bias Power Supply Effi ciency 3.3V OFF ON C mf L 5.6mH SW PMP f SET LT348 SHDN V OT GND MON R4 k V OT 4V AT 3mA C mf C3.mF R3 W R M R 8k C4.mF 348 TA4a EFFICIENCY (%) I (ma) C: MRATA X7R GRMBR7C5KAB C: MRATA X7R GRM3MR7H5KA88B C3, C4: AVX 635C4KATA L: COILCRAFT ME3-56ML OR EQIVALENT 348 TA4b RELATED PARTS PART NMBER DESCRIPTION COMMENTS LT93/LT93A A (I SW ),.MHz/.MHz High Effi ciency Step-p :.6V to 6V, V OT(MAX) = 34V, I Q = 4.mA/5.5mA, I SD < µa, DC/DC Converters ThinSOT TM Package LT346.3A (I SW ),.3MHz High Effi ciency Step-p DC/DC Converter LT346/LT346A.3A (I SW ),.3MHz/3MHz High Effi ciency Step-p DC/DC Converters with Integrated Schottky LT3465/LT3465A Constant Current,.MHz/.7MHz High Effi ciency White LED Boost Regulator with Integrated Schottky ThinSOT is a trademark on Linear Technology Corporation. :.5V to 6V, V OT(MAX) = 38V, I Q = ma, I SD < µa, ThinSOT Package :.5V to 6V, V OT(MAX) = 38V, I Q =.8mA, I SD < µa, SC7 and ThinSOT Package :.7V to 6V, V OT(MAX) = 34V, I Q =.9mA, I SD < µa, ThinSOT Package LT 7 REV A PRINTED IN SA Linear Technology Corporation 63 McCarthy Blvd., Milpitas, CA (48) 43-9 FAX: (48) LINEAR TECHNOLOGY CORPORATION 6