V OUT 5V, 0.7A 10V CHEMI-CON SXE SERIES

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1 FEATRES Wide Input Voltage Range: V to 75V High Switch Voltage: V Low Quiescent Current:.5mA Internal A Switch Shutdown Mode Draws Only µa Supply Current Isolated Flyback Regulation Mode for Fully Floating Outputs Can Be Externally Synchronized Available in MiniDIP and TO- Packages Same Pinout as LT7 APPLICATI O S Telecom 5V Supply at.7a from 8V 9V Supply at ma from 5V All Applications sing LT7 (See Below for Specification Differences) LT8 and LT7 Major Specification Differences LT8C LT7HV V to 75V V to 6V V SW V 75V Switch Current Limit A.5A Quiescent Current.5mA 6mA Operating Frequency 6kHz khz Flyback Reference Voltage 6..6 (5kΩ/R FB ) 6.5 (7kΩ/R FB ) SER NOTE: This data sheet is only intended to provide specifications, graphs, and a general functional description of the LT8. Application circuits are included to show the capability of the LT8. A complete design manual (AN9) and Switcher CAD (LTC Switching Power Supply Design Program) should be obtained to assist in developing new designs. This manual contains a comprehensive discussion of both the LT7 and the external components used with it, as well as complete formulas for calculating the values of these components. The manual can also be used for the LT8 by factoring in the lower switch current rating. LT8 A High Voltage, Efficiency Switching Voltage Regulator DESCRIPTIO The LT8 is a monolithic high voltage switching regulator. It can be operated in all standard switching configurations including buck, boost, flyback, forward, and inverting. A A high efficiency switch is included on the die along with all oscillator, control, and protection circuitry. The LT8 operates with supply voltages from V to 75V, switch voltage up to V and draws only.5ma quiescent current. It can deliver load power up to W with no external power devices. By utilizing current-mode switching techniques, it provides excellent AC and DC load and line regulation. An externally activated shutdown mode reduces total supply current to µa typical for standby operation. Totally isolated and regulated outputs can be generated by using the optional isolated flyback regulation mode built into the LT8, without the need for optocouplers or extra transformer windings. The LT8 has a unique feature to provide high voltage short-circuit protection. When the FB pin is pulled down to.6v and the current out of the pin reaches approximately 5µA, the switching frequency will shift down from 6kHz to khz. The LT8 is nearly identical to the lower voltage LT7. For the major differences in specifications, see the table on the left. µf 8V CHEMI-CON SXE SERIES V TO 7V Negative-to-Positive Telecom 5V Supply GND LT8 V C **5µH.7k VSW.µF FB *D Q N5.µF.k 8 TA.8k V OT 5V,.7A 7µF V CHEMI-CON SXE SERIES * MOTOROLA MR (V, A) ** 69 TRNS OF #8 AWG WIRE ON A MICROMETALS T6 TYPE 5 CORE. NOTE: THIS CORE IS LOW COST, BT HAS HIGHER CORE LOSS AND IS LARGER THAN NECESSARY FOR LOWER CRRENT APPLICATIONS. FOR SMALLER INDCTORS OR HIGHER EFFICIENCY, SE A LOW LOSS CORE SCH AS MAGNETICS INC. KOOL Mµ OR MOLYPERMALLOY. NOTE: MAXIMM OTPT CRRENT IS A FNCTION OF INPT VOLTAGE. SEE THE GRAPH ON THE RIGHT. MAXIMM OTPT CRRENT (A) Telecom 5V Supply Maximum Output Current vs Input Voltage f = 5kHz I SW LIMIT =.7A L=55µH L=5µH L=5µH L=5µH L=5µH L=µH INPT VOLTAGE (V) 8 TA

