Datasheet MAX1735 (Maxim) - 8

200mA, Negative-Output, Low-Dropout Linear Regulator in SOT23
selected ambient temperatures. The working principle is
MAXIMUM OUTPUT CURRENT
that the SOT23-5 package is small enough that in a typi-
vs. INPUT-OUTPUT VOLTAGE DIFFERENTIAL
cal application circuit at room temperature, the package 250 cannot dissipate enough power to allow -6.5V to be reg- ulated to -1.25V at -200mA output (more than 1200mW). MAXIMUM CONTINUOUS CURRENT 200 As ambient temperature falls, the available power dissi- TA = +50 pation increases to allow for a greater operating region. T 150 A = +70 ° T C The equation for the family of curves follows: A
MAX1735
= +85 °C °C T 70 100 AT MAXIMUM P A MAX − − JUNCTION TEMP θ DEVICE IN DROPOUT (TJ = +150°C) |I | JA OUT = 50 |V −V | OUT IN MAXIMUM OUTPUT CURRENT (mA) where |IOUT| is in mA, |VOUT - VIN| in V, PMAX (571mW) MAX SUPPLY VOLTAGE – MIN OUTPUT VOLTAGE 0 is the absolute maximum rated power dissipation at 0 1 2 3 4 5 6 +70°C for the SOT23-5, and θJA (0.140°C/mW) is the INPUT-OUTPUT VOLTAGE DIFFERENTIAL (V) approximate junction-to-ambient thermal resistance of the SOT23-5 in a typical application. Figure 4. Output Current and In-Out Voltage Differential A key to reducing Operating Region Bounded by Available Power Dissipation at θCA, thereby increasing thermal con- Selected Ambient Temperatures ductivity to the PC board, is to provide large PC board pads and traces for IN.
Operating Region and Power Dissipation
Maximum power dissipation of the MAX1735 depends
__________Applications Information
on the thermal resistance of the case and the circuit
Capacitor Selection and
board, the temperature difference between the die
Regulator Stability
junction and ambient air, and the rate of air flow (see Capacitors are required at the input and output of the also Thermal Overload Protection). The maximum MAX1735. Connect a 1µF or greater capacitor between power that can be dissipated by the device is: IN and GND. This input capacitor serves only to lower the source impedance of the input supply in transient T T T T JMAX A JMAX A conditions; a smaller value can be used when the regu- PMAX = − = − θ + θ θ lator is powered from a low-impedance source, such as JC CA JA another regulated supply or low-impedance batteries. where the numerator expresses the temperature differ- For output voltages between -2.5V and -5.5V, connect ence between the maximum allowed die junction a 1µF or greater capacitor between OUT and GND. For (+150°C) and the surrounding air, θJC (junction to case) voltages between -1.25V and -2.5V, use a 2.2µF or is the thermal resistance of the package, and θCA (case greater output capacitor. The maximum value of the to ambient) is the thermal resistance from the package output capacitor to guarantee stability is 10µF. through the PC board, traces, and other material to the The output capacitor’s value and equivalent series surrounding air. The former is a characteristic solely of resistance (ESR) affect stability and output noise. To the device in its package, and the latter is completely ensure stability and optimum transient response, output defined by PC board layout and airflow. It is important to capacitor ESR should be 0.1Ω or less for output volt- note that the ability to dissipate power is as much a func- ages from -1.25V to -2.45V and 0.2Ω or less for output tion of the PC board layout and air flow as the packaged voltages between -2.5V and -5.5V. Inexpensive sur- part itself. Hence, a manufacturer can reliably provide a face-mount ceramic capacitors typically have very-low value for θJC, but not accurately provide a value for the ESR and are commonly available in values up to 10µF. total thermal resistance θJA. θJA is the sum of θJC and θ Other low-ESR capacitors, such as surface-mount tan- CA, and the manufacturer can seldom reliably predict talum, may also be used. Do not use low-cost alu- the thermal characteristics of the application circuit. minum electrolytic capacitors due to their large size Figure 4 shows the estimated allowable power dissipa- and relatively high ESR. Lastly, make sure the input and tion for a MAX1735 mounted on a typical PC board at output capacitors are as close to the IC as possible to ambient temperatures of +50°C, +70°C, and +85°C. minimize the impact of PC board trace impedance. Figure 4 shows the maximum continuous output current for a particular input-to-output voltage differential, for
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