USER’S MANUAL
ISL85033-12VEVAL3Z
Wide VIN Negative VOUT Buck-Boost Regulator With Up to 5A Output Current
AN1636
Rev 1.00
July 27, 2011
Hardware Description
The ISL85033 is a wide input range, dual standard buck
converter that can be configured to provide a negative output
voltage. The ISL85033-12VEVAL3Z evaluation board is used in
this application note to demonstrate the negative
configuration. In this configuration, V
IN
can range from 4.5V to
(28V - V
OUT
).
The ISL85033 is offered in a 4mmx4mm 28 Ld TQFN package
with 1mm maximum height. The complete converter occupies
6.25cm
2
area.
Evaluating the Other Output Voltages
The ISL85033-12VEVAL3Z kit output is preset to -12V;
however, output voltages can be adjusted from -3.3V to -15V.
The output voltage programming resistor, R3, depends on the
desired output voltage of the regulator. The value for the
feedback resistor is typically between 0 and 142k as
shown in Equation 1.
VOUT
R3
=
R2
-------------- –
1
VFB
(EQ. 1)
Key Features
• Wide Input Voltage Range from 4.5V to (28V - V
OUT
)
• Adjustable and Synchronizable Negative Output Voltage with
Continuous Output Current up to 5A, depending on V
OUT
and
V
IN
• Current Mode Control
• Adjustable Switching Frequency from 300kHz to 2MHz
• Power-Good Detection (referenced to V
OUT
)
• Externally Adjustable Soft-start Time
• Overcurrent and Hiccup Mode Short Circuit Protection,
Thermal Overload Protection, UVLO
• Boot Undervoltage Detection
• Default Out-of-phase Channels Reduce Voltage Ripple and
Component Size. User-selectable Phase Operation.
Table 1 summarizes the external component selection for
V
OUT
= -5V, V
OUT
= -12V, and V
OUT
= -15V.
TABLE 1. COMPONENT SELECTION FOR V
OUT
V
OUT
-5V
EXTERNAL COMPONENTS
R3 = 43.2k; R5 = 20k; C4 = 2.7nF
C20 = 22µF (1210 ceramic)
C17 = 22µF (1210 ceramic)
C19 = 68µF (15mΩ polymer EEFUD0K680R)
R3 = 112k; R5 = 120k; C4 = 4.7nF
C14, C17 = 22µF (2210 ceramic)
C12, C19 = 68µF (70mΩ TaNbO2 TPSD686M020R0070)
C20 = DNP
R3 = 142k; R5 = 120k; C4 = 4.7nF
C14, C17 = 22µF (2210 ceramic)
C12, C19 = 68µF (70mΩ TaNbO2 TPSD686M020R0070)
C20 = DNP
-12V
-15V
Recommended Equipment
The following materials are recommended to perform testing:
• 0V to 30V power supply with at least 7A source current
capability or a 5V battery
• Electronic loads capable of sinking current up to 7A
• Digital multimeters (DMMs)
• 100MHz quad-trace oscilloscope
• Signal generator
The curves in Figure 1 indicate the maximum output current
the converter can deliver as a function of the input voltage and
the selected output voltage configuration.
Figures 2, 3 and 4 show the efficiency for different V
IN
, V
OUT
and load combinations.
Quick Setup Guide
1. Ensure that the circuit is correctly connected to the supply
and loads prior to applying any power.
2. Connect the bias supply to VIN1, the plus terminal to VIN1,
and the negative return to GND1.
3. Verify that the position is ON for S1.
4. Turn on the power supply.
5. Verify the output voltage is -12V for V
OUT
.
AN1636 Rev 1.00
July 27, 2011
Page 1 of 7
ISL85033-12VEVAL3Z
6
5
4
I
OUT
(A)
3
2
1
0
95
90
EFFICIENCY
(%)
85
80
75
70
65
60
5
7
9
11
13
15
V
IN
(V)
17
19
21
23
25
V
IN
= 5V
V
IN
= 12V
V
OUT
= -5V
V
OUT
= -12V
V
OUT
= -15V
0
0.5
1.0
I
OUT
(A)
1.5
2.0
2.5
FIGURE 1. ISL85033-12VEVAL3Z BUCK BOOST: RECOMMENDED
MAXIMUM I
OUT
(V
IN
)
95
90
85
80
75
70
65
60
0
V
IN
= 5V
V
IN
= 12V
V
IN
= 15V
FIGURE 2. ISL85033-12VEVAL3Z EFFICIENCY: V
OUT
= -15V
95
90
EFFICIENCY
(%)
85
80
75
70
65
60
V
IN
= 15V
V
IN
= 6V
V
IN
= 12V
EFFICIENCY
(%)
V
IN
= 5V
0.5
1.0
I
OUT
(A)
1.5
2.0
2.5
0
0.5
1.0
I
OUT
(A)
1.5
2.0
2.5
FIGURE 3. ISL85033-12VEVAL3Z EFFICIENCY: V
OUT
= -12V
FIGURE 4. ISL85033-12VEVAL3Z EFFICIENCY: V
OUT
= -5V
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
V
OUT
< |2.5V| (i.e., -0.8V), the switching frequency and
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.
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.
TABLE 2. SWITCH SETTINGS
S1
ON
OFF
Enable V
OUT
Disable V
OUT
ON/OFF CONTROL
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
the most negative potential on the board (V
OUT
), the switching
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.
NOTE: If driven externally, the EN signal (the same as all input
logic signals) is referenced to V
OUT
(the most negative potential
on the board) and not to GND. Therefore, the source cannot be
GND referenced, and input level shifting is required.
AN1636 Rev 1.00
July 27, 2011
Page 2 of 7
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