2 LT8 ABSOLTE AXI RATI GS W W W Supply Voltage... 75V Switch Output Voltage... V Feedback Pin Voltage (Transient, ms)... ±5V Storage Temperature Range C to 5 C Lead Temperature (Soldering, sec)... C Operating Junction Temperature Range LT8M C to 5 C LT8I... C to 5 C LT8C... C to C PACKAGE/ORDER I FOR W ATIO GND V C FB NC TOP VIEW J8 PACKAGE 8-LEAD CERAMIC DIP 8 E 7 V SW 6 E 5 N8 PACKAGE 8-LEAD PLASTIC DIP T JMAX = 5 C, θ JA = C/W (MJ8) T JMAX = C, θ JA = 9 C/W (CN8) T JMAX = 5 C, θ JA = 9 C/W (IN8) ORDER PART NMBER LT8MJ8 LT8CN8 LT8IN8 FRONT VIEW 5 Q PACKAGE 5-LEAD DD V SW GND FB V C T JMAX = C, θ JA = C/W (CQ) T JMAX = 5 C, θ JA = C/ W (IQ) NOTE: θ JA VARIES FROM 5 C/W TO 5 C/W DEPENDING ON BOARD COMPOSITION. ORDER PART NMBER LT8CQ LT8IQ FRONT VIEW 5 T PACKAGE 5-LEAD TO- V SW GND FB V C T JMAX = C, θ JA = 75 C/W, θ JC = 8 C/W (CT) T JMAX = 5 C, θ JA = 75 C/W, θ JC = 8 C/W (IT) ORDER PART NMBER LT8CT LT8IT ELECTRICAL CHARA CTERISTICS = 5V, V C =.5V, V FB = V REF, output pin open, unless otherwise specified. SYMBOL PARAMETER CONDITIONS MIN TYP MAX NITS V REF Reference Voltage Measured at Feedback Pin...6 V V C =.8V...7 V I B Feedback Input Current V FB = V REF 5 75 na na g m Error Amplifier I C = ±5µA 6 µmho Transconductance 7 µmho Error Amplifier Source or V C =.5V 5 µa Sink Current µa Error Amplifier Clamp Hi Clamp, V FB = V.8. V Voltage Lo Clamp, V FB =.5V...6 V Reference Voltage Line Regulation V V MAX, V C =.8V. %/V A V Error Amplifier Voltage Gain.9V V C.V 5 65 V/V Minimum Input Voltage.6. V

3 LT8 ELECTRICAL CHARA CTERISTICS = 5V, V C =.5V, V FB = V REF, output pin open, unless otherwise specified. SYMBOL PARAMETER CONDITIONS MIN TYP MAX NITS I Q Supply Current V V MAX, V C =.6V.5 7. ma Control Pin Threshold Duty Cycle =.7.9. V.5.5 V Normal/Flyback Threshold V on Feedback Pin f Switching Frequency khz 5 75 khz 8µA I FB 5µA khz BV Output Switch Breakdown Voltage V V MAX, I SW =.5mA 5 V Control Voltage to Switch.5 A/V Current Transconductance V FB Flyback Reference Voltage I FB = 6µA V 6.5 V Change in Flyback Reference Voltage 6µA I FB µa V Flyback Reference Voltage Line Regulation I FB = 6µA, V V MAX.. %/V Flyback Amplifier Transconductance (g m ) I C = ±µa 5 5 µmho Flyback Amplifier Source V C =.6V Source 5 7 µa and Sink Current I FB = 6µA Sink 5 9 µa V SAT Output Switch On Resistance (Note ) I SW =.7A (LT8C), I SW =.5A (LT8M).8. Ω I LIM Switch Current Limit Duty Cycle = %.7.6 A (LT8C) Duty Cycle 5%..6 A Duty Cycle = 8% (Note ).8. A Switch Current Limit Duty Cycle = %.85.8 A (LT8I) Duty Cycle 5%.8.8 A Duty Cycle = 8% (Note ).65.6 A Switch Current Limit Duty Cycle = %.75. A (LT8M) Duty Cycle 5%.7. A Duty Cycle = 8% (Note ).6.8 A I IN Supply Current Increase 5 5 ma/a I SW During Switch-On Time DC MAX Maximum Switch Duty Cycle % Flyback Sense Delay Time.5 µs Shutdown Mode Supply Current V V MAX, V C =.5V 5 µa Shutdown Mode V V MAX mv Threshold Voltage 5 mv The denotes the specifications which apply over the operating temperature range. Note : Measured with V C in hi clamp, V FB =.8V. Note : For duty cycles (DC) between 5% and 8%, minimum guaranteed switch current decreases linearly.

