AP1538
18V 3A 300KHz BUCK CONVERTER
AP1538 Rev. 5 1 of 13 OCTOBER 2009
DS31451 www.diodes.com © Diodes Incorporated
• Input voltage : 3.6V to 18V
• Output voltage : 0.8V to VCC
• Output current : up to 3A
• Duty ratio: 0% to 99% PWM control
• Oscillation frequency: 300KHz typical
• Soft-start like, Current limit and Enable function
• Thermal Shutdown function
• Built-in internal SW P-channel MOS
• SOP-8L and SOP-8L-DEP: Available in “Green”
Molding Compound (No Br, Sb)
• Lead Free Finish / RoHS Compliant (Note 1)
AP1538 consists of step-down switching regulator with
PWM control. These devices include a reference voltage source,
oscillation circuit, error amplifier, internal PMOS.
AP1538 provides low-ripple power, high efficiency, and excellent
transient characteristics. The PWM control circuit is able to vary
the duty ratio linearly from 0 up to 99%. This converter also
contains an error amplifier circuit as well as a soft-start circuit that
prevents overshoot at startup. An enable function, an over
current protect fun ction and a sh ort circuit protect function are built
inside, and when OCP or SCP happens, the operation frequency
will be reduced from 300KHz to 50KHz. Also, an internal
compensation block is built in to minimum external component
count.
With the addition of an internal P-channel Power MOS, a coil,
capacitors, and a diode connected externally, these ICs can
function as step-down switching regulators. They serve as ideal
power supply units for portable devices when coupled with the
SOP-8L and SOP-8L-DEP mini-packages, providing such
outstanding features as low current consumption. Since this
converter can accommodate an input voltage up to 18V, it is also
suitable for the operation via an AC adapter.
• PC Motherboard
• LCD Monitor
• Graphic Card
• DVD-Video Player
• Telecom Equipment
• ADSL Modem
• Printer and other Peripheral Equipment
• Microprocessor core supply
Typical Application Circuit
AP 1538 +
-
VSS
EN
Cc RA
RB
D1**
FB
Vcc Output
CVcc
VOUT = 5V/3A
VIN
ROCSET
OCSET COUT
CIN
Option*
COCSET
CEN
REN
Option
100K
L1
22uH
6.8K
1.3K
3K
0.1uF
+
-
C
0.1uF 470uF***
470uF
0.1uF
Notes: * Typical feedback compensation (Cc): 5600pF.
** Suggested DIODES Power Schottky P/N: B540 series or PDS540.
*** Suggested COUT for VOUT < 1V; 680uF.
VOUT
= VFB x (1+RA/RB)
RB= 0.7K~5K ohm
Features General Description
Applications
AP1538
18V 3A 300KHz BUCK CONVERTER
AP1538 Rev. 5 2 of 13 OCTOBER 2009
DS31451 www.diodes.com © Diodes Incorporated
Ordering Information
AP 1538 XXX G - 13
Package Packing
S : SOP-8L
Green
13 : Tape & Reel
G : Green
SDP : SOP-8L-DEP
Device Package
Code Packaging
(Note 2) 13” Tape and Reel
Quantity Part Number Suffix
AP1538SG-13 S SOP-8L 2500/Tape & Reel -13
AP1538SDPG-13 SDP SOP-8L-DEP 2500/Tape & Reel -13
Notes: 1. EU Directive 2002/95/EC (RoHS). All applicable RoHS exemptions app lied. Please visit our website at
http://www.diodes.com/products/lead_free.html
2. Pad layout as shown on Diodes Inc. suggested pad layout document AP02001, which can be found on our website at
http://www.diodes.com/datasheets/ap02001.pdf.
Pin Assignments
(1) SOP-8L (2) SOP-8L-DEP (Dual Exposed Pads)
1
2
3
4
8
7
6
5
FB
Output
OCSET
Output
EN
Vss
Vss
VCC
( Top View )
1
2
3
4
8
7
6
5
FB
Output
OCSET
Output
EN
Vss
Vss
VCC
1
2
( Top View )
(Note 3)
Notes: 3. Exposed pad 1 is connected to VSS and exposed pad 2 is connected to Output. The board layout for exposed pads needs to be considered to
avoid short circuit.
