5

LT1612

Figure 2. Recommended Component Placement. Traces

Pin Is Kept Low. Lead Length to Battery Should Be Kept Short

Layout Hints

The LT1612 switches current at high speed, mandating

careful attention to layout for proper performance.

You

will not get advertised performance with careless layouts.

Figure 2 shows recommended component placement for

a buck (step-down) converter. Follow this closely in your

PC layout. Note the direct path of the switching loops.

Input capacitor C1 must be placed close (< 5mm) to the IC

oscillation.

The ground terminal of input capacitor C1 should tie close

to Pin 4 of the LT1612. Doing this reduces dI/dt in the

ground copper which keeps high frequency spikes to a

minimum. The DC/DC converter ground should tie to the

PC board ground plane at one place only, to avoid intro-

ducing dI/dt in the ground plane.

OPERATIO

the Circuit of Figure 1

Burst Mode Operation Defeat

To maintain high efficiency at light loads, the LT1612 will

automatically shift into Burst Mode operation (MODE = 0V

or floating). In this mode of operation the oscillator and

switch drive circuitry is alternately turned on and off,

reducing quiescent current to 160

µA. This reduces power

consumption but also adds low frequency voltage ripple to

the output. Figure 3 shows switching waveforms for a 5V

to 3.3V converter running in Burst Mode operation. Output

is pulled high, Burst Mode operation will be inhibited and

the oscillator runs continuously with no low frequency

ripple at the output. See Figures 4 and 5.

Figure 4. Transient Response for the Circuit of Figure 1

with the MODE Pin Tied to Ground or Floating

Figure 5. With the MODE Pin Tied High, Low

Frequency Output Voltage Ripple Is No Longer Present

1612 F03

20mV/DIV

AC COUPLED

IL

200mA/DIV

5

µs/DIV

1612 F04

200mV/DIV

AC COUPLED

10mA TO 310mA

0.1ms/DIV

200mA/DIV

1612 F05

200mV/DIV

AC COUPLED

10mA TO 300mA

0.1ms/DIV

200mA/DIV

1

2

8

7

3

4

6

5

L1

C2

R1

C3

LT1612

GND

SHDN

1612 F02

MODE

MULTIPLE

VIAs

C1

R2