LTC1751/LTC1751-3.3/LTC1751-5 Micropower, Regulated Charge Pump DC/DC Converters U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO 5V Output Current: 100mA (VIN ≥ 3V) 3.3V Output Current: 80mA (VIN ≥ 2.5V) Ultralow Power: 20µA Quiescent Current Regulated Output Voltage: 3.3V ±4%, 5V ±4%, ADJ No Inductors Short-Circuit/Thermal Protection VIN Range: 2V to 5.5V 800kHz Switching Frequency Very Low Shutdown Current: <2µA Shutdown Disconnects Load from VIN PowerGood/Undervoltage Output Adjustable Soft-Start Time Available in an 8-Pin MSOP Package U APPLICATIO S ■ ■ ■ ■ ■ Li-Ion Battery Backup Supplies Local 3V and 5V Conversion Smart Card Readers PCMCIA Local 5V Supplies White LED Backlighting The LTC®1751 family are micropower charge pump DC/ DC converters that produce a regulated output voltage at up to 100mA. The input voltage range is 2V to 5.5V. Extremely low operating current (20µA typical with no load) and low external parts count (one flying capacitor and two small bypass capacitors at VIN and VOUT) make them ideally suited for small, battery-powered applications. The LTC1751 family operate as Burst ModeTM switched capacitor voltage doublers to achieve ultralow quiescent current. They have thermal shutdown capability and can survive a continuous short circuit from VOUT to GND. The PGOOD pin on the LTC1751-3.3 and LTC1751-5 indicates when the output voltage has reached its final value and if the output has an undervoltage fault condition. The FB pin of the adjustable LTC1751 can be used to program the desired output voltage or current. An optional soft-start capacitor may be used at the SS pin to prevent excessive inrush current during start-up. The LTC1751 family is available in an 8-pin MSOP package. , LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a trademark of Linear Technology Corporation. U TYPICAL APPLICATIO Output Voltage vs Input Voltage 5.2 VIN 2.7V TO 5.5V 3 C2 10µF OFF ON VIN VOUT 2 7 1 SHDN PGOOD LTC1751-5 8 6 SS C+ 4 GND C– 5 R1 100k PGOOD CFLY 1µF VOUT 5V ±4% C1 I 10µF OUT ≤ 100mA, VIN ≥ 3V IOUT ≤ 50mA, VIN ≥ 2.7V OUTPUT VOLTAGE (V) Regulated 5V Output from a 2.7V to 5.5V Input IOUT = 50mA CFLY = 1µF COUT = 10µF 5.1 TA = 85°C TA = 25°C 5.0 TA = –40°C 4.9 1751 TA01 CFLY = MURATA GRM39X5R105K6.3AJ C1, C2 = MURATA GRM40X5R106K6.3AJ 4.8 2.5 3.0 3.5 4.0 4.5 INPUT VOLTAGE (V) 5.0 5.5 1751 TA02 1 LTC1751/LTC1751-3.3/LTC1751-5 W W W AXI U U ABSOLUTE RATI GS U U W PACKAGE/ORDER I FOR ATIO (Note 1) VIN to GND .................................................. – 0.3V to 6V PGOOD, FB, VOUT to GND ........................... – 0.3V to 6V SS, SHDN to GND ........................ – 0.3V to (VIN + 0.3V) VOUT Short-Circuit Duration ............................. Indefinite IOUT (Note 2)....................................................... 125mA Operating 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 TOP VIEW FB/PGOOD* VOUT VIN GND 1 2 3 4 8 7 6 5 SS SHDN C+ C– MS8 PACKAGE 8-LEAD PLASTIC MSOP LTC1751EMS8 LTC1751EMS8-3.3 LTC1751EMS8-5 MS8 PART MARKING TJMAX = 150°C, θJA = 160°C/W LTKL LTKN LTKP *PGOOD ON LTC1751-3.3/LTC1751-5 FB ON LTC1751 Consult factory for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full specified temperature range, otherwise specifications are at TA = 25°C. CFLY = 1µF, CIN = 10µF, COUT = 10µF unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 4.4 V 3.3 3.3 3.43 3.43 V V 18 40 µA LTC1751-3.3 VIN Input Supply Voltage ● 2 VOUT Output Voltage 2V ≤ VIN ≤ 4.4V, IOUT ≤ 40mA 2.5V ≤ VIN ≤ 4.4V, IOUT ≤ 80mA ● ● 3.17 3.17 ICC Operating Supply Current 2V ≤ VIN ≤ 4.4V, IOUT = 0mA, SHDN = VIN ● VR Output Ripple VIN = 2.5V, IOUT = 40mA 68 mVP-P η Efficiency VIN = 2V, IOUT = 40mA 80 % LTC1751-5 VIN Input Supply Voltage ● 2.7 VOUT Output Voltage ICC VR η 5.5 V 2.7V ≤ VIN ≤ 5.5V, IOUT ≤ 50mA 3V ≤ VIN ≤ 5.5V, IOUT ≤ 100mA ● ● 4.8 4.8 5 5 5.2 5.2 V V Operating Supply Current 2.7V ≤ VIN ≤ 5.5V, IOUT = 0mA, SHDN = VIN ● Output Ripple VIN = 3V, IOUT = 50mA 20 75 50 mVP-P Efficiency VIN = 3V, IOUT = 50mA 82 % µA LTC1751 VIN Input Supply Voltage ICC Operating Supply Current 2V ≤ VIN ≤ 5.5V, IOUT = 0mA, SHDN = VIN (Note 4 ) ● VFB FB Regulation Voltage 2V ≤ VIN ≤ 5.5V, IOUT ≤ 20mA ● 1.157 IFB FB Input Current VFB = 1.3V ● – 50 ROUT Open-Loop Charge Pump Strength VIN = 2V, VOUT = 3.3V (Note 5) VIN = 2.7V, VOUT = 5V (Note 5) ● ● 2 ● 2 5.5 V 16 40 µA 1.205 1.253 V 50 nA 8.5 6.0 20 12 Ω Ω LTC1751/LTC1751-3.3/LTC1751-5 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full specified temperature range, otherwise specifications are at TA = 25°C. CFLY = 1µF, CIN = 10µF, COUT = 10µF unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS LTC1751-3.3/LTC1751-5 UVL PGOOD Undervoltage Low Threshold Relative to Regulated VOUT (Note 6) ● –11 –7 –3 % UVH PGOOD Undervoltage High Threshold Relative to Regulated VOUT (Note 6) ● –8 – 4.5 –2 % VOL PGOOD Low Output Voltage IPGOOD = – 500µA ● 0.4 V IOH PGOOD High Output Leakage VPGOOD = 5.5V ● 1 µA VIN ≤ 3.6V, VOUT = 0V, VSHDN = 0V 3.6V < VIN, VOUT = 0V, VSHDN = 0V ● ● 2 5 µA µA LTC1751/LTC1751-3.3/LTC1751-5 ISHDN Shutdown Supply Current VIH SHDN Input Threshold (High) ● VIL SHDN Input Threshold (Low) ● IIH SHDN Input Current (High) SHDN = VIN ● IIL SHDN Input Current (Low) SHDN = 0V ● tr VOUT Rise Time VIN = 3V, IOUT = 0mA, 10% to 90% (Note 6) fOSC Switching Frequency Oscillator Free Running Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: Based on long term current density limitations. Note 3: The LTC1751EMS8-X is 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. 0.01 1.5 V 0.3 V –1 1 µA –1 1 µA 0.6ms/nF • CSS sec 800 kHz Note 4: The no load input current will be approximately ICC plus twice the standing current in the resistive output divider. Note 5: ROUT ≡ (2VIN – VOUT)/IOUT. Note 6: See Figure 2. U W TYPICAL PERFOR A CE CHARACTERISTICS (LTC1751-3.3) 3.40 TA = 25°C CFLY = 1µF 3.35 OUTPUT VOLTAGE (V) OUTPUT VOLTAGE (V) 3.40 VIN = 2.5V 3.30 VIN = 2V 3.25 3.20 0 25 50 75 100 LOAD CURRENT (mA) 125 No Load Supply Current vs Input Voltage Output Voltage vs Input Voltage 150 1751 G01 IOUT = 40mA CFLY = 1µF COUT = 10µF 3.35 40 TA = –40°C TA = 25°C TA = 85°C 3.30 3.25 3.20 2.0 2.5 3.5 4.0 3.0 INPUT VOLTAGE (V) 4.5 1751 G02 SUPPLY CURRENT (µA) Output Voltage vs Load Current IOUT = 0mA CFLY = 1µF VSHDN = VIN 30 TA = 85°C 20 TA = 25°C TA = –40°C 10 0 2.0 2.5 3.5 4.0 3.0 INPUT VOLTAGE (V) 4.5 1751 G03 3 LTC1751/LTC1751-3.3/LTC1751-5 U W TYPICAL PERFOR A CE CHARACTERISTICS (LTC1751-3.3) Short-Circuit Output Current vs Input Voltage Power Efficiency vs Load Current 100 250 VIN = 2V 70 OUTPUT CURRENT (mA) EFFICIENCY (%) TA = 25°C 90 CFLY = 1µF = 10µF C 80 OUT VIN = 2.75V 60 50 VIN = 3.3V 40 VIN = 4.4V 30 20 TA = 25°C CFLY = 1µF 200 150 100 10 0 0.001 0.01 0.1 1 10 LOAD CURRENT (mA) 50 2.0 100 2.5 3.5 4.0 3.0 INPUT VOLTAGE (V) 1751 G04 Start-Up 1751 G05 Output Ripple SHDN 2V/DIV PGOOD 5V/DIV 4.5 Load Transient Response IOUT 40mA/DIV VOUT AC COUPLED 50mV/DIV VOUT 1V/DIV VOUT AC COUPLED 50mV/DIV CSS = 10nF 2ms/DIV VIN = 2.5V IOUT = 80mA COUT = 10µF 1751 G06 5µs/DIV 1751 G07 VIN = 2.5V 50µs/DIV (LTC1751-5) No Load Supply Current vs Input Voltage Output Voltage vs Output Current 40 TA = 25°C CFLY = 1µF 5.1 5.0 VIN = 3V VIN = 2.7V 4.9 4.8 SUPPLY CURRENT (µA) OUTPUT VOLTAGE (V) 5.2 0 100 150 50 OUTPUT CURRENT (mA) 200 1751 G09 4 CFLY = 1µF IOUT = 0 VSHDN = VIN 30 TA = 85°C TA = 25°C 20 TA = –40°C 10 0 2.5 3.0 3.5 4.0 4.5 INPUT VOLTAGE (V) 5.0 5.5 1751 G10 1751 G08 LTC1751/LTC1751-3.3/LTC1751-5 U W TYPICAL PERFOR A CE CHARACTERISTICS (LTC1751-5) Short-Circuit Output Current vs Input Voltage Power Efficiency vs Load Current 100 250 VIN = 2.7V 70 OUTPUT CURRENT (mA) EFFICIENCY (%) TA = 25°C 90 CFLY = 1µF COUT = 10µF 80 VIN = 4.1V 60 VIN = 5.5V 50 40 30 20 TA = 25°C CFLY = 1µF 200 150 100 10 0 0.001 0.01 0.1 1 10 LOAD CURRENT (mA) 50 100 2.0 2.5 3.0 3.5 4.0 4.5 INPUT VOLTAGE (V) 1751 G11 Start-Up 5.0 5.5 1751 G12 Output Ripple Load Transient Response SHDN 2V/DIV IOUT 50mA/DIV PGOOD 5V/DIV VOUT AC COUPLED 50mV/DIV VOUT AC COUPLED 50mV/DIV VOUT 2V/DIV 2ms/DIV 1751 G13 VIN = 3V IOUT = 100mA COUT = 10µF 5µs/DIV 1751 G14 VIN = 3V 50µs/DIV 1751 G15 U CSS = 10nF U U PI FU CTIO S PGOOD (Pin 1) (LTC1751-3.3/LTC1751-5): Output Voltage Status Indicator. On start-up, this open-drain pin remains low until the output voltage, VOUT, is within 4.5% (typ) of its final value. Once VOUT is valid, PGOOD becomes high-Z. If, due to a fault condition, VOUT falls 7% (typ) below its correct regulation level, PGOOD pulls low. PGOOD may be pulled up through an external resistor to any appropriate reference level. FB (Pin 1) (LTC1751): The voltage on this pin is compared to the internal reference voltage (1.205V) by the error comparator to keep the output in regulation. An external resistor divider is required between VOUT and FB to program the output voltage. VOUT (Pin 2): Regulated Output Voltage. For best performance, VOUT should be bypassed with a 6.8µF (min) low ESR capacitor as close to the pin as possible . VIN (Pin 3): Input Supply Voltage. VIN should be bypassed with a 6.8µF (min) low ESR capacitor. GND (Pin 4): Ground. Should be tied to a ground plane for best performance. C – (Pin 5): Flying Capacitor Negative Terminal. C + (PIN 6): Flying Capacitor Positive Terminal. SHDN (Pin 7): Active Low Shutdown Input. A low on SHDN disables the device. SHDN must not be allowed to float. SS (Pin 8): Soft-Start Programming Pin. A capacitor on SS programs the start-up time of the charge pump so that large start-up input current is eliminated. 