LT1074CK datasheet(8/16 Pages) LINER

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LT1074/LT1076

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

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

GND

LT1074

LT1074•PD02

HIGH CURRENT

RETURN PATH

NEGATIVE OUTPUT NODE

WHERE LOAD REGULATION

WILL BE MEASURED

∆

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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