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AP434/434L Dual Operational Amplifier and VoltageReference ( Preliminary )
ANP014

Application Note
AP1513 300KHz, 2A High Efficiency PWM Buck DC/DC Converter
This application note contains new product information. Diodes, Inc. reserves the right to modify the product specification without notice. No liability is
assumed as a result of the use of this product. No rights under any patent accompany the sale of the product.
1/9
ANP014 App. Note 1
May 2006

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Contents



1. Features
2. Introduction
3. Regulator Design Procedure
4. Design
Example






© Diodes Incorporated


ANP014
Application Note
AP1513 300KHz, 2A High Efficiency PWM Buck DC/DC Converter


2/9
ANP014 App. Note 1
May 2006

www.diodes.com
1.0 Features

Small Board Size
-
Entire circuit can fit in less than 1 square inch of PCB space


Low Implementation Cost
- Fewer than 4 discrete components required

ON
/OFF Control
-
Be controlled by external logic level signal


Thermal Shut-Down and Current Limit
- Thermal shutdown function built in and current limit level can be set by outside resistor

Simple Feedback Compensation
-
Lead compensation using external capacitor


Immediate Implementation
- Schematic, bill-of-materials and board layout available from Anachip

2.0 Introduction

This application note discusses simple ways to select all necessary components to implement a
step-down (BUCK) regulator and gives a design example. In this example, the AP1513 monolithic IC is used
to design a cost-effective and high-efficiency miniature switching buck regulator. Please refer to the
datasheet for more complete information, as pin descriptions and specifications for the AP1513 will not be
repeated here.

This demonstration board allows the designer to evaluate the performance of the AP1513 series buck
regulator in a typical application circuit. The user needs only to supply an input voltage and a load. The
demonstration board can be configured to evaluate adjustable output voltage settings by two resistors.
Operation at different voltages and currents may be accomplished by proper component selection and
replacement.








© Diodes Incorporated


ANP014
Application Note
AP1513 300KHz, 2A High Efficiency PWM Buck DC/DC Converter


3/9
ANP014 App. Note 1
May 2006

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3.0 Regulator Design Procedure

3.0 .1

Given Power Specification
V
IN (max)
= Maximum Input Voltage
V
IN (min)
= Minimum Input Voltage
V
OUT
= Regulated Output Voltage
V
RIPPLE
= Ripple Voltage (peak-to-peak), typical value is 0.6% of the output voltage
I
LOAD(max)
= Maximum Load Current
I
LOAD(min)
= Minimum Load Current before the circuit becomes discontinuous, typical value is 10% of
the Maximum Load Current

F
= Switching Frequency (fixed at a nominal 300 kHz)


3.0.2 Programming Output Voltage (refer to 4.0.4 Demo Board Schematic P7)


The output voltage is programmed by selection of the divider R1 and R2. The designer should use
resistors R1 and R2 with ±1% tolerance in order to obtain best accuracy of output voltage. The output
voltage can be calculated from the following formula:


V
out
= 0.8 x (1 + R1 / R2)

Select a value for R2 between 0.7K and 5K. The lower resistor values minimize noise pickup in the
sensitive feedback pin.


3.0.3 Programming Current Limit Level (refer to 4.0.4 Demo Board Schematic P7)

Select a value for R4 to set the current limit level by using this formula:
R
I
R
I
OCSET
OCSET
on
DS
LOAD
×
=
×
)
(


In this application we use R4 to be the R
OCSET
and in the example we use 3.0K resistor, the R
DS(ON)
is
100m and the I
OCSET
is 90uA, so we limit the maximum load current to 2.7A.