4 LT8 TYPICAL PERFOR Suggested Core Size and Inductance for Telecom 5V Supply LOAD CRRENT ma ma ma 6mA 8mA TYPE 5 POWDERED IRON T8 5µH T5 5µH T6 5µH T6 5µH T8 5µH A W KOOL Mµ OR MOLY- PERMALLOY T8 µh T8 5µH T5 5µH T5 µh T8 5µH 8 GA CECHARA CTERISTICS EFFICIENCY (%) Telecom 5V Supply Efficiency = V = V = 6V = 7V POWER OTPT (W) L = 5µH R =.8Ω NOTE: THIS GRAPH IS BASED ON LOW CORE LOSS PERMALLOY INDCTOR. IF POWDERED IRON CORE INDCTOR IS SED, THE CORE LOSS IS TYPICALLY mw HIGHER. 8 G FREQENCY (khz) Telecom 5V Supply Short-Circuit Frequency Shift-Down T A = 5 C 5 OTPT VOLTAGE (V) 8 G 7 6 Short-Circuit Frequency Shift-Down vs Feedback Current T A = C T A = 55 C Switch Current Limit Maximum Duty Cycle FREQENCY (khz) 5 T A = 5 C SWITCH CRRENT (A) T J = 55 C T J = 5 C T J = 5 C DTY CYCLE (%) FEEDBACK CRRENT (µa) DTY CYCLE (%) TEMPERATRE ( C) 8 G 8 G 8 G5. Flyback Blanking Time.9 Minimum Input Voltage. Switch Saturation Voltage TIME (µs) MINIMM INPT VOLTAGE (V) I SW = A I SW = A SWITCH SATRATION VOLTAGE (V) T J = 5 C T J = C T J = 5 C T J = 5 C JNCTION TEMPERATRE ( C) TEMPERATRE ( C) SWITCH CRRENT (A) 8 G6 8 G7 8 G8

5 LT8 TYPICAL PERFOR A W CE CHARA CTERISTICS FLYBACK VOLTAGE (V) FEEDBACK BIAS CRRENT (na) Isolated Mode Flyback Reference Voltage R FB = k R FB = 6k R FB = k TEMPERATRE ( C) Feedback Bias Current vs Temperature 8 G TEMPERATRE ( C) FREQENCY (khz) V C PIN VOLTAGE (mv) Reference Voltage and Switching Frequency vs Temperature V REF FREQ TEMPERATRE ( C) Normal/Feedback Mode Threshold on Feedback Pin FEEDBACK PIN VOLTAGE (AT THRESHOLD) FEEDBACK PIN CRRENT (AT THRESHOLD) 8 G TEMPERATRE ( C) REFERENCE VOLTAGE (V) V C PIN CRRENT (µa) REFERENCE VOLTAGE CHANGE (mv) SPPLY CRRENT (µa) Line Regulation T J = 55 C T J = 5 C T J = 5 C INPT VOLTAGE (V) Shutdown Mode Supply Current 55 C T J 5 C T J = 5 C 8 G V C PIN VOLTAGE (mv) 8 G 8 G6 8 G7 SPPLY CRRENT (µa) 6 8 Supply Current vs Supply Voltage (Shutdown Mode) V C = 5mV V C = V SPPLY VOLTAGE (V) 8 G5 DRIVER CRRENT (ma) Driver Current* vs Switch Current SWITCH CRRENT (A) * AVERAGE SPPLY CRRENT = I Q DC(.9 I SW 5 I SW ) I Q = QIESCENT CRRENT, DC = DTY CYCLE, I SW = SWITCH CRRENT 8 G SPPLY CRRENT (ma) Supply Current vs Input Voltage** 9% DTY CYCLE 5% DTY CYCLE % DTY CYCLE % DTY CYCLE INPT VOLTAGE (V) **NDER VERY LOW OTPT CRRENT CONDITIONS, DTY CYCLE FOR MOST CIRCITS WILL APPROACH % OR LESS. 8 G 5