5 LTC1751/LTC1751-3.3/LTC1751-5 W W SI PLIFIED BLOCK DIAGRA S LTC1751-3.3/LTC1751-5 READY PGOOD + 1 –+ 2µA – 8 SS 7 SHDN –+ – VREF + UNDERV VOUT 2 + – CONTROL COMP1 CHARGE PUMP VIN 3 6 C+ GND 4 5 C– 1751 BD1 LTC1751 2µA FB 1 8 SS 7 SHDN VREF VOUT 2 + – CONTROL COMP1 CHARGE PUMP VIN 3 6 C+ GND 4 5 C– 1751 BD2 6 LTC1751/LTC1751-3.3/LTC1751-5 U W U U APPLICATIO S I FOR ATIO Operation (Refer to Simplified Block Diagrams) The LTC1751 family uses a switched capacitor charge pump to boost VIN to a regulated output voltage. Regulation is achieved by sensing the output voltage through a resistor divider and enabling the charge pump when the divided output drops below the lower trip point of COMP1. When the charge pump is enabled, a 2-phase nonoverlapping clock activates the charge pump switches. The flying capacitor is charged to VIN on phase 1 of the clock. On phase 2 of the clock, it is stacked in series with VIN and connected to VOUT. This sequence of charging and discharging the flying capacitor continues at the clock frequency until the divided output voltage reaches the upper trip point of COMP1. Once this happens the charge pump is disabled. When the charge pump is disabled the device typically draws less than 20µA from VIN thus providing high efficiency under low load conditions. In shutdown mode all circuitry is turned off and the LTC1751 draws only leakage current from the VIN supply. Furthermore, VOUT is disconnected from VIN. The SHDN pin is a CMOS input with a threshold voltage of approximately 0.8V. The LTC1751 is in shutdown when a logic low is applied to the SHDN pin. The quiescent supply current of the LTC1751 will be slightly higher if the SHDN pin is driven high with a voltage that is below VIN than if it is driven all the way to VIN. Since the SHDN pin is a high impedance CMOS input it should never be allowed to float. To ensure that its state is defined it must always be driven with a valid logic level. Power Efficiency The efficiency (η) of the LTC1751 family is similar to that of a linear regulator with an effective input voltage of twice the actual input voltage. This occurs because the input current for a voltage doubling charge pump is approximately twice the output current. In an ideal regulated doubler the power efficiency would be given by: η= POUT VOUT • IOUT VOUT = = PIN VIN • 2IOUT 2VIN At moderate to high output power, the switching losses and quiescent current of the LTC1751 are negligible and the expression is valid. For example, an LTC1751-5 with VIN = 3V, IOUT = 50mA and VOUT regulating to 5V, has a measured efficiency of 82% which is in close agreement with the theoretical 83.3% calculation. The LTC1751 product family continues to maintain good efficiency even at fairly light loads because of its inherently low power design. Short-Circuit/Thermal Protection During short-circuit conditions, the LTC1751 will draw between 200mA and 400mA from VIN causing a rise in the junction temperature. On-chip thermal shutdown circuitry disables the charge pump once the junction temperature exceeds approximately 160°C and re-enables the charge pump once the junction temperature drops back to approximately 150°C. The device will cycle in and out of thermal shutdown indefinitely without latchup or damage until the short circuit on VOUT is removed. VIN, VOUT Capacitor Selection The style and value of capacitors used with the LTC1751 family determine several important parameters such as output ripple, charge pump strength and minimum start-up time. To reduce noise and ripple, it is recommended that low ESR (< 0.1Ω) capacitors be used for both CIN and COUT. These capacitors should be either ceramic or tantalum and should be 6.8µF or