10

LT1372/LT1377

APPLICATIO S I FOR ATIO

(magnetic) radiation is minimized by keeping output di-

ode, switch pin, and output bypass capacitor leads as

short as possible. E field radiation is kept low by minimiz-

ing the length and area of all traces connected to the switch

pin. A ground plane should always be used under the

switcher circuitry to prevent interplane coupling.

The high speed switching current path is shown schemati-

cally in Figure 3. Minimum lead length in this path is

essential to ensure clean switching and low EMI. The path

including the switch, output diode, and output capacitor is

the only one containing nanosecond rise and fall times.

Keep this path as short as possible.

LOAD

L1

SWITCH

NODE

LT1372 • F03

HIGH

FREQUENCY

CIRCULATING

PATH

Figure 3

More Help

For more detailed information on switching regulator

circuits, please see Application Note 19. Linear Technol-

ogy also offers a computer software program, SwitcherCAD,

to assist in designing switching converters. SwitcherCAD

will be updated in late 1995 for the LT1372 and LT1377. In

addition, our applications department is always ready to

lend a helping hand.

Frequency Compensation

Loop frequency compensation is performed on the output

The main pole is formed by the series capacitor and the

output impedance (

≈500kΩ) of the error amplifier. The

pole falls in the range of 2Hz to 20Hz. The series resistor

creates a “zero” at 1kHz to 5kHz, which improves loop

stability and transient response. A second capacitor,

typically one-tenth the size of the main compensation

capacitor, is sometimes used to reduce the switching

output voltage ripple attenuated by the output divider and

multiplied by the error amplifier. Without the second

(

≈1500µmho)

maximum output load current and will also be increased

if poor quality (high ESR) output capacitors are used. The

addition of a 0.0047

switching frequency ripple to only a few millivolts. A low

margin may be inadequate.

Switch Node Considerations

For maximum efficiency, switch rise and fall time are

made as short as possible. To prevent radiation and high

frequency resonance problems, proper layout of the com-

ponents connected to the switch node is essential. B field