PAM2305
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1A STEP-DOWN DC-DC CONVERTER
Description
The PAM2305 is a step-down current-mode, DC-DC converter. At
heavy load, the constant frequency PWM control performs excellent
stability and transient response. To ensure the longest battery life in
portable applications, the PAM2305 provides a power-saving Pulse-
Skipping Modulation (PSM) mode to reduce quiescent current under
light load operation to save power.
The PAM2305 supports a range of input voltages from 2.5V to 5.5V,
allowing the use of a single Li+/Li-polymer cell, multiple Alkaline/NiMH
cell, USB, and other standard power sources. The output voltage is
adjustable from 0.6V to the input voltage, while the part number suffix
PAM2305-XX indicates pre-set output voltage of 3.3V, 2.8V, 2.5V,
1.8V, 1.5V, 1.2V or adjustable. All versions employ internal power
switch and synchronous rectifier to minimize external part count and
realize high efficiency. During shutdown, the input is disconnected
from the output and the shutdown current is less than 0.1µA. Other
key features include under-voltage lockout to prevent deep battery
discharge.
The PAM2305 is available in TSOT25, DFN2x2-6 Pin and QFN3x3-16
Pin packages.
Features
• Efficiency up to 96%
• Only 40µA(typ) Quiescent Current
• Output Current: Up to 1A
• Internal Synchronous Rectifier
• 1.5MHz Switching Frequency
• Soft Start
• Under-Voltage Lockout
• Short Circuit Protection
• Thermal Shutdown
• 5-pin Small TSOT25, DFN2x2-6 Pin and QFN3x3-16 Pin
Packages
• Pb-Free Package
Applications
• Cellular Phone
• Portable Electronics
• Wireless Devices
• Cordless Phone
• Computer Peripherals
• Battery Powered Widgets
• Electronic Scales
• Digital Frame
Pin Assignments
Top View
TSOT25
PAM2305
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Typical Applications Circuit
Fixed Output Voltage Adjustable Output Voltage
⎟
⎠
⎞
⎜
⎝
⎛+×= 2R
1R
16.0
VO
Pin Descriptions
Pin
Name
Package Name Function
TSOT25 DFN2x2-6L QFN3x3-16L
VIN 1 3 9, 10, 11, 12 Chip main power supply pin.
GND 2 5 1, 2, 3, 5 Ground.
EN 3 2 7
Enable Control Input. Force this pin voltage above 1.5V, enables the chip, and
below 0.3V shuts down the device.
VOUT/FB 4 6 4
VOUT: Output voltage feedback pin, an internal resistive divider divides the output
voltage down for comparison to the internal reference voltage.
FB: Feedback voltage to internal error amplifier, the threshold voltage is 0.6V.
SW 5 4 13, 14, 15 The drains of the internal main and synchronous power MOSFET.
NC — 1 6, 8, 16 No connection.
Functional Block Diagram
PAM2305
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Absolute Maximum Ratings (@TA = +25°C, unless otherwise specified.)
These are stress ratings only and functional operation is not implied. Exposure to absolute maximum ratings for prolonged time periods may
affect device reliability. All voltages are with respect to ground.
Parameter Rating Unit
Input Voltage -0.3 to +6.0 V
EN, FB Pin Voltage -0.3 to VIN V
SW Pin Voltage -0.3 to (VIN +0.3) V
Junction Temperature 150 °C
Storage Temperature Range -65 to +150 °C
Soldering Temperature 300, 5sec °C
Recommended Operating Conditions (@TA = +25°C, unless otherwise specified.)
Parameter Rating Unit
Supply Voltage 2.5 to 5.5 V
Operation Temperature Range -40 to +85 °C
Junction Temperature Range -40 to +125
Thermal Information
Parameter Package Symbol Max Unit
Thermal Resistance (Junction to Case)
TSOT25 (Note 1)
θJC
130
°C/W
DFN2x2-6 25
QFN3x3-16 14
Thermal Resistance (Junction to Ambient)
TSOT25
θJA
250
DFN2x2-6 68
QFN3x3-16 35
Internal Power Dissipation
TSOT25
PD
400
mW
DFN2x2-6 980
QFN3x3-16 1470
Note: 1. The maximun output current for TSOT25 package is limited by internal power dissipation capacity as described in
Application Information here inafter.
PAM2305
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Electrical Characteristics (@TA = +25°C, VIN = 3.6V, VO = 1.8V, CIN = 10µF, COUT = 10µF, L = 4.7µH, unless otherwise specified.)
