Datasheet LT1117, LT1117-2.85, LT1117-3.3, LT1117-5 (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | 800mA Low Dropout Positive Regulators Adjustable and Fixed 2.85V,3.3V, 5V |
| Pages / Page | 16 / 9 — APPLICATIONS INFORMATION. Ripple Rejection. Table 1. COPPER AREA. THERMAL … |
| File Format / Size | PDF / 193 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Ripple Rejection. Table 1. COPPER AREA. THERMAL RESISTANCE. TOPSIDE*. BACKSIDE
Model Line for this Datasheet
- LT1117CM#PBF LT1117CM#PBF LT1117CM#TRPBF LT1117CM#TRPBF LT1117CM-2.85#PBF LT1117CM-2.85#PBF LT1117CM-2.85#TRPBF LT1117CM-2.85#TRPBF LT1117CM-3.3#PBF LT1117CM-3.3#PBF LT1117CM-3.3#TRPBF LT1117CM-3.3#TRPBF LT1117CM-5#PBF LT1117CM-5#PBF LT1117CM-5#TRPBF LT1117CM-5#TRPBF LT1117CST#PBF LT1117CST#PBF LT1117CST#TRPBF LT1117CST#TRPBF LT1117CST-2.85#PBF LT1117CST-2.85#PBF LT1117CST-2.85#TRPBF LT1117CST-2.85#TRPBF LT1117CST-3.3#PBF LT1117CST-3.3#PBF LT1117CST-3.3#TRPBF LT1117CST-3.3#TRPBF LT1117CST-5#PBF LT1117CST-5#PBF LT1117CST-5#TRPBF LT1117CST-5#TRPBF LT1117IST#PBF LT1117IST#PBF LT1117IST#TRPBF LT1117IST#TRPBF LT1117IST-2.85#PBF LT1117IST-2.85#PBF LT1117IST-2.85#TRPBF LT1117IST-2.85#TRPBF LT1117IST-3.3#PBF LT1117IST-3.3#PBF LT1117IST-3.3#TRPBF LT1117IST-3.3#TRPBF LT1117IST-5#PBF LT1117IST-5#PBF LT1117IST-5#TRPBF LT1117IST-5#TRPBF
Text Version of Document
LT1117/LT1117-2.85 LT1117-3.3/LT1117-5
APPLICATIONS INFORMATION
Experiments have shown that the heat spreading copper The power dissipation of the LT1117 is equal to: layer does not need to be electrically connected to the P tab of the device. The PC material can be very effective at D = ( VIN – VOUT )( IOUT ) transmitting heat between the pad area, attached to the Maximum junction temperature will be equal to: tab of the device, and a ground plane layer either inside TJ = TA(MAX) + PD (Thermal Resistance (junction-to- or on the opposite side of the board. Although the actual ambient)) thermal resistance of the PC material is high, the Length/ Area ratio of the thermal resistor between layers is small. Maximum junction temperature must not exceed 125°C. The data in Table 1 was taken using 1/16" FR-4 board with
Ripple Rejection
1oz. copper foil. It can be used as a rough guideline in estimating thermal resistance. The curves for Ripple Rejection were generated using an adjustable device with the adjust pin bypassed. These
Table 1.
curves will hold true for all values of output voltage. For
COPPER AREA THERMAL RESISTANCE
proper bypassing, and ripple rejection approaching the
TOPSIDE* BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT)
values shown, the impedance of the adjust pin capacitor, 2500 Sq. mm 2500 Sq. mm 2500 Sq. mm 45°C/W at the ripple frequency, should be < R1. R1 is normally in 1000 Sq. mm 2500 Sq. mm 2500 Sq. mm 45°C/W the range of 100Ω to 200Ω. The size of the required adjust 225 Sq. mm 2500 Sq. mm 2500 Sq. mm 53°C/W pin capacitor is a function of the input ripple frequency. At 100 Sq. mm 2500 Sq. mm 2500 Sq. mm 59°C/W 120Hz, with R1 = 100Ω, the adjust pin capacitor should 1000 Sq. mm 1000 Sq. mm 1000 Sq. mm 52°C/W be >13μF. At 10kHz only 0.16μF is needed. 1000 Sq. mm 0 1000 Sq. mm 55°C/W For fixed voltage devices, and adjustable devices without * Tab of device attached to topside copper an adjust pin capacitor, the output ripple will increase as The thermal resistance for each application will be the ratio of the output voltage to the reference voltage affected by thermal interactions with other components (VOUT/VREF). For example, with the output voltage equal on the board. Some experimentation will be necessary to to 5V, the output ripple will be increased by the ratio of determine the actual value. 5V/1.25V. It will increase by a factor of four. Ripple rejection will be degraded by 12dB from the value shown on the curve. 1117fd 9
APPLICATIONS INFORMATION
Experiments have shown that the heat spreading copper The power dissipation of the LT1117 is equal to: layer does not need to be electrically connected to the P tab of the device. The PC material can be very effective at D = ( VIN – VOUT )( IOUT ) transmitting heat between the pad area, attached to the Maximum junction temperature will be equal to: tab of the device, and a ground plane layer either inside TJ = TA(MAX) + PD (Thermal Resistance (junction-to- or on the opposite side of the board. Although the actual ambient)) thermal resistance of the PC material is high, the Length/ Area ratio of the thermal resistor between layers is small. Maximum junction temperature must not exceed 125°C. The data in Table 1 was taken using 1/16" FR-4 board with
Ripple Rejection
1oz. copper foil. It can be used as a rough guideline in estimating thermal resistance. The curves for Ripple Rejection were generated using an adjustable device with the adjust pin bypassed. These
Table 1.
curves will hold true for all values of output voltage. For
COPPER AREA THERMAL RESISTANCE
proper bypassing, and ripple rejection approaching the
TOPSIDE* BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT)
values shown, the impedance of the adjust pin capacitor, 2500 Sq. mm 2500 Sq. mm 2500 Sq. mm 45°C/W at the ripple frequency, should be < R1. R1 is normally in 1000 Sq. mm 2500 Sq. mm 2500 Sq. mm 45°C/W the range of 100Ω to 200Ω. The size of the required adjust 225 Sq. mm 2500 Sq. mm 2500 Sq. mm 53°C/W pin capacitor is a function of the input ripple frequency. At 100 Sq. mm 2500 Sq. mm 2500 Sq. mm 59°C/W 120Hz, with R1 = 100Ω, the adjust pin capacitor should 1000 Sq. mm 1000 Sq. mm 1000 Sq. mm 52°C/W be >13μF. At 10kHz only 0.16μF is needed. 1000 Sq. mm 0 1000 Sq. mm 55°C/W For fixed voltage devices, and adjustable devices without * Tab of device attached to topside copper an adjust pin capacitor, the output ripple will increase as The thermal resistance for each application will be the ratio of the output voltage to the reference voltage affected by thermal interactions with other components (VOUT/VREF). For example, with the output voltage equal on the board. Some experimentation will be necessary to to 5V, the output ripple will be increased by the ratio of determine the actual value. 5V/1.25V. It will increase by a factor of four. Ripple rejection will be degraded by 12dB from the value shown on the curve. 1117fd 9
