ISW=IOUT1−D+IRIPPLE2=IOUT1−D+ VIN⋅D

2⋅L⋅fSW

(29)

While its average current level is equal to:

ISW=IOUT1−D

(30)

This is due to the fact that the current flowing through the internal power switch is delivered to the output only

during the OFF phase.

The switch peak current must be lower than the minimum current limit of the overcurrent protection and the

average current must be lower than the rated DC current of the device.

In addition to these constraints, the thermal considerations summarized in Section 6.4 Thermal considerations

must also be evaluated.

Figure 23. Inverting buck-boost schematic

IC GND

GND PLANE

C9

100nF

Q1

2STR2160

3

2

JP1

1

3

R13

0.82

C6

100nF

R15

1k

TP2

SYNCH

R7

15k

TP4

VLED+

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

D2 BZX384-C30

TP3

VIN

TP10

GND

TP6

GND

R8

47k

D1 STPS3L60

+

R9

27k

R4

0R

C11

3.3nF

C4

1uF

R16

100k

C10

22nF

L1 47uH

R17

22k

C7

100nF

C5

2.2uF

TP9

VLED-

TP1

DIM

R6

1k

TP7

ICGND

Figure 23. Inverting buck-boost schematic shows the schematic circuit for a LED current source based on

inverting buck-boost topology. The input voltage ranges from 4.5 to 32 V, with typical expected 13 V and it can

drive a string composed of 6 white LEDs with 0.3 A DC.

This schematic also includes additional circuitry:

•

A level shifter network for proper dimming operation, since the ALED6000 local ground (ICGND) is referred

to the negative output voltage given by the voltage drop across the LEDs and the sensins resistor. This

function is implemented by Q1, R15, R16 and R17. However, due to limited control loop bandwidth, the

dimming operation in buck-boost topology is quite limited in terms of minimum achievable DIM pulse

•

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

is implemented by a Zener diode, D2, R7 and the sensing resistor, RSNS = R13

In case the LED string is disconnected the maximum output voltage is given by:

VOUT,MAX=VFB+VD2

(31)

Eq. (31), in addition to Eq. (27), must be verified in order avoid the ALED6000 to be exposed to electrical stress

outside the allowed range.

The series resistor, R7, must be selected in order to limit the maximum current flowing through D2, as shown in

equation below:

ID2= VFB

RSNS+R7

(32)

The compensation network design strategy for buck-boost topology is described in Section 8.4 Compensation

strategy for alternative topologies .

ALED6000

Inverting buck-boost

DS13018 - Rev 4

page 29/45