12

LTC1159/LTC1159-3.3/LTC1159-5

APPLICATIO S I FOR ATIO

0.15

µF

CAP

P-DRIVE

P-GND

LTC1159

P-GATE

SHDN2

IRFZ34

VN2222LL

0.1

µF

100pF

5M

1N4148

IRF9Z34

1N5819

0.01

µF

100

Ω

100

Ω

+

100

µF

50V

+

150

µF

16V

OS-CON

+

1

µF

0V = NORMAL

>3V = SHUTDOWN

3300pF

390pF

1k

N-GATE

S-GND

R1

24.9k

LTC1159 • F06

R2

215k

()

1 + R2

R1

100

µH

0.039

Ω

1.25

comparator at 0V common mode, increasing the off-time

approximately 40% and requiring the use of a smaller

Inverting Regular Applications

The LTC1159 can also be used to obtain negative output

voltages from positive inputs. In these inverting applica-

tions, the current sense resistor connects to ground while

the LTC1159 and N-channel MOSFET connections, which

would normally go to ground, instead ride on the negative

output. This allows the negative output voltage to be set by

the same process as in conventional applications, using

either the internal divider (LTC1159-3.3, LTC1159-5) or an

external divider with the adjustable version.

Figure 15 in the Typical Applications shows a synchronous

12V to –12V converter which can supply up to 1A with

the Figure 15 circuit, the entire 12V output voltage is placed

across the driver and control circuits since the LTC1159

ground pins are at –12V. During start-up or short-circuit

conditions, operating power is supplied by the internal

4.5V regulator. The shutdown signal is level-shifted to the

negative output rail by Q3, and Q4 ensures that Q1 and Q2

remain off during the entire shutdown sequence.

Efficiency Considerations

The percent efficiency of a switching regulator is equal to

the output power divided by the input power times 100%.

It is often useful to analyze individual losses to determine

what is limiting the efficiency and which change would

produce the most improvement. Percent efficiency can be

expressed as:

%Efficiency = 100 – (L1 + L2 + L3 + ...)

where L1, L2, etc., are the individual losses as a percentage

of input power.

Although all dissipative elements in the circuit produce

losses, four main sources usually account for most of the

transition losses.

the electrical characteristics which excludes MOSFET

and control circuit currents. The MOSFET driver current

results from switching the gate capacitance of the power

MOSFETs. Each time a MOSFET gate is switched from