greater. Aluminum capacitors are not recommended because of their high ESR. If the source impedance to VIN is very low, up to several megahertz, CIN may not be needed. Alternatively, a somewhat smaller value of input capacitor may be adequate, but will not be as effective in preventing ripple on the VIN pin. The value of COUT controls the amount of output ripple. Increasing the size of COUT to 10µF or greater will reduce the output ripple at the expense of higher minimum turn on time and higher start-up current. See the section Output Ripple. 7 LTC1751/LTC1751-3.3/LTC1751-5 U W U U APPLICATIO S I FOR ATIO Flying Capacitor Selection Warning: A polarized capacitor such as tantalum or aluminum should never be used for the flying capacitor since its voltage can reverse upon start-up of the LTC1751. Low ESR ceramic capacitors should always be used for the flying capacitor. The flying capacitor controls the strength of the charge pump. In order to achieve the rated output current, it is necessary to have at least 0.6µF of capacitance for the flying capacitor. Capacitors of different materials lose their capacitance with higher temperature and voltage at different rates. For example, a ceramic capacitor made of X7R material will retain most of its capacitance from – 40°C to 85°C, whereas, a Z5U or Y5V style capacitor will lose considerable capacitance over that range. Z5U and Y5V capacitors may also have a very strong voltage coefficient causing them to lose 50% or more of their capacitance when the rated voltage is applied. The capacitor manufacturer’s data sheet should be consulted to determine what value of capacitor is needed to ensure 0.6µF at all temperatures and voltages. Generally an X7R ceramic capacitor is recommended for the flying capacitor with a minimum value of 1µF. For very low load applications, it may be reduced to 0.01µF-0.68µF. A smaller flying capacitor delivers less charge per clock cycle to the output capacitor resulting in lower output ripple. The output ripple is reduced at the expense of maximum output current and efficiency. The theoretical minimum output resistance of a voltage doubling charge pump is given by: ROUT (MIN) ≡ 2VIN – VOUT 1 = IOUT fC Where f if the switching frequency and C is the value of the flying capacitor. (Using units of MHz and µF is convenient since they cancel each other.) Note that the charge pump will typically be weaker than the theoretical limit due to additional switch resistance. However, for light load applications, the above expression can be used as a guideline in determining a starting capacitor value. 8 Below is a list of ceramic capacitor manufacturers and how to contact them: AVX www.avxcorp.com Kemet www.kemet.com Murata www.murata.com Taiyo Yuden www.t-yuden.com Vishay www.vishay.com Output Ripple Low frequency regulation mode ripple exists due to the hysteresis in the sense comparator and propagation delays in the charge pump control circuits. The amplitude and frequency of this ripple are heavily dependent on the load current, the input voltage and the output capacitor size. For large VIN the ripple voltage can become substantial because the increased strength of the charge pump causes fast edges that may outpace the regulation circuitry. In some cases, rather than bursting, a single output cycle