Datasheet LT1945 (Analog Devices) - 7
| Manufacturer | Analog Devices |
| Description | Dual Micropower DC/DC Converter with Positive and Negative Outputs |
| Pages / Page | 8 / 7 — APPLICATIONS INFORMATION. Diode Selection. Table 2. Recommended … |
| File Format / Size | PDF / 129 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Diode Selection. Table 2. Recommended Capacitors. CAPACITOR TYPE. VENDOR
Model Line for this Datasheet
Text Version of Document
LT1945
APPLICATIONS INFORMATION
of related parts.
Diode Selection Table 2. Recommended Capacitors
For most LT1945 applications, the Zetex ZHCS400 sur-
CAPACITOR TYPE VENDOR
face mount Schottky diode (0.4A, 40V) is an ideal choice. Ceramic Taiyo Yuden Schottky diodes, with their low forward voltage drop and (408) 573-4150 fast switching speed, are the best match for the LT1945. www.t-yuden.com The Motorola MBR0520, MBR0530, or MBR0540 can also Ceramic AVX be used. Many different manufacturers make equivalent (803) 448-9411 www.avxcorp.com parts, but make sure that the component is rated to handle Ceramic Murata at least 0.35A. (714) 852-2001 www.murata.com
Lowering Output Voltage Ripple Setting the Output Voltages
Using low ESR capacitors will help minimize the output Set the output voltage for Switcher 1 (negative output ripple voltage, but proper selection of the inductor and the voltage ) by choosing the appropriate values for feedback output capacitor also plays a big role. The LT1945 provides resistors R1 and R2. energy to the load in bursts by ramping up the inductor current, then delivering that current to the load. If too large V –1.23V R OUT 1= of an inductor value or too small of a capacitor value is 1.23V + used, the output ripple voltage will increase because the 2 • 10 6 ( − ) R2 capacitor will be slightly overcharged each burst cycle. Set the output voltage for Switcher 2 (positive output To reduce the output ripple, increase the output capacitor voltage) by choosing the appropriate values for feedback value or add a 4.7pF feed-forward capacitor in the feedback resistors R1B and R2B (see Figure 1). network of the LT1945 (see the circuits in the Typical Ap- plications section). Adding this small, inexpensive 4.7pF ⎛ V ⎞ capacitor will greatly reduce the output voltage ripple. R = R OUT 1 2 −1 ⎝⎜ 1 23 ⎠⎟ .
PACKAGE DESCRIPTION MS Package 10-Lead Plastic MSOP
3.00 p 0.102 0.889 p 0.127 (Reference LTC DWG # 05-08-1661) (.118 p .004) 0.497 p 0.076 (NOTE 3) (.035 p .005) (.0196 p .003) 10 9 8 7 6 REF 5.23 3.2 – 3.45 3.00 p 0.102 (.206) 4.88 p 0.10 (.126 – .136) DETAIL “A” (.118 p .004) MIN (.192 p .004) 0.254 NOTE 4 (.010) 0o – 6o TYP GAUGE PLANE 0.305 p 0.038 0.50 (.0120 p .0015) (.0197) 1 2 3 4 5 TYP BSC 0.53 p 0.01 RECOMMENDED SOLDER PAD LAYOUT (.021 p .006) 1.10 0.86 (.043) (.034) DETAIL “A” MAX REF 0.18 (.007) SEATING NOTE: PLANE 0.17 – 0.27 0.13 p 0.05 1. DIMENSIONS IN MILLIMETER/(INCH) (.007 – .011) (.005 p .002) 2. DRAWING NOT TO SCALE 0.50 MSOP (MS) 0402 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. (.0197) MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE TYP 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 1945fa 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 representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. 7
APPLICATIONS INFORMATION
of related parts.
Diode Selection Table 2. Recommended Capacitors
For most LT1945 applications, the Zetex ZHCS400 sur-
CAPACITOR TYPE VENDOR
face mount Schottky diode (0.4A, 40V) is an ideal choice. Ceramic Taiyo Yuden Schottky diodes, with their low forward voltage drop and (408) 573-4150 fast switching speed, are the best match for the LT1945. www.t-yuden.com The Motorola MBR0520, MBR0530, or MBR0540 can also Ceramic AVX be used. Many different manufacturers make equivalent (803) 448-9411 www.avxcorp.com parts, but make sure that the component is rated to handle Ceramic Murata at least 0.35A. (714) 852-2001 www.murata.com
Lowering Output Voltage Ripple Setting the Output Voltages
Using low ESR capacitors will help minimize the output Set the output voltage for Switcher 1 (negative output ripple voltage, but proper selection of the inductor and the voltage ) by choosing the appropriate values for feedback output capacitor also plays a big role. The LT1945 provides resistors R1 and R2. energy to the load in bursts by ramping up the inductor current, then delivering that current to the load. If too large V –1.23V R OUT 1= of an inductor value or too small of a capacitor value is 1.23V + used, the output ripple voltage will increase because the 2 • 10 6 ( − ) R2 capacitor will be slightly overcharged each burst cycle. Set the output voltage for Switcher 2 (positive output To reduce the output ripple, increase the output capacitor voltage) by choosing the appropriate values for feedback value or add a 4.7pF feed-forward capacitor in the feedback resistors R1B and R2B (see Figure 1). network of the LT1945 (see the circuits in the Typical Ap- plications section). Adding this small, inexpensive 4.7pF ⎛ V ⎞ capacitor will greatly reduce the output voltage ripple. R = R OUT 1 2 −1 ⎝⎜ 1 23 ⎠⎟ .
PACKAGE DESCRIPTION MS Package 10-Lead Plastic MSOP
3.00 p 0.102 0.889 p 0.127 (Reference LTC DWG # 05-08-1661) (.118 p .004) 0.497 p 0.076 (NOTE 3) (.035 p .005) (.0196 p .003) 10 9 8 7 6 REF 5.23 3.2 – 3.45 3.00 p 0.102 (.206) 4.88 p 0.10 (.126 – .136) DETAIL “A” (.118 p .004) MIN (.192 p .004) 0.254 NOTE 4 (.010) 0o – 6o TYP GAUGE PLANE 0.305 p 0.038 0.50 (.0120 p .0015) (.0197) 1 2 3 4 5 TYP BSC 0.53 p 0.01 RECOMMENDED SOLDER PAD LAYOUT (.021 p .006) 1.10 0.86 (.043) (.034) DETAIL “A” MAX REF 0.18 (.007) SEATING NOTE: PLANE 0.17 – 0.27 0.13 p 0.05 1. DIMENSIONS IN MILLIMETER/(INCH) (.007 – .011) (.005 p .002) 2. DRAWING NOT TO SCALE 0.50 MSOP (MS) 0402 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. (.0197) MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE TYP 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 1945fa 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 representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. 7
