Diodes PAM2804 User Manual

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Description

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PAM2804

1A STEP-DOWN CONSTANT CURRENT, HIGH EFFICIENCY LED DRIVER

Pin Assignments

The PAM2804 is a step-down constant current LED driver. When the

input voltage down to lower than LED forward voltage, then PAM2804

run into LDO mode.

The PAM2804 supports a range of input voltages from 2.5V to 6.0V,

allowing the use of a single Li+/Li-polymer cell, 3AA or 4AA cell, USB,

and other standard power sources.

The FB voltage is only 0.1V to achieve high efficiency.

PAM2804 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 1µA. Other key features include under-

voltage lockout to prevent deep battery discharge of the Li+ battery.

Features

• Efficiency up to 93%

• 180µA(typ) Quiescent Current

• Output Current: Up to 1A

• Internal Synchronous Rectifier

• 1.5MHz Switching Frequency

• Soft-Start

• Under-Voltage Lockout

• Short LED Protection

• Open LED Protection

• Thermal Shutdown

• 5-Pin Small TSOT25 Packages

• Pb-Free Package

Typical Applications Circuit

Top View

TSOT25

Applications

• 3AA or 4AA Batteries Powered Flashlight

• 1 Cell Li-Ion Battery Powered Flashlight

I

= 0.1/RS

PAM2804

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Pin Description

Pin

Number

1

2
3
4

5 FB Feedback voltage to internal error amplifier, the threshold voltage is 0.1V.

Pin

Name

EN

GND Ground

SW The drains of the internal main and synchronous power MOSFET.

VIN Chip main power supply pin.

Enable control input. Force this pin voltage above 1.5V, enables the chip,
and below 0.3V shuts down the device.

Function

Block Diagram

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Absolute Maximum Ratings (@T

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 Pin Voltage -0.3 to +6.5

EN, FB Pin Voltage

SW Pin Voltage

Junction Temperature Range 150

Storage Temperature Range -65 to +150

Soldering Temperature 300, 5sec

PAM2804

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= +25°C, unless otherwise specified.)

A

-0.3 to V

-0.3 to (V

IN

+0.3)

IN

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V

°C

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PAM2804

Recommended Operating Conditions (@T

Parameter Rating Unit

Supply Voltage 2.5 to 6.0

Operation Temperature Range -40 to +85

Junction Temperature Range -40 to +125

= +25°C, unless otherwise specified.)

A

°C

Thermal Information

Parameter Symbol Package Max Unit

Thermal Resistance (Junction to Case)

Thermal Resistance (Junction to Ambient)

Internal Power Dissipation

Note: 1. The maximum output current for TSOT25 package is limited by internal power dissipation capacity as described in Application Information herein after.

θ

JC

θ

JA

P

D

TSOT25 (Note 1) 130

TSOT25 250

TSOT25 400 mW

°C/W

Electrical Characteristics

(@TA = +25°C, VIN = 4.2V, Real WLED load, L = 4.7µH, CIN = 10µF, CO = 10µF, unless otherwise specified.)

Parameter Symbol Test Conditions Min Typ Max Units

Input Voltage Range

Regulated Feedback Voltage

Peak Inductor Current

Quiescent Current

Shutdown Current

Oscillator Frequency

Drain-Source On-State Resistance

SW Leakage Current

High Efficiency η 93 %

EN Threshold High

EN Threshold Low

EN Leakage Current

Over Temperature Protection OTP 150 °C

OTP Hysteresis OTH 30 °C

V

f

R

DS(ON)

I

V

V

V

I

I

OSC

LSW

I

PAM2804

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IN

0.095 0.100 0.105 V

FB

VIN = 5V

PK

No Load

I

Q

VEN = 0V

SD

VO = 100%

IDS = 100mA

±0.01 1 µA

1.5 V

EH

0.3 V

EL

±0.01 µA

EN

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P MOSFET 0.30 0.45 Ω
N MOSFET 0.35 0.50 Ω

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2.5 6.0 V

1.5 A

180 µA

1 µA

1.2 1.5 1.8 MHz

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Typical Performance Characteristics (@T

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PAM2804

= +25°C, L = 4.7µF, CIN = 10µF, CO = 10µF, unless otherwise specified.)

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PAM2804

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Typical Performance Characteristics (cont.) @T

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PAM2804

= +25°C, L = 4.7µF, CIN = 10µF, CO = 10µF, unless otherwise specified.)

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PAM2804

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Application Information

The basic PAM2804 application circuit is shown in Page 1. External component selection is determined by the load requirement, selecting L first

and then C

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 V

current as shown in equation 1. A reasonable starting point for setting ripple current is ΔI

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.

Using Ceramic Input 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 PAM2804. 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:

is the step-down converter quiescent current. The term tsw is used to estimate the full load step-down converter switching losses.

I

Q

For the condition where the step-down converter is in dropout at 100% duty cycle, the total device dissipation reduces to:

and C

IN

I

L

=

OUT

1

()( )

2

IP

OD

2

OD

.

or V

IN

= 400mA (40% of 1A).

L

⎛
⎜

1

OUT

−=Δ

⎜
⎝

V

Lf

+=

⎞

V

OUT

⎟

Equation (1)

⎟

V

IN

⎠

()

−+

RVVRV

V

IN

VIRIP

INQH)ON(DS

L)ON(DSOINH)ON(DSO

()

t

SW

IF

++

OS

VI

INQ

also increases the ripple

OUT

Since R

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:

Where T

junction to the ambient. Based on the standard JEDEC for a two layers thermal test board, the thermal resistance θ

250°C/W. The maximum power dissipation at T

, quiescent current, and switching losses all vary with input voltage, the total losses should be investigated over the complete input

DS(ON)

−

TT

=

P

D

is the maximum allowable junction temperature +125°C. TA is the ambient temperature and θJA is the thermal resistance from the

J(MAX)

()

P

D

A)MAX(J

JA

of TSOT25 package is

JA

= +25°C can be calculated by following formula:

A

W4.0W/C250/C25C125

=°°−°=

θ

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PAM2804

Application Information

Setting the Output Current

The internal feedback(FB) voltage is 0.1V (Typical). The output current is calculated as below:

The output Current is given by the following table.

As the input voltage approaches the LED forward voltage, the PAM2804 turns the P-Chan nel transistor continuously on. In this mode the
Voltage drop on LED is equal to the input voltage minus the voltage drop across the P-Channel transistor, Inductor and current resistor:

where R

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 PAM2804. 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 V

3. Does the (+) plate of C

4. Keep the switching node, SW, away from the sensitive VFB node.

5. Keep the (–) plates of C

PAM2804

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=

/1.0

I

(Ω) I

R

S

0.286 350

0.143 700

0.1 1000

DS(ON)

FB

R

SLED

(mA)

LED

ILED

VV

= P-Channel switch ON resistance, ILED = LED current, RL = Inductor DC Resistance, RS = Inductor DC Resistance.

pin connect directly to the current sense resistor? The current sense resistor to GND trace should be kept short, direct and wide.

connect to VIN as closely as possible? This capacitor provides the AC current to the internal power MOSFETs.

IN

and C

IN

as close as possible.

OUT

++−=

RRR

SL)ON(DSINLEDDROP

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Ordering Information

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PAM2804

Part Number Marking Package Type Standard Package

PAM2804AAB010

Refer to Marking

Information Below

TSOT25 3000 Units/Tape&Reel

Marking Information

Top View

TSOT25

PAM2804

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Package Outline Dimensions (All dimensions in mm.)

TSOT25

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PAM2804

PAM2804

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IMPORTANT NOTICE

LIFE SUPPORT

PAM2804

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