Datasheet LT1074, LT1076 (Analog Devices) - 8

LT1074/LT1076
U U PI DESCRIPTIO S VIN PIN
(∆V V)( ) GND OUT The VIN pin is both the supply voltage for internal control ∆VOUT = circuitry and one end of the high current switch. It is 2 2 . 1 important, especially at low input voltages, that this pin be To ensure good load regulation, the ground pin must be bypassed with a low ESR, and low inductance capacitor to connected directly to the proper output node, so that no prevent transient steps or spikes from causing erratic high currents flow in this path. The output divider resistor operation. At full switch current of 5A, the switching should also be connected to this low current connection transients at the regulator input can get very large as line as shown in Figure 2. 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. LT1074 ( FB dl GND dt ) (LP) R2 STEP = (I (ESR) SW ) RAMP = (I (T HIGH CURRENT NEGATIVE OUTPUT NODE ON ) SW ) RETURN PATH WHERE LOAD REGULATION C WILL BE MEASURED LT1074•PD01 LT1074•PD02
Figure 1. Input Capacitor Ripple Figure 2. Proper Ground Pin Connection
L
FEEDBACK PIN
P = Total inductance in input bypass connections and capacitor. The feedback pin is the inverting input of an error amplifier “Spike” height (dI/dt • L which controls the regulator output by adjusting duty P) is approximately 2V per inch of lead length for LT1074 and 0.8V per inch for cycle. The noninverting input is internally connected to a LT1076. trimmed 2.21V reference. Input bias current is typically 0.5µA when the error amplifier is balanced (IOUT = 0). The “Step” for ESR = 0.05Ω and ISW = 5A is 0.25V. error amplifier has asymmetrical GM for large input sig- “Ramp” for C = 200µF, TON = 5µs, and ISW = 5A, nals to reduce startup overshoot. This makes the amplifier is 0.12V. more sensitive to large ripple voltages at the feedback pin. Input current on the VIN Pin in shutdown mode is the sum 100mVp-p ripple at the feedback pin will create a 14mV of actual supply current (≈140µA, with a maximum of offset in the amplifier, equivalent to a 0.7% output voltage 300µA), and switch leakage current. Consult factory for shift. To avoid output errors, output ripple (P-P) should be special testing if shutdown mode input current is critical. less than 4% of DC output voltage at the point where the output divider is connected.
GROUND PIN
See the “Error Amplifier” section for more details. It might seem unusual to describe a ground pin, but in the
Frequency Shifting at the Feedback Pin
case of regulators, the ground pin must be connected properly to ensure good load regulation. The internal The error amplifier feedback pin (FB) is used to downshift reference voltage is referenced to the ground pin; so any the oscillator frequency when the regulator output voltage error in ground pin voltage will be multiplied at the output; is low. This is done to guarantee that output short-circuit sn1074 1074fds 8