AP1538
18V 3A 300KHz BUCK CONVERTER
AP1538 Rev. 5 3 of 13 OCTOBER 2009
DS31451 www.diodes.com © Diodes Incorporated
Pin Descriptions
Pin Name Pin No. Description
FB 1 Feedback pin
EN 2
Power-off pin
H: Normal operation
(Step-down operation)
L: Step-down operation stopped
(All circuits deactivated)
OCSET 3 Add an external resistor to set max output current
VCC 4 IC power supply pin
Output 5, 6
Switch Pin. Co nnect external in du ctor/diode here. Minimize tra ce
area at th is pi n t o red uc e EMI
VSS 7, 8 GND Pin
Block Diagram
Oscillation
Circuit Reference Voltage
So urc e with S o ft Start
PWM-Switched
Control Circuit
+
-
VssVEN
EN
90uA
OCSET
FB
Vcc
Thermal
Shutdown
Output
AP1538
18V 3A 300KHz BUCK CONVERTER
AP1538 Rev. 5 4 of 13 OCTOBER 2009
DS31451 www.diodes.com © Diodes Incorporated
Absolute Maximum Ratings
Symbol Parameter Rating Unit
ESD HBM Human Body Model ESD Protection 5.5 KV
ESD MM Machine Model ESD Protection 200 V
VCC VCC Pin Voltage VSS - 0.3 to VSS + 20 V
VFB Feedback Pin Voltage VSS - 0.3 to VCC V
VEN EN Pin Vo ltage VSS - 0.3 to VCC V
VOUT Switch Pin Voltage VSS - 0.3 to VCC V
PD Power Dissipation Internally limited mW
TJ Operating Junction Temperature Range -40 to +125 oC
TST Storage Temperature Range -65 to +150 oC
Caution: The absolute maximum ratings are rated values exceeding which the product could suffer physical damage.
These values must therefore not be exceeded under any conditions.
Recommended Operating Conditions
Symbol Parameter Min Max Unit
VIN Input Voltage (Note 4) 3.6 18 V
IOUT Output Current 0 3 A
TA Operating Ambient Temperature -20 +85 oC
Notes: 4. For the operations in low input voltage, AP1538 can tolerate down to 3.6V but max output current loading will be less than 3A. For nominal
applications in such low input voltage range, especiall y lower than 4V, a higher ROCSET with larger heat sink is recommended.
AP1538
18V 3A 300KHz BUCK CONVERTER
AP1538 Rev. 5 5 of 13 OCTOBER 2009
DS31451 www.diodes.com © Diodes Incorporated
Electrical Characteristics
(VIN = 12V, TA = 25°C, unless otherwise specified)
Symbol Parameter Conditions Min Typ. Max Unit
VFB Feedback Voltage IOUT = 0.1A 0.784 0.8 0. 816 V
IFB Feedback Bias Current IOUT = 0.1A - 0.1 0.5 µA
ISHDN Current Consumption During
Power Off VEN = 0V - 10 - µA
∆VOUT
/ VIN Line Regulation VIN = 5V~18V - 1 2 %
∆VOUT
/ VOUT Load Regulation IOUT = 0.1 to 3A - 0.2 0.5 %
fOSC Oscillation Frequency Measure waveform at SW pin 240 300 400 KHz
fOSC1 Frequency of Current Limit or
Short Circuit Protection Measure waveform at SW pin - 50 - KHz
VIH EN Pin Input Voltage Evaluate oscillation at SW pin 2.0 - - V
VIL Evaluate oscillation stop at SW pin - - 0.8
ISH EN Pin Input Leakage Current EN Pin High - 20 - µA
ISL EN Pin Low - -10 - µA
IOCSET OCSET P in Bias Current -- 75 90 105 µA
RDS(ON) Internal MOSFET RDS(ON) VIN=5V, VFB=0V - 110 -
mΩ
VIN=12V, VFB=0V - 70 -
EFFI Efficiency VIN=12V, VOUT = 5V
IOUT=3
A
- 92 - %
TSHDN Thermal shutdown threshold - 150 - °C
THYS Thermal shutdown hysteresis - 55 - °C
θJC Thermal Resistance
Junction-to-Case SOP-8L (Note 5) - 24 - oC/W
SOP-8L-DEP (Note 5) - 26 -
Notes: 5. Test condition for SOP-8L and SOP-8L-DEP: Devices mounted on 2oz copper, minimum recommended pad layout on top & bottom l a yer with
thermal vias, double sided FR-4 PCB.
