Application Note 1636

2

AN1636.1

July 27, 2011

If the output voltage desired is -0.8V, then R3 is shorted. The

value for R2 is typically between 1k and 10k. Note that if

compensation must be changed to accommodate the 300kHz

operation, due to minimum on-time limitation. Please refer to

the ISL85033 data sheet for further information.

Frequency Control

The ISL85033-12VEVAL3Z evaluation board has an FS pin that

controls the frequency of operation. Programmable frequency

allows for optimization between efficiency and external

component size. Default switching frequency is 500kHz when FS

is tied to VCC (R9 = 0). By removing R9 and connecting R10 to

frequency could be changed from 300kHz (R10 = 383k) to 2MHz

(R10 = 40.2k). See the ISL85033 data sheet for information

about calculating the value of R10. Do not leave this pin floating.

Disabling/Enabling Function

The ISL85033-12VEVAL3Z evaluation board contains an S1

switch that enables or disables both channels, thus allowing a

low quiescent current state. Table 2 details this function.

NOTE: If driven externally, the EN signal (the same as all input

on the board) and not to GND. Therefore, the source cannot be

GND referenced, and input level shifting is required.

FIGURE 1. ISL85033-12VEVAL3Z BUCK BOOST: RECOMMENDED

0

1

2

3

4

5

6

5

7

9

11

13

15

17

19

21

23

25

60

65

70

75

80

85

90

95

0

0.5

1.0

1.5

2.0

2.5

0

0.5

1.0

1.5

2.0

2.5

60

65

70

75

80

85

90

95

60

65

70

75

80

85

90

95

0

0.5

1.0

1.5

2.0

2.5

TABLE 2. SWITCH SETTINGS

S1

ON/OFF CONTROL

ON

OFF