5

LT1611

OPERATIO

boost converter, generating a negative output voltage,

which is directly regulated. The circuit schematic is de-

tailed in Figure 3. Only one inductor is required, and the

two diodes can be in a single SOT-23 package. Output

noise is the same as in a boost converter, because current

is delivered to the output only during the time when the

LT1611’s internal switch is off.

If D2 is replaced by an inductor, as shown in Figure 4, a

higher performance solution results. This converter topol-

ogy was developed by Professor S. Cuk of the California

Institute of Technology in the 1970s. A low ripple voltage

results with this topology due to inductor L2 in series with

the output. Abrupt changes in output capacitor current are

eliminated because the output inductor delivers current to

the output during both the off-time and the on-time of the

LT1611 switch. With proper layout and high quality output

The operation of Cuk’s topology is shown in Figures 5

and 6. During the first switching phase, the LT1611’s

switch, represented by Q1, is on. There are two current

loops in operation. The first loop begins at input capacitor

C1, flows through L1, Q1 and back to C1. The second loop

flows from output capacitor C3, through L2, C2, Q1 and

L1 and L2 current. C2 functions as a voltage level shifter,

across it.

When Q1 turns off during the second phase of switching,

Now current in the first loop, begining at C1, flows through

L1, C2, D1 and back to C1. Current in the second loop flows

from C3 through L2, D1 and back to C3. Load current

continues to be supplied by L2 and C3.

An important layout issue arises due to the chopped

nature of the currents flowing in Q1 and D1. If they are both

tied directly to the ground plane before being combined,

switching noise will be introduced into the ground plane.

It is almost impossible to get rid of this noise, once present

in the ground plane. The solution is to tie D1’s cathode to

the ground pin of the LT1611 before the combined cur-

rents are dumped into the ground plane as drawn in

Figures 4, 5 and 6. This single layout technique can

virtually eliminate high frequency “spike” noise so often

present on switching regulator outputs.

Output ripple voltage appears as a triangular waveform

of L2 multiplied by the equivalent series resistance (ESR)

of output capacitor C3. Increasing the inductance of L1

and L2 lowers the ripple current, which leads to lower

output voltage ripple. Decreasing the ESR of C3, by using

ceramic or other low ESR type capacitors, lowers output

ripple voltage. Output ripple voltage can be reduced to

arbitrarily low levels by using large value inductors and

low ESR, high value capacitors.

1611 F03

SW

L1

D1

D2

GND

LT1611

C1

C3

C2

1

µF

R2

10k

R1

NFB

SHDN

SHUTDOWN

+

1611 F04

SW

L1

L2

D1

GND

LT1611

C1

C3

C2

1

µF

R2

10k

R1

NFB

+

Figure 3. Direct Regulation of Negative Output

Using Boost Converter with Charge Pump

Figure 4. L2 Replaces D2 to Make Low Output Ripple

Inverting Topology. Coupled or Uncoupled Inductors Can

Be Used. Follow Phasing If Coupled for Best Results