1

Application Note 1636

Author: Paul Orfanu

Buck-Boost Regulator With Up to 5A Output Current

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

The ISL85033 is offered in a 4mmx4mm 28 Ld TQFN package

with 1mm maximum height. The complete converter occupies

Key Features

• Adjustable and Synchronizable Negative Output Voltage with

• Current Mode Control

• Adjustable Switching Frequency from 300kHz to 2MHz

• 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.

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

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.

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

Ω and 142kΩ as

shown in Equation 1.

Table 1 summarizes the external component selection for

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.

and load combinations.

EXTERNAL COMPONENTS

-5V

R3 = 43.2k; R5 = 20k; C4 = 2.7nF

C20 = 22µF (1210 ceramic)

C17 = 22µF (1210 ceramic)

C19 = 68µF (15mΩ polymer EEFUD0K680R)

-12V

R3 = 112k; R5 = 120k; C4 = 4.7nF

C14, C17 = 22µF (2210 ceramic)

C12, C19 = 68µF (70mΩ TaNbO2 TPSD686M020R0070)

C20 = DNP

-15V

R3 = 142k; R5 = 120k; C4 = 4.7nF

C14, C17 = 22µF (2210 ceramic)

C12, C19 = 68µF (70mΩ TaNbO2 TPSD686M020R0070)

C20 = DNP

R3

R2

VOUT

VFB

--------------

1

–

⎝⎠

⎛⎞

=

(EQ. 1)

CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.

Copyright Intersil Americas Inc. 2011. All Rights Reserved.

1-888-INTERSIL or 1-888-468-3774

All other trademarks mentioned are the property of their respective owners.

July 27, 2011

AN1636.1