LTC3452EUF datasheet(12/16 Pages) LINER

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LTC3452

3452f

Optional Schottky Diodes

Schottky diodes across the synchronous switches B and

D are not required, but provide a lower drop during the

break-before-make time (typically 20ns) of the NMOS to

PMOS transition, improving efficiency. Use a Schottky

diode such as an MBRM120T3 or equivalent. Do not use

ordinary rectifier diodes, since the slow recovery times

will compromise efficiency.

Closing the Feedback Loop

The LTC3452 incorporates voltage mode PWM control.

The control to output gain varies with operation region

(Buck, Boost, Buck/Boost), but is usually no greater than

15. The output filter exhibits a double pole response

given by:

FILTER POLE

OUT

••

•

where COUT is the output filter capacitor.

The output filter zero is given by:

FILTER ZERO

ESR

OUT

••

•

where RESR is the capacitor equivalent series resistance.

A troublesome feature in Boost mode is the right-half

plane zero (RHP), and is given by:

IL V

RHPZ

OUT

2• •

• •

The loop gain is typically rolled off before the RHP zero

frequency.

A simple Type I compensation network can be incorpo-

rated to stabilize the loop but at a cost of reduced band-

width and slower transient response. To ensure proper

phase margin, the loop is required to be crossed over a

decade before the LC double pole.

The unity-gain frequency of the error amplifier with the

Type I compensation is given by:

2•

•

where gm is the error amp transconductance (typically

1/5.2k) and CVC is the external capacitor to GND at the

VC pin. For the white LED application, a 0.1µF or greater

capacitor value is recommended.

Paralleling LED Outputs for Higher Current

Two or more LED output pins can be connected together

in parallel to achieve higher output current in fewer than 7

LEDs. For a very high power LED such as a LumiLED, all

7 outputs can be connected in parallel for maximum total

output current, as shown in the back page application of

this data sheet.

Maximum LED Current

As described in the Operation section, the maximum

output LED currents are equal to:

and

MAXL

ISETL

MAXH

ISETH

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

256

768

Since the maximum LED current for the low power bank is

25mA, this sets a minimum limit on RISETL of:

MINL =

⎛

⎝⎜

⎞

⎠⎟

Ω

256

8192

Similarly, for the high power bank:

MINH =

⎛

⎝⎜

⎞

⎠⎟

Ω

768

150

4096

In addition, since the maximum continuous output current

for the buck-boost is limited to 425mA, this may impose

higher resistor value minimums if all outputs are used.

APPLICATIO S I FOR ATIO

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