may be enough to boost the output voltage into or possibly beyond regulation. In these cases the average output voltage will climb slightly. For large input voltages a larger output capacitor will ensure that bursting always occurs, thus mitigating possible DC problems. Generally the regulation ripple has a sawtooth shape associated with it. A high frequency ripple component may also be present on the output capacitor due to the charge transfer action of the charge pump. In this case, the output can display a voltage pulse during the output-charging phase. This pulse results from the product of the charging current and the ESR of the output capacitor. It is proportional to the input voltage, the value of the flying capacitor and the ESR of the output capacitor. For example, typical combined output ripple for an LTC1751-5 with VIN = 3V under maximum load is 75mVP-P with a low ESR 10µF output capacitor. A smaller output capacitor and/or larger output current load will result in higher ripple due to higher output voltage slew rates. LTC1751/LTC1751-3.3/LTC1751-5 U W U U APPLICATIO S I FOR ATIO There are several ways to reduce output voltage ripple. For applications requiring VIN to exceed 3.3V or for applications requiring < 100mV of peak-to-peak ripple, a larger COUT capacitor (22µF or greater) is recommended. A larger capacitor will reduce both the low and high frequency ripple due to the lower charging and discharging slew rates as well as the lower ESR typically found with higher value (larger case size) capacitors. A low ESR ceramic output capacitor will minimize the high frequency ripple, but will not reduce the low frequency ripple unless a high capacitance value is used. An R-C filter may also be used to reduce high frequency voltages spikes (see Figure 1). 1Ω VOUT LTC1751-X + 10µF TANT + VOUT 5V 10µF TANT 1751 F01 Figure 1. Output Ripple Reduction Technique Note that when using a larger output capacitor the minimum turn-on time of the device will increase. Soft-Start The LTC1751 family has built-in soft-start circuitry to prevent excessive current flow at VIN during start-up. The soft-start time is programmed by the value of the capacitor at the SS pin. Typically a 2µA current is forced out of SS causing a ramp voltage on the SS pin. The regulation loop follows this ramp voltage until the output reaches the correct regulation level. SS is automatically pulled to ground whenever SHDN is low. The typical rise time is given by the expression: various parameters such as temperature, output loading, charge pump and flying capacitor values and input voltage. PGOOD and Undervoltage Detection The PGOOD pin on the LTC1751-3.3/LTC1751-5 performs two functions. On start-up, it indicates when the output has reached its final regulation level. After start-up, it indicates when a fault condition, such as excessive loading, has pulled the output out of regulation. Once the LTC1751-3.3/LTC1751-5 are enabled via the SHDN pin, VOUT ramps to its final regulation value slowly by following the SS pin. The PGOOD pin switches from low impedance to high impedance after VOUT reaches its regulation value. If VOUT is subsequently pulled below its correct regulation level, the PGOOD pin pulls low again indicating that a fault exists. Alternatively, if there is a short