AP1538
18V 3A 300KHz BUCK CONVERTER
AP1538 Rev. 5 6 of 13 OCTOBER 2009
DS31451 www.diodes.com © Diodes Incorporated
Typical Performance Characteristics
V
IN
vs. Frequency
(V
OUT
= 3.3V, I
OUT
= 0.2 A)
306
308
310
312
314
316
318
320
322
324
326
0 2 4 6 8 10 12 14 16 18
V
IN
(V)
F
OSC
(kHz)
V
IN
vs. V
FB
(V
OUT
= 3.3V, I
OUT
= 0.2 A)
0.780
0.785
0.790
0.795
0.800
0.805
0.810
0.815
0.820
024681012141618
V
IN
(V)
V
FB
(V)
Line Regulat ion
(V
OUT
= 3.3V, I
OUT
= 0.2A)
3.00
3.05
3.10
3.15
3.20
3.25
3.30
3.35
3.40
024681012141618
V
IN
(V)
V
OUT
(V)
Load Regulation
(V
IN
= 12V)
3.25
3.26
3.27
3.28
3.29
3.30
3.31
3.32
3.33
3.34
3.35
0.0 0.5 1.0 1.5 2.0 2.5 3.0
I
OUT
(A)
V
OUT
(V)
Efficiency
(VIN = 12V)
0
10
20
30
40
50
60
70
80
90
100
0.0 1.0 2.0 3.0
IOUT (A)
Ef ficie ncy (%)
VOUT=3.3V
VOUT=5V
Switch Current Limit
(T
A
= 25C)
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
0 2 4 6 8 10 12 14 16 18
V
IN
(V)
Current Li m it (A)
ROCSET=6.8k
ROCSET=3.9k
AP1538
18V 3A 300KHz BUCK CONVERTER
AP1538 Rev. 5 7 of 13 OCTOBER 2009
DS31451 www.diodes.com © Diodes Incorporated
Typical Performance Characteristics (Continued)
Current Li m i t vs. Tem perat ure
(V
IN
=12V, R
OCSET
=6.8k)
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
-25 0 25 50 75 100
Ambient Temperature (C)
Current Limit (A)
V
OUT Ripple
(VIN = 12V, VOUT = 3.3V, IOUT = 0.1A)
V
OUT Ripple
(VIN = 12V, VOUT = 3. 3V, IOUT = 3.0A)
Load Transient Response
(VIN = 12V, VOUT = 3.3V, IOUT = 0. 1~3A)
AP1538
18V 3A 300KHz BUCK CONVERTER
AP1538 Rev. 5 9 of 13 OCTOBER 2009
DS31451 www.diodes.com © Diodes Incorporated
Functional Description
PWM Control
The AP1538 is a DC/DC converter that employs pulse width
modulation (PWM) scheme. Its pulse width varies in the range of
0% to 99%, based on the output current loading. The output
ripple voltage caused by the PWM high frequency switching can
easily be reduced through an output filter. Therefore, this
converter provides a low ripple output supply over a broad range
of input voltage & output current loading
Under Voltage Lockout
The under voltage lockout circuit of the AP1538 assures that the
high-side MOSFET driver remains in the off state whenever the
supply voltage drops below 3.3V. Normal operation resumes
once VCC rises above 3.5V.
Current Limit Protection
The current limit threshold is set by external resistor ROCSET
connected from VCC supply to OCSET pin. The internal sink
current IOCSET (90uA typical) across this resistor sets the voltage
at OCSET pin. When the PWM voltage is less than the voltage at
OCSET, an over-current c ondition is trigger ed.
The current limit threshold is given by the following equation:
RIRI OCSETOCSETDS(ON)PEAK ×=×
2)I(Δ
+> II OUT(MAX)PEAK
where,
IN
OUTOUTIN
V
V
Lfs
VV
I×
×
−
=Δ
IPEAK is the output peak current; RDS (ON) is the MOSFET ON
resistance; FS is the PWM frequency (300KHz typical). Also, the
inductor value will affect the ripple current ΔI.
The above equation is recommended for input voltage range of
5V to 18V. For input voltage lower than 5V or ambient
temperature over 100°C, higher ROCSET is recommended.
The recommen ded mini mum R OCSET va lue is summarized b elow :
VOUT
(V) VIN (V)
5V 12V 18V
0.8 4.7K 3.0K 3.3K
1.0 5.6K 3.0K 3.9K
1.2 5.6K 3.0K 3.9K
1.8 5.6K 3.0K 3.9K
2.5 5.6K 3.0K 3.9K
3.3 5.6K 3.0K 3.9K
5.0 N/A 3.0K 4.7K
Inductor Selection
For most designs, the operates with inductors of 22µH to
33µH. The inductor value can be derived from the following
equation:
IN
OUTOUTIN
V
V
fs
VV
L×
Δ×
−
=I
Where ΔIL is inductor Ripple Current. Large value inductors lower
ripple current and small value inductors result in high ripple
current. Choose inductor ripple current approximately 15% of the
maximum load current 3A, ∆IL=0.45A. The DC current rating of
the inductor should be at least equal to th e maximum load current
plus half the ripple current to prevent core saturation
(3A+0.225A).
Input Cap acitor Selection
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. The RMS current rating requirement for
the input capacitor of a buck regulator is approximately 1⁄2 the
DC load current. A low ESR input capacitor sized for maximum
RMS curr ent must be used. A 47 0µF low ESR c apacitor f or most
applications is sufficient.