6 LT8 TYPICAL PERFOR A W CE CHARA CTERISTICS TRANSCONDCTANCE (µmho) Error Amplifier Transconductance 5 5 g m = I (V C PIN) V (FB PIN) TEMPERATRE ( C) V C VOLTAGE (mv) Shutdown Thresholds VOLTAGE AT V C PIN CRRENT (OT OF V C PIN) TEMPERATRE ( C) VC CRRENT (µa) IDLE SPPLY CRRENT (ma) Idle Supply Current vs Temperature V C =.6V = 75V = V TEMPERATRE ( C) 8 G8 8 G9 8 G FEEDBACK VOLTAGE (mv) Feedback Pin Clamp Voltage 8 7 T J = 55 C 6 T J = 5 C 5 T J = 5 C FEEDBACK CRRENT (ma) 8 G SWITCH CRRENT (µa) Switch Off Characteristics A. = V B. = 5V C. = V D. = 55V E. = 75V A B C D E SWITCH VOLTAGE (V) 8 G V C PIN CRRENT (µa) V C Pin Characteristics T J = 5 C V FB =.5V (CRRENT INTO V C PIN) V FB =.8V (CRRENT OT OF V C PIN) TRANSCONDCTANCE (µmho) Transconductance of Error Amplifier θ g m PHASE (DEG) V C PIN VOLTAGE (V) 8 G k k k M M FREQENCY (Hz) 8 G 6

7 LT8 BLOCKDAGRA I W 6.V SWITCH OT.V REG FLYBACK ERROR AMP OSC 6kHz khz LOGIC DRIVER MODE SELECT ANTI-SAT COMP FB ERROR AMP V C GAIN 5 CRRENT AMP.Ω.Ω SHTDOWN CIRCIT E* E.V REF.5V * ALWAYS CONNECT E TO GROND PIN ON MiniDIP PACKAGE. EMITTERS TIED TO GROND ON TO- PACKAGE. GND 8 BD OPERATIO The LT8 is a current mode switcher. This means that switch duty cycle is directly controlled by switch current rather than by output voltage. Referring to the block diagram, the switch is turned on at the start of each oscillator cycle. It is turned off when switch current reaches a predetermined level. Control of output voltage is obtained by using the output of a voltage sensing error amplifier to set current trip level. This technique has several advantages. First, it has immediate response to input voltage variations, unlike ordinary switchers which have notoriously poor line transient response. Second, it reduces the 9 phase shift at mid-frequencies in the energy storage inductor. This greatly simplifies closedloop frequency compensation under widely varying input voltage or output load conditions. Finally, it allows simple pulse-by-pulse current limiting to provide maximum switch protection under output overload or short conditions. A low dropout internal regulator provides a.v supply for all internal circuitry on the LT8. This low dropout design allows input voltage to vary from V to 75V with virtually no change in device performance. A 6kHz oscillator is the basic clock for all internal timing. It turns on the output switch via the logic and driver circuitry. Special adaptive anti-sat circuitry detects onset of saturation in the power switch and adjusts driver current instantaneously to limit switch saturation. This minimizes driver dissipation and provides very rapid turn-off of the switch. A.V bandgap reference biases the positive input of the error amplifier. The negative input is brought out for output voltage sensing. This feedback pin has a second function: when pulled low with an external resistor and with I FB of 6µA to µa, it programs the LT8 to 7

