Figure 26. Positive buck-boost topology

ALED6000

VIN

VOUT

The main drawback of this topology, compared to inverting buck-boost, is the slightly lower efficiency. In the

positive buck-boost Eq. (27) is not applied, so the maximum input voltage is the maximum operating input voltage

for the ALED6000, as shown in Table 5. Electrical characteristics. In addition, the maximum output voltage can be

higher than 61 V, assuming that the external MOS FET and Schottky diode are properly rated.

The other considerations summarized in Section 8.1 Inverting buck-boost are also applied to this topology.

Figure 26. Positive buck-boost topology shows the schematic circuit for an LED current source based on positive

buck-boost topology. The expected input voltage is 24 V, with dynamic variation in 9 - 60 V range and it can drive

a string composed of 8 white LEDs with 0.53 A DC.

Figure 27. Positive buck-boost schematic

R19

0

C7

100nF

R9

27k

C16

N.M.

TP8

VBIAS

L2

4.7uH

TP1

DIM

R1

100k

TP7

GND

C11

1nF

D3

BZX384-B30

R4

0

R6 1.5k

L1 47uH

+

C12

TP6

GND

R3

0R

Q1

R15

100R

C8

N.M.

U1

ALED6000

DIM 12

VIN_2

3

SS

6

SYNCH

7

FSW

10

ILIM

11

COMP 8

EN

5

FB 9

GND

16

VCC

4

LX_1 13

LX_2 14

VIN_1

2

VBIAS 1

BOOT 15

TP

17

R5 N.M.

D2 STPS3L60

D4

BAS70

TP5

EN

R12

100k

D1

STPS3L60

C9

68nF

R2

0R

TP2

SYNCH

R16

N.M.

D5

BZX384-B12

TP9 VLED-

TP3

VIN

TP4 VLED+

C13

N.M.

R13

0.47

C15

2.2nF

R8

47k

R7

10k

R18

N.M.

This schematic also includes two additional circuits:

•

A level shifter / clamping network for safe MOS driving and protection

•

An external overvoltage protection which avoids the IC damage in case of LED open row fault. This function

is implemented by a Zener diode, D3, R7 and the sensing resistor, RSNS = R13

ALED6000

Positive buck-boost

DS13018 - Rev 4

page 31/45