Output Ca pacitor Selection
The output capacitor is required to filter the output voltage and
provides regulator loop stability. The important capacitor
parameters are t he 100K H z Equ i va lent Series R esistance (ESR),
the RMS ripples current rating, voltage rating and capacitance
value. For the output capacitor, the ESR value is the most
important parameter. The output ripple can be calculated from
the following formula.
ESR
ΔIV LRIPPLE
×=
The bulk capacitor’s ESR will determine the output ripple voltage
and the initial voltage drop after a high slew-rate transient.
An aluminum electrolytic capacitor's ESR value is related to the
capacitance and its voltage rating. In most case, 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 requ ired fo r low ou tput ripple v oltage .
PCB Layout Guide
If you need low TC & TJ or large PD (Power Di ssipation), Th e dual
SW pins(5& 6) and Vss pins(7& 8)on the SOP-8L package are
internally connected to die pad, The evaluation board should be
allowed for maximum copper area at output (SW) pins.
1. Connect FB circuits (R1, R2, C1) as closely as possible and
keep away from inductor flux for pure VFB.
2. Connect C3 to Vcc and Vss pin as closely as possible to get
good power filter effect.
3. Connect R4 to Vcc and OCSET pin as closely as possible.
4. Connect ground side of the C2 & D1 & C4 as closely as
possible and use ground plane for best performance.
AP1538
18V 3A 300KHz BUCK CONVERTER
AP1538 Rev. 5 10 of 13 OCTOBER 2009
DS31451 www.diodes.com © Diodes Incorporated
Top Side Layout Guide
Bottom Side Layout Guide
Functional Description (Continued)
Use vias to conduct the heat into the backside of
PCB layer. The heat sink at output (SW) pins should
be allowed for maximum solder-painted area.
Keep the gap of exposed pads from short circuit. Recommended exposed-pads gap: 30~40mil
(0.75~1mm)
Brown: IC exposed pads.
Red: recommended lay out.
Reference pads layout dimension:
Output: 90 x 50 mil
V
ss: 90 x 60 mil
AP1538
18V 3A 300KHz BUCK CONVERTER
AP1538 Rev. 5 11 of 13 OCTOBER 2009
DS31451 www.diodes.com © Diodes Incorporated
AP1538 +
-
VSS
EN
C1 R1
R2
FB
Vcc Output
C3
VOUT
VIN
R4
OCSET C4
C2 Option
C7
C6
R5
Option
L1
+
-
C8
D1
Figure: Layout numbering comparison.
Marking Information
(1) SOP-8L
( Top View )
AP1538
YY WW
XX
Logo
Part Number
5
8
41
WW : Week : 01~52; 52
YY : Year : 08, 09,10~
G : Green
X : Internal Code
represents 52 and 53 week
(2) SOP-8L-DEP
( Top View )
AP1538
YY WW
XX
Logo
Part No.
5
8
41
E WW : Week : 01~52; 52
YY : Year : 08, 09,10~
G : Green
X : Internal Code
represents 52 and 53 week
SOP-8L-DEP
Functional Description (Continued)
AP1538
18V 3A 300KHz BUCK CONVERTER
AP1538 Rev. 5 12 of 13 OCTOBER 2009
DS31451 www.diodes.com © Diodes Incorporated
Package Information ( All Dimensions in mm )
(1) Package type: SOP-8L
1.27typ 0.3/0.5
7°~9°
4.85/4.95
3.85/3.95
5.90/6.10
0.15/0.25
7°~9°
0.62/0.82
0.10/0.20
Gauge Plane
0.254
Seating Plane
0.35max. 45°
Detail "A"
Detail "A"
1.30/1.50
1.75max.
Land Pattern Recommendation
(Unit: mm)
6x-1.27
8x-1.55
8x-0.60
5.4
0°/8°
(2) Package type: SOP-8L-DEP
1
9°(all side)
0.35Typ.
45°
4
5
8
R0.1
0.3/0.5
1.27Typ.
4.85/4.95
Seating Plane
Gauge Plane
5.90/6.10
0.62/0.82
0.254
0/0.13
View"A"
View"A"
1.4/1.5
1.63Max.
3.85/3.95
Land Pattem Recommendation
(Unit: mm)
EXPOSED PAD
Bottom view
EXPOSED PAD
14
58
1.15Typ.
2.05Typ.
2.15Typ. 0.685
0.713
0.20
0.9
C
L
1.7 1.7
C
L
14
5
8
8x-0.60
5.4
2.35
6x-1.27
1.8 1.8
1.0
C
L
0.951.65
C
L
Top View
0.15/0.25
8x-1.55
0°/8°
AP1538
18V 3A 300KHz BUCK CONVERTER
AP1538 Rev. 5 13 of 13 OCTOBER 2009
DS31451 www.diodes.com © Diodes Incorporated
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