circuit on VOUT preventing it from ever reaching its correct regulation level, the PGOOD pin will remain low. The lower fault threshold, UVL, is preprogrammed to recognize errors of – 7% below nominal VOUT. The upper fault threshold, UVH, is preprogrammed at – 4.5% below nominal. Figure 2 shows an example of the PGOOD pin with a normal start-up followed by an undervoltage fault. Using an external pull-up resistor, the PGOOD pin can be pulled high from any available voltage supply, including the LTC1751-3.3/LTC1751-5 VOUT pin. If PGOOD is not used it may be connected to GND. SHDN PGOOD tr = 0.6ms/nF • CSS For example, with a 4.7nF capacitor the 10% to 90% rise time will be approximately 2.8ms. If the output charge storage capacitor is 10µF, then the average output current for an LTC1751-5 will be 4V/2.8ms • 10µF or 14mA, giving 28mA at the VIN pin. The soft-start feature is optional. If there is no capacitor on SS, the output voltage of the LTC1751 will ramp up as quickly as possible. The start-up time will depend on tr VOUT 90% UVL UVH 10% TIME 17515 F02 Figure 2. PGOOD During Start-Up and Undervoltage 9 LTC1751/LTC1751-3.3/LTC1751-5 U W U U APPLICATIO S I FOR ATIO While the LTC1751-3.3/LTC1751-5 versions have internal resistive dividers to program the output voltage, the programmable LTC1751 may be set to an arbitrary voltage via an external resistive divider. Since it employs a voltage doubling charge pump, it is not possible to achieve output voltages greater than twice the available input voltage. Figure 3 shows the required voltage divider connection. The voltage divider ratio is given by the expression: Typical ROUT values as a function of input voltage are shown in Figure 5. 10 TA = 25°C CFLY = 1µF 8 OUTPUT RESISTANCE (Ω) Programming the LTC1751 Output Voltage (FB Pin) IOUT = 100mA 6 IOUT = 50mA 4 2 R1 V = OUT – 1 R2 1.205V 0 2.0 2 VOUT R1 1 FB ( ) 4 1751 F03 Figure 3. Programming the Adjustable LTC1751 The sum of the voltage divider resistors can be made large to keep the quiescent current to a minimum. Any standing current in the output divider (given by 1.205V/R2) will be reflected by a factor of 2 in the input current. Typical values for total voltage divider resistance can range from several kΩs up to 1MΩ. Maximum Available Output Current For the adjustable LTC1751, the maximum available output current and voltage can be calculated from the effective open-loop output resistance, ROUT, and effective output voltage, 2VIN(MIN). From Figure 4 the available current is given by: IOUT = 2VIN – VOUT ROUT Figure 5. Typical ROUT vs Input Voltage Due to high switching frequency and high transient currents produced by the LTC1751 product family, careful board layout is necessary. A true ground plane and short connections to all capacitors will improve performance and ensure proper regulation under all conditions. Figure 6 shows the recommended layout configuration. Thermal Management For higher input voltages and maximum output current, there can be substantial power dissipation in the LTC1751. If the junction temperature increases above approximately 160°C, the thermal shutdown circuitry will automatically deactivate the output. To reduce the maximum junction temperature, a good thermal connection to the PC board is recommended. Connecting the GND pin (Pin 