© Diodes Incorporated


ANP014
Application Note
AP1513 300KHz, 2A High Efficiency PWM Buck DC/DC Converter


4/9
ANP014 App. Note 1
May 2006

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3.0.4 Inductor Selection
A.
The minimum inductor
can be calculated from the following design formula table:
L
(min)
Calculation
Step-down (buck) regulator
Duty

(
)
V
V
V
V
V
F
SAT
IN
F
OUT
+ +
(min)


TT
OFF
ON

(
)
V
V
V
V
V
OUT
SAT
IN
F
OUT
+
(min)




L
(min)
[
]
I
T
V
V
V
LOAD
ON
OUT
SAT
IN
(min)
(max)
(min)
2
×
×

V
SAT
= Internal switch saturation voltage of the AP1513 =
R
I
on
DS
LOAD
)
(
×
V

V
F
= Forward voltage drop of output rectifier D1 = 0.5V

B.
The inductor must be designed so that it does not saturate or significantly saturate at DC current bias of
I
PK

.
(
I
PK
= Peak inductor or switch current =
I
I
LOAD
LOAD
(min)
(max)
+
)


3.0.5 Output Capacitor Selection

A.
The output capacitor is required to filter the output and provide regulator loop stability. When selecting
an output capacitor, the important capacitor parameters are; the 100kHz Equivalent Series Resistance
(ESR), the RMS ripples current rating, voltage rating, and capacitance value. For the output capacitor,
the ESR value is the most important parameter. The ESR can be calculated from the following formula:




×
=
I
V
LOAD
RIPPLE
ESR
(min)
2
------------------------ (3)

An aluminum electrolytic capacitor's ESR value is related to the capacitance and its voltage rating. In
most cases, higher voltage electrolytic capacitors have lower ESR values. Most of the time, capacitors with
much higher voltage ratings may be needed to provide the low ESR values required for low output ripple
voltage. If the selected capacitor's ESR is extremely low, it results in an oscillation at the output. It is
recommended to replace this low ESR capacitor by using two general standard capacitors in parallel.

B.
The capacitor voltage rating should be at least 1.5 times greater than the output voltage, and often much
higher voltage ratings are needed to satisfy the low ESR requirements needed for low output ripple
voltage.











© Diodes Incorporated


ANP014
Application Note
AP1513 300KHz, 2A High Efficiency PWM Buck DC/DC Converter


5/9
ANP014 App. Note 1
May 2006

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3.0.6 Output Rectifier Selection


A.
The current rating of the output rectifier D1 must be greater than the peak switch current I
PK
. The
reverse voltage rating of the output rectifier D1 should be at least 1.25 times the maximum input voltage.


B.
The output rectifier D1 must be fast (short reverse recovery time) and must be located close to the
AP1513 using short leads and short printed circuit traces.

Because of their fast switching speed and low
forward voltage drop, Schottky Diodes provide the best performance and efficiency, and should be the
first choice, especially in low output voltage applications.


3.0.7 Input Capacitor Selection

A.
The RMS current rating of the input capacitor can be calculated from the following formula table. The
capacitor manufactured by the datasheet must be checked to assure that this current rating is not
exceeded.

Calculation
Step-down (buck) regulator
T
on
/(T
on
+T
off
)
I
PK

I
I
LOAD
LOAD
(min)
(max)
+

I
m

I
I
LOAD
LOAD
(min)
(max)
I
L
I
LOAD(min)
2
×

I
rms
IN
)
(

(
) ( )



+
×
×
2
3
1
I
I
I
L
m
PK

B.

This capacitor should be located close to the IC using short leads and the voltage rating should be
approximately 1.5 times the maximum input voltage.

4.0 Design Example
4.0.1 Summary of Target Specifications

Input Power
V
IN (max)
= +12V;
= +12V
V
IN (min)
Regulated Output Power
V
OUT
= + 5V;
I
LOAD(max)
= 2A;
I
LOAD(min)
= 0.2A
Output Ripple Voltage V
RIPPLE
50 mV peak-to-peak
Output Voltage Load Regulation
0.6% (0.2A to 2A)
Efficiency
87% minimum at full load

Switching Frequency
F = 300kHz ± 15 %



© Diodes Incorporated


ANP014
Application Note
AP1513 300KHz, 2A High Efficiency PWM Buck DC/DC Converter


6/9
ANP014 App. Note 1
May