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LT1074CT Datasheet(PDF) 8 Page - Linear Technology |
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LT1074CT Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  LT1074/LT1076 8 Figure 1. Input Capacitor Ripple Figure 2. Proper Ground Pin Connection VIN PIN The VIN pin is both the supply voltage for internal control circuitry and one end of the high current switch. It is important, especially at low input voltages, that this pin be bypassed with a low ESR, and low inductance capacitor to prevent transient steps or spikes from causing erratic operation. At full switch current of 5A, the switching transients at the regulator input can get very large as shown in Figure 1. Place the input capacitor very close to the regulator and connect it with wide traces to avoid extra inductance. Use radial lead capacitors. FEEDBACK PIN The feedback pin is the inverting input of an error amplifier which controls the regulator output by adjusting duty cycle. The noninverting input is internally connected to a trimmed 2.21V reference. Input bias current is typically 0.5 µA when the error amplifier is balanced (IOUT = 0). The error amplifier has asymmetrical GM for large input sig- nals to reduce startup overshoot. This makes the amplifier more sensitive to large ripple voltages at the feedback pin. 100mVp-p ripple at the feedback pin will create a 14mV offset in the amplifier, equivalent to a 0.7% output voltage shift. To avoid output errors, output ripple (P-P) should be less than 4% of DC output voltage at the point where the output divider is connected. See the “Error Amplifier” section for more details. Frequency Shifting at the Feedback Pin The error amplifier feedback pin (FB) is used to downshift the oscillator frequency when the regulator output voltage is low. This is done to guarantee that output short-circuit DESCRIPTIO S PI LP = Total inductance in input bypass connections and capacitor. “Spike” height (dI/dt • LP) is approximately 2V per inch of lead length for LT1074 and 0.8V per inch for LT1076. “Step” for ESR = 0.05 Ω and ISW = 5A is 0.25V. “Ramp” for C = 200 µF, TON = 5µs, and ISW = 5A, is 0.12V. Input current on the VIN Pin in shutdown mode is the sum of actual supply current ( ≈140µA, with a maximum of 300 µA), and switch leakage current. Consult factory for special testing if shutdown mode input current is critical. GROUND PIN It might seem unusual to describe a ground pin, but in the case of regulators, the ground pin must be connected properly to ensure good load regulation. The internal reference voltage is referenced to the ground pin; so any error in ground pin voltage will be multiplied at the output; LT1074•PD01 () ISW () ESR STEP = () ISW () TON RAMP = ()()L P dl dt C FB GND LT1074 LT1074•PD02 R2 HIGH CURRENT RETURN PATH NEGATIVE OUTPUT NODE WHERE LOAD REGULATION WILL BE MEASURED ∆ ∆ V VV OUT GND OUT = ()( ) 221 . To ensure good load regulation, the ground pin must be connected directly to the proper output node, so that no high currents flow in this path. The output divider resistor should also be connected to this low current connection line as shown in Figure 2. |
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