4) to a ground plane, and maintaining a solid ground plane under the device on two layers of the PC board, will reduce the thermal resistance of the package and PC board system considerably. VIN ROUT + – 4.5 Layout Considerations R2 GND 3.5 4.0 3.0 INPUT VOLTAGE (V) 1751 F05 VOUT 1.205V 1 + R1 R2 COUT 2.5 2VIN + VOUT SHDN VOUT – 1751 F04 Figure 4. Equivalent Open-Loop Circuit 10 GND Figure 6. Recommended Layout 17515 F03 LTC1751/LTC1751-3.3/LTC1751-5 U TYPICAL APPLICATIO S USB Port to Regulated 5V Power Supply with Soft-Start 2-Cell NiCd or NiMH to 3.3V with Low Standby Current 1µF 1µF 5 1Ω 3 10µF 6 C– C+ VIN VOUT 5 2 2-CELL NiCd OR NiMH 10µF LTC1751-5 7 3 VOUT = 5V 100k SHDN + + 10µF 7 OFF ON 8 1nF 1 SS PGOOD 8 PGOOD GND 6 C– C+ VIN VOUT 2 3.3V 40mA 10µF LTC1751-3.3 100k SHDN 1 SS PGOOD PGOOD GND 1nF 4 4 1751 TA04 1751 TA06 Boosted Constant Current Source 1µF 5 VIN 3 C+ VIN 10µF OFF ON 6 C– VOUT IL = 1.205V RX VOUT ≤ 2 VIN 2 10µF LTC1751 7 8 LOAD SHDN 1 FB SS GND RX 4 1751 TA07 Low Power Battery Backup with Auto Switchover and No Reverse Current Si4435DY 1µF 5 75k 3 VIN = 5V 10µF IN4148 3-CELL NiCd BATTERY + 10µF + + 6 C– C+ VIN VOUT 2 VOUT = 5V IOUT ≤ 100mA 10µF LTC1751-5 100k 1 PGOOD 8 HIGH = BACKUP MODE 7 SS SHDN GND 330pF 4 1.43M BAT54C 7 4 3 LTC1540 – 8 + 6 475k 10k 5 HYST 2 1 1M 1751 TA05 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. 11 LTC1751/LTC1751-3.3/LTC1751-5 U TYPICAL APPLICATIO Current Mode White or Blue LED Driver with PWM Brightness Control C4 1µF 6 3V TO 4.5V Li-Ion BATTERY 3 C1 10µF 17ms VSHDN 7 t 8 5 C+ C– VIN VOUT LTC1751 SHDN SS FB GND UP TO 6 LEDS 2 1 C2 10µF 82Ω 4 82Ω 82Ω 82Ω 82Ω 82Ω 1751 TA03 C3 680pF U PACKAGE DESCRIPTIO Dimensions in inches (millimeters) unless otherwise noted. MS8 Package 8-Lead Plastic MSOP (LTC DWG # 05-08-1660) 0.043 (1.10) MAX 0.007 (0.18) 0.034 (0.86) REF 0.118 ± 0.004* (3.00 ± 0.102) 8 7 6 5 0° – 6° TYP 0.021 ± 0.006 (0.53 ± 0.015) SEATING PLANE 0.009 – 0.015 (0.22 – 0.38) 0.0256 (0.65) BSC 0.118 ± 0.004** (3.00 ± 0.102) 0.193 ± 0.006 0.005 ± 0.002 (4.90 ± 0.15) (0.13 ± 0.05) MSOP (MS8) 1100 * DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE ** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE 1 2 3 4 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC1144 Charge Pump Inverter with Shutdown VIN = 2V to 18V, 15V to –15V Supply LTC1262 12V, 30mA Flash Memory Prog. Supply Regulated 12V ±5% Output, IQ = 500µA LTC1514/LTC1515 Buck/Boost Charge Pumps with IQ = 60µA 50mA Output at 3V, 3.3V or 5V; 2V to 10V Input LTC1516 Micropower 5V Charge Pump IQ = 12µA, Up to 50mA Output, VIN = 2V to 5V LTC1517-5/LTC1517-3.3 Micropower 5V/3.3V Doubler Charge Pumps IQ = 6µA, Up to 20mA Output LTC1522 Micropower 5V Doubler Charge Pump IQ = 6µA, Up to 20mA Output LTC1555/LTC1556 SIM Card Interface Step-Up/Step-Down Charge Pump, VIN = 2.7V to 10V LTC1682 Low Noise Doubler Charge Pump Output Noise = 60µVRMS, 2.5V to 5.5V Output LTC1754-5 Micropower 5V Doubler Charge Pump IQ = 13µA, Up to 50mA Output, SOT-23 Package LTC1755 Smart Card Interface Buck/Boost Charge Pump, IQ = 60µA, VIN = 2.7V to 6V LTC3200 Constant Frequency Doubler Charge Pump Low Noise, 5V Output or Adjustable 12 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com 1751f LT/TP 0401 4K • PRINTED IN USA LINEAR TECHNOLOGY CORPORATION 2000