Microsemi

Integrated Products Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 8

Copyright

© 2000

Rev. 1.1b, 2005-03-03

LX1741

High Efficiency High Voltage Boost Controller

I N T E GRA T ED

PR ODUC T S

APPLICATION IN FORMATION (CONTINUED )

Output voltage ripple is a function of the inductor value

boost converter regulation scheme. When the switch is

first turned on, the peak-to-peak voltage ripple is a

function of the output droop (as the inductor current

typically 10mV), and the output overshoot (when the

stored energy in the inductor is delivered to the load at the

end of the charging cycle). Therefore the total ripple

voltage is

The initial droop can be estimated as follows where the

0.5v value in the denominator is an estimate of the voltage

()

()

0.5

V

I

I

C

L

∆V

IN

OUT

PK

OUT

DROOP

−

×

×

=

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

The output overshoot can be estimated as follows

where the 0.5 value in the denominator is an estimate of

the voltage drop across the diode:

()

()

IN

OUT

2

OUT

PK

OUT

2

1

OVERSHOOT

V

0.5

V

I

I

C

L

∆V

−

+

−

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

=

()

()

mV

8

2

0.5

3.0

35mA

200mA

F

4.7

H

47

=

−

×

×

⎟

⎠

⎞

⎜

⎝

⎛

µ

µ

=

()

()

mV

4

.

9

0

.

3

0.5

0

.

8

1

mA

5

3

mA

200

F

4.7

H

47

∆V

2

2

1

OVERSHOOT

=

−

+

−

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

µ

µ

×

=

Increasing the output capacitor value results in the

reduction of the output voltage ripple voltage. Low ESR

capacitors are recommended to reduce ripple caused by the

switching current.

Multi-layer ceramic capacitors with

X5R or X7R dielectric are a superior choice featuring small

size, very low ESR, and a temperature stable dielectric.

Low ESR electrolytic capacitors such as solid tantalum or

OS-CON types are also acceptable. Moreover, adding a

capacitor from the output to the feedback pin (C2) allows

the internal feedback circuitry to respond faster which

further minimizes output voltage ripple and reduces noise

coupling into the high impedance feedback input.

A Schottky diode is recommended for most applications

(e.g. Microsemi UPS5819). The low forward voltage drop

and fast recovery time associated with this device supports

the switching demands associated with this circuit topology.

The designer is encouraged to consider the diode’s average

and peak current ratings with respect to the application’s

output and peak inductor current requirements. Further, the

diode’s reverse breakdown voltage characteristic must be

capable of withstanding a negative voltage transition that is

The LX1741 can drive up to 100mA of gate drive

current. An N-channel MOSFET with a relatively low

required to optimize overall circuit performance.

The

LXE1741 Evaluation Board uses a Fairchild FDV303. This

NMOS device was chosen because it demonstrates an

The LX1741 produces high slew-rate voltage and current

waveforms hence; the designer should take this into

consideration when laying out the circuit. Minimizing trace

lengths from the IC to the inductor, transistor, diode, input

and output capacitors, and feedback connection (i.e., pin 6)

are typical considerations. Moreover, the designer should

maximize the DC input and output trace widths to

accommodate peak current levels associated with this

circuit.