Parameter Symbol Test Conditions Min Typ Max Units
Input Voltage Range VIN 2.5 5.5 V
Regulated Feedback Voltage VFB 0.588 0.6 0.612 V
Reference Voltage Line Regulation ΔVFB 0.3 %/V
Regulated Output Voltage Accuracy VO I
O = 100mA -3 +3 %
Peak Inductor Current IPK V
IN = 3V,VFB = 0.5V or VO = 90% 1.5 A
Output Voltage Line Regulation LNR VIN = 2.5V to 5V, IO = 10mA 0.2 0.5 %/V
Output Voltage Load Regulation LDR IO = 1mA to 800mA 0.5 1.5 %
Quiescent Current IQ No load 40 70 µA
Shutdown Current ISD V
EN = 0V 0.1 1 µA
Oscillator Frequency fOSC VO = 100% 1.2 1.5 1.8 MHz
VFB = 0V or VO = 0V 500 kHz
Drain-Source On-State Resistance RDS(ON) I
DS = 100mA P MOSFET 0.3 0.45 Ω
N MOSFET 0.35 0.5 Ω
SW Leakage Current ILSW ±0.01 1 µA
High Efficiency η 96 %
EN Threshold High VEH 1.5 V
EN Threshold Low VEL 0.3 V
EN Leakage Current IEN ±0.01 µA
Over Temperature Protection OTP 150 °C
OTP Hysteresis OTH 30 °C
PAM2305
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Application Information
The basic PAM2305 application circuit is shown in Page 2. External component selection is determined by the load requirement, selecting L first
and then CIN and COUT.
Inductor Selection
For most applications, the value of the inductor will fall in the range of 1μH to 4.7μH. Its value is chosen based on the desired ripple current.
Large value inductors lower ripple current and small value inductors result in higher ripple currents. Higher VIN or VOUT also increases the ripple
current as shown in Equation 1. A reasonable starting point for setting ripple current is ΔIL = 400mA (40% of 1A).
()( )
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛−=
ΔV
V
1
V
Lf
1
I
IN
OUT
OUTL Equation (1)
The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation.
Thus, a 1.4A rated inductor should be enough for most applications (1A + 400mA). For better efficiency, choose a low DC-resistance inductor.
VO 1.2V 1.5V 1.8V 2.5V 3.3V
L 2.2µH 2.2µH 2.2µH 4.7µH 4.7µH
CIN and COUT Selection
In continuous mode, the source current of the top MOSFET is a square wave of duty cycle VOUT/VIN. To prevent large voltage transients, a low
ESR input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by:
()
[]
V
VVV
II
required
C
IN
2/1
OUTINOUT
OMAXRMS
IN
−
≅
This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT /2. This simple worst-case condition is commonly used for design because even
significant deviations do not offer much relief. Note that the capacitor manufacturer's ripple current ratings are often based on 2000 hours of life.
This makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. Consult the
manufacturer if there is any question.
The selection of COUT is driven by the required effective series resistance (ESR).
Typically, once the ESR requirement for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE (P-P) requirement. The
output ripple ΔVOUT is determined by:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛+
Δ
≅
ΔC
f8
1
ESR
iV
OUT
LOUT
Where f = operating frequency, COUT = output capacitance and ΔIL = ripple current in the inductor. For a fixed output voltage, the output ripple is
highest at maximum input voltage since ΔIL increases with input voltage.
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple current, high voltage rating and
low ESR make them ideal for switching regulator applications. Using ceramic capacitors can achieve very low output ripple and small circuit size.
When choosing the input and output ceramic capacitors, choose the X5R or X7R dielectric formulations. These dielectrics have the best
temperature and voltage characteristics of all the ceramics for a given value and size.
Thermal Consideration
Thermal protection limits power dissipation in the PAM2305. When the junction temperature exceeds +150°C, the OTP (Over Temperature
Protection) starts the thermal shutdown and turns the pass transistor off. The pass transistor resumes operation after the junction temperature
drops below +120°C.
For continuous operation, the junction temperature should be maintained below +125°C.
The power dissipation is defined as:
(
)
(
)
VIIF
t
IV
RVVRV
PINQOS
SW
IN
L)ON(DSOINH)ON(DSO
2
OD ++= −+
IQ is the step-down converter quiescent current. The term tsw is used to estimate the full load step-down converter switching losses.
PAM2305
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Application Information (cont.)
For the condition where the step-down converter is in dropout at 100% duty cycle, the total device dissipation reduces to:
VIRIP INQH)ON(DS
2
OD +=
Since RDS(ON), quiescent current, and switching losses all vary with input voltage, the total losses should be investigated over the complete input
voltage range. The maximum power dissipation depends on the thermal resistance of IC package, PCB layout, the rate of surrounding airflow
and temperature difference between junction and ambient. The maximum power dissipation can be calculated by the following formula:
θ
−
=
JA
A)MAX(J
D
TT
P
Where TJ(max) is the maximum allowable junction temperature +125°C. TA is the ambient temperature and θJA is the thermal resistance from the
junction to the ambient. Based on the standard JEDEC for a two layers thermal test board, the thermal resistance θJA of TSOT25 package is
250°C/W, DFN2X2 102°C/W, and QFN3X3 68°C/W, respectively. The maximum power dissipation at TA = +25°C can be calculated by following
formula:
SOT-25 package:
P
D =(125°C - 25°C)/250°C/W = 0.4W
DFN2*2 package:
P
D =(125°C - 25°C)/102°C/W = 0.984W
QFN3*3 package:
P
D =(125°C - 25°C)/68°C/W = 1.47W
Setting the Output Voltage
The internal reference is 0.6V (Typical). The output voltage is calculated as below:
⎟
⎠
⎞
⎜
⎝
⎛+×= 2R
1R
16.0
VO
The output voltage is given by Table 1.
Table 1: Resistor selection for output voltage setting.
VO R1 R2
1.2V 100k 100k
1.5V 150k 100k
1.8V 200k 100k
2.5V 380k 120k
3.3V 540k 120k
100% Duty Cycle Operation
As the input voltage approaches the output voltage, the converter turns the P-Channel transistor continuously on. In this mode the output voltage
is equal to the input voltage minus the voltage drop across the P-Channel transistor:
V
OUT = VIN – ILOAD (RDS(ON) = RL)
where RDS(ON) = P-Channel switch ON resistance, ILOAD = Output current, RL = Inductor DC resistance.
UVLO and Soft-Start
The reference and the circuit remain reset until the VIN crosses its UVLO threshold.
The PAM2305 has an internal soft-start circuit that limits the in-rush current during start-up. This prevents possible voltage drops of the input
voltage and eliminates the output voltage overshoot. The soft-start acts as a digital circuit to increase the switch current in several steps to the P-
Channel current limit (1500mA).
Short Circuit Protection
The switch peak current is limited cycle-by-cycle to a typical value of 1500mA. In the event of an output voltage short circuit, the device operates
with a frequency of 400kHz and minimum duty cycle, therefore the average input current is typically 200mA.
PAM2305
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Application Information (cont.)
Thermal Shutdown
When the die temperature exceeds +150°C, a reset occurs and the reset remains until the temperature decrease to +120°C, at which time the
circuit can be restarted.
PCB Layout Check List
When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the PAM2305. These items are
also illustrated graphically in Figure 1. Check the following in your layout:
1. The power traces, consisting of the GND trace, the SW trace and the VIN trace should be kept short, direct and wide.
2. Does the VFB pin connect directly to the feedback resistors? The resistive divider R1/R2 must be connected between the (+) plate of
COUT and ground.
3. Does the (+) plat e of CIN connect to VIN as closely as possible? This capacitor provides the AC current to the internal power
MOSFETs.
4. Keep the switching node, SW, away from the sensitive VFB node.
5. Keep the (–) plates of CIN and COUT as close as possible.
Figure 1. PAM2305 Suggested Layout
PAM2305
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Ordering Information
Part Number Output Voltage Part Marking Package Type Standard Package
PAM2305AAB330 3.3V BEKYW TSOT25 3000 Units/Tape&Reel
PAM2305AAB280 2.8V BEHYW TSOT25 3000 Units/Tape&Reel
PAM2305AAB250 2.5V BEGYW TSOT25 3000 Units/Tape&Reel
PAM2305AAB180 1.8V BEEYW TSOT25 3000 Units/Tape&Reel
PAM2305AAB150 1.5V BECYW TSOT25 3000 Units/Tape&Reel
PAM2305AAB120 1.2V BEBYW TSOT25 3000 Units/Tape&Reel
PAM2305AABADJ ADJ BEAYW TSOT25 3000 Units/Tape&Reel
PAM2305BJE330 3.3V P2305K QFN3x3 3000 Units/Tape&Reel
PAM2305BJE280 2.8V P2305H QFN3x3 3000 Units/Tape&Reel
PAM2305BJE250 2.5V P2305G QFN3x3 3000 Units/Tape&Reel
PAM2305BJE180 1.8V P2305E QFN3x3 3000 Units/Tape&Reel
PAM2305BJE150 1.5V P2305C QFN3x3 3000 Units/Tape&Reel
PAM2305BJE120 1.2V P2305B QFN3x3 3000 Units/Tape&Reel
PAM2305BJEADJ ADJ P2305A QFN3x3 3000 Units/Tape&Reel
PAM2305CGF330 3.3V BEKYW DFN2x2-6 3000 Units/Tape&Reel
PAM2305CGF280 2.8V BEHYW DFN2x2 3000 Units/Tape&Reel
PAM2305CGF250 2.5V BEGYW DFN2x2 3000 Units/Tape&Reel
PAM2305CGF180 1.8V BEEYW DFN2x2 3000 Units/Tape&Reel
PAM2305CGF150 1.5V BECYW DFN2x2 3000 Units/Tape&Reel
PAM2305CGF120 1.2V BEBYW DFN2x2 3000 Units/Tape&Reel
PAM2305CGFADJ ADJ BEAYW DFN2x2 3000 Units/Tape&Reel
PAM2305
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Package Outline Dimensions (cont.) (All dimensions in mm.)
Notes: 2. Controlling dimensions are in millimeters (angles in degress).
3. Coplanarity applies to the exposed pad as well as the terminals.
4. DAP is 1.90 x 1.90mm.
PAM2305
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written approval of the Chief Executive Officer of Diodes Incorporated. As used herein:
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