8 LT8 OPERATIO 8 disconnect the main error amplifier output and connects the output of the flyback amplifier to the comparator input. The LT8 will then regulate the value of the flyback pulse with respect to the supply voltage. This flyback pulse is directly proportional to output voltage in the traditional transformer coupled flyback topology regulator. By regulating the amplitude of the flyback pulse, the output voltage can be regulated with no direct connection between input and output. The output is fully floating up to the breakdown voltage of the transformer windings. Multiple floating outputs are easily obtained with additional windings. A special delay network inside the LT8 ignores the leakage inductance spike at the leading edge of the flyback pulse to improve output regulation. When I FB drawn out of the FB pin reaches 5µA, the LT8 shifts the switching frequency down to khz. This unique feature provides high voltage short-circuit protection in systems like the telecom 5V supplies with input voltages down to 7V; lower frequency is needed under short-circuit conditions with current mode switchers because minimum on time cannot be forced below the internally set blanking time. Referring to the telecom 5V supply circuit on the front page, with output shorted to ground, the V FB stays at.6v when sourcing I FB up to ma. If the FB pin is forced to source more than ma, the frequency shifting function may be defeated. Therefore, the minimum suggested value for R FB is k and the maximum suggested value is.k. Also, no capacitance more than nf should be used on the FB pin, because it may cause unstable switching frequency in this low frequency mode. The error signal developed at the comparator input is brought out externally. This pin (V C ) has four different functions. It is used for frequency compensation, current limit adjustment, soft starting, and total regulator shutdown. During normal regulator operation this pin sits at a voltage between.9v (low output current) and V (high output current). The error amplifiers are current output (g m ) types, so this voltage can be externally clamped for adjusting current limit. Likewise, a capacitor-coupled external clamp will provide soft start. Switch duty cycle goes to zero if the V C pin is pulled to ground through a diode, placing the LT8 in an idle mode. Pulling the V C pin below.5v causes total regulator shutdown, with only µa supply current for shutdown circuitry biasing. See AN9 for full application details. Extra Pins on the MiniDIP Packages The minidip LT8 has the emitters of the power transistor brought out separately from the ground pin. This eliminates errors due to ground pin voltage drops and allows the user to reduce switch current limit by a factor of : by leaving the second emitter (E) disconnected. The first emitter (E) should always be connected to the ground pin. Note that switch on resistance doubles when E is left open, so efficiency will suffer somewhat when switch currents exceed ma. Also, note that chip dissipation will actually increase with E open during normal load operation, even though dissipation in current limit mode will decrease. See Thermal Considerations. Thermal Considerations When sing the MiniDIP Packages The low supply current and high switch efficiency of the LT8 allow it to be used without a heat sink in most applications when the TO- package is selected. This package is rated at 5 C/W. The minidips, however, are rated at C/W in ceramic (J) and 9 /W in plastic (N). Care should be taken for minidip applications to ensure that the worst case input voltage and load current conditions do not cause excessive die temperatures. The following formulas can be used as a rough guide to calculate LT8 power dissipation. For more details, the reader is referred to Application Note 9 (AN9), Efficiency Calculations section. Average supply current (including driver current) is: I IN.5mA I SW (. DC/8) I SW = switch current DC = switch duty cycle Switch power dissipation is given by: P SW = (I SW ) R SW DC R SW = LT8 switch on resistance (.Ω maximum)

9 LT8 OPERATIO Total power dissipation is the sum of supply current times input voltage plus switch power: P TOT = (I IN )( ) P SW In a typical example, using negative-to-positive converter to generate 5V at.5a from a 5V input, duty cycle is approximately %, and switch current is about.5a, yielding: I IN =.5mA.5(. DC/8) = 8.7mA P SW = (.5).Ω (.) =.6W P TOT = (5V)(8.7mA).6 =.W Temperature rise in a plastic minidip would be 9 C/W times.w, or approximately 9 C. The maximum ambient temperature would be limited to C (commercial temperature limit) minus 9 C, or 6 C. In most applications, full load current is used to calculate die temperature. However, if overload conditions must also be accounted for, four approaches are possible. First, if loss of regulated output is acceptable under overload conditions, the internal thermal limit of the LT8 will protect the die in most applications by shutting off switch current. Thermal limit is not a tested parameter, however, and should be considered only for noncritical applications with temporary overloads. A second approach is to use the larger TO- (T) package which, even without a heat sink, may limit die temperatures to safe levels under overload conditions. In critical situations, heat sinking of these packages is required; especially if overload conditions must be tolerated for extended periods of time. The third approach for lower current applications is to leave the second switch emitter (minidip only) open. This increases switch on resistance by :, but reduces switch current limit by : also, resulting in a net : reduction in I R switch dissipation under current limit conditions. The fourth approach is to clamp the V C pin to a voltage less than its internal clamp level of V. The LT8 switch current limit is zero at approximately V on the V C pin and.6a at V on the V C pin. Peak switch current can be externally clamped between these two levels with a diode. See AN9 for details. LT8 Synchronizing The LT8 can be externally synchronized in the frequency range of 75kHz to 9kHz. This is accomplished as shown in the accompanying figures. Synchronizing occurs when the V C pin is pulled to ground with an external transistor. To avoid disturbing the DC characteristics of the internal error amplifier, the width of the synchronizing pulse should be under µs. C sets the pulse width at.6µs. The effect of a synchronizing pulse on the LT8 amplifier offset can be calculated from: V OS = KT q ( V ts )( fs ) I C C R I C KT/q = 6mV at 5 C t S = pulse width f S = pulse frequency I C = LT8 V C source current ( µa) V C = LT8 operating V C voltage (V to V) R = resistor used to set mid-frequency zero in LT8 frequency compensation network. With t S =.6µs, f S = 8kHz, V C =.5V, and R = k, offset voltage shift is 5mV. This is not particularly bothersome, but note that high offset could result if R were reduced to a much lower value. Also, the synchronizing transistor must sink higher currents with low values of R, so larger drives may have to be used. The transistor must be capable of pulling the V C pin to within mv of ground to ensure synchronizing. GND LT8 C R V C Synchronizing the LT8 VN* D N8 R.k C 5pF D N8 *SILICONIX OR EQIVALENT FROM 5V LOGIC 8 OP 9

10 LT8 TYPICAL APPLICATI O S Totally Isolated Converter V to 7V 5µF 5V MR V SW LT8 FB GND V C k L PRI 5µH.: MR MR µf µf 5V AT.A COM 5V AT.A.µF 7k MINIMM LOAD OF.5A IS REQIRED FOR EACH OTPT. (SEE AN9) 8 TA Boost Converter 5V mh, A MR VOT 9V AT ma V SW 78.8k 5µF LT8 FB GND V C µf.7k.µf.k.µf 8 TA

11 LT8 PACKAGE DESCRIPTIO Dimensions in inches (milimeters) unless otherwise noted. J8 Package 8-Lead Ceramic DIP.9. ( ).8.8 (..6).85 ±.5 (9.779 ±.65) 5..5 (.58.) HALF LEAD OPTION.5.65 (..65) FLL LEAD OPTION CORNER LEADS OPTION ( PLCS).5.65 (..65)..6 (.6.66).5.6 (.8.5). (5.8) MAX.5.75 MIN. ±. (.5 ±.5).5 (.7) MIN.5 (.65) RAD TYP.5 (.87) MAX ( ).. ( ).5.65 (..65) N8 Package 8-Lead Plastic DIP. ±. (. ±.7). (.6) MAX (.9.8) ( ).65 (.65) TYP.5 ±.5 (. ±.8). ±. (.5 ±.5).5 (.75) MIN.8 ±. (.57 ±.76). (.58) MIN.5 ±. (6.5 ±.5) Q Package 5-Lead Plastic DD.6 (.5). ±.5 (.85 ±.8) 5 TYP.75 ±.8 (.5 ±.).5 ±.8 (.7 ±.)... ( ).59 (.99) TYP..8. (...).5 ±.8 (.667 ±.)... ( ). ±.8 (.8 ±.).67 ±. (.7 ±.5). ±.5 (.559 ±.7).5 ±. (.7 ±.5) 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 LT8 PACKAGE DESCRIPTIO Dimensions in inches (milimeters) unless otherwise noted. T Package 5-Lead TO-.8. ( ).9.5 (.5.886) DIA.79.5 (.7.9) (.9.699).5.55 ( ) (. 6.5) (.996.9).97.5 ( ).6.5 (.68.7).6 ±. (5.75 ±.58).7.78 ( ) (.8.956).8.5 (.7.889).5.5 (.8.65).. ( ).55.9 (.97.86).79.5 (.7.9) T5 (FORMED) 9 LT/GP 9 K REV Linear Technology Corporation 6 McCarthy Blvd., Milpitas, CA (8) -9 FAX: (8) -57 TELEX: LINEAR TECHNOLOGY CORPORATION 99