一,产品概述(General Description)         
     The JTM2803 is a set-up DC-DC converter that delivers a regulated output current. The device switches at a 1.0MHz constant frequency, allowing for the
use of small value external inductor and ceramic capacitors.The JTM2803 is targeted to be used for driving loads up to 1A from a two-cell alkaline battery.
The LED current can be auto adjusted by internal smart module and programmed by the external current sense resistor, Rs,connected between the feedback pin (FB) and ground. A low 95mV(TYP.) feedback voltage reduces the power
loss in the Rs for better efficiency. With its internal 2A,100mΩ NMOS switch, the device can provide high efficiency even at heavy load. During the shutdown
mode, the feedback resistor Rs and the load are completely disconnected and the current consumption is reduced to 1uA.The JTM2803 is available in the 6-lead
SOT-23-6L package.

二.产品特点(Features)
LED Power Efficiency: up to 90%
Current Accuracy: 5%(Vin =3.6V to 1.8V
@Vf =3.7V)
Low Start-Up Voltage: 0.9V
Low Hold Voltage:0.75V
1MHz Switching Frequency
Uses small, Low Profile External Components
Low RDS(ON) : 100mΩ (TYP.)
Over Temperature Protection
Low Profile SOT-23-6L Package

三,应用范围 (Applications)
White LED Torch (Flashlight)
White LED Camera Flash
DSC(Digital Still Camera)Flash
Cellular Camera Phone Flash
PDA Camera Flash
Camcorder Torch(Flashlight) Lamp

四.下载产品资料PDF文档

jtm2803

五,产品封装图 (Package)

Pin No.	Pin Name	Pin Function
1	SW	Switching Pin.
2	GND	Ground Pin.
3	FB	Feedback Pin.
4	EN	Chip Enable(Active High).
5	VOUT	Output Voltage Pin.
6	VIN	Input Supply.

六.电路原理图


七,功能概述
Inductor Selection
The JTM2803 can use small value inductors due to its switching frequency of 1MHz. The value of inductor will
focus in the range of 2.2uH to 4.7uH for most JTM2803 applications. In typical high current white LED applications,
it is recommended to use a 4.7uH inductor. The inductor should have low DCR (DC resistance) to minimize the IR
power loss, and it requires a current rating of 2A to handle the peak inductor current without saturating.
 
Capacitor Selection
An input capacitor is required to reduce the input ripple and noise for proper operation of the JTM2803. For good input
decoupling,  Low  ESR(equivalent  series  resistance) capacitors should be used at the input. At least 2.2uF input
capacitor  is  recommended  for  most  applications. A minimum output capacitor value of 6.8uF is recommended under normal operating conditions, while a 10uF-22uF capacitor may be required for higher power LED current. A reasonable value of the output capacitor depends on the LED current. The ESR of the output capacitor is the important parameter to determine the output voltage ripple of the converter, so low ESR capacitors should be used at the output to reduce the output voltage ripple. The small size of ceramic capacitors is an excellent choice for JTM2803 applications. The X5R and
X7R types are preferred because they maintain capacitance over wide voltage and temperature ranges.

Diode Selection
It's indispensable to use a Schottky diode rated at 2A with the JTM2803. Using a Schottky diode with a lower
forward voltage drop is better to improve the power LED efficiency, and its voltage rating should be greater than
the output voltage. In application, the ON Semiconductor MBRA210LT3G is recommended.
 
LED Current Setting
The LED current is set by the single external Rs resistor connected to the FB pin as shown in the typical
application circuit on page 1. The typical FB reference is internally regulated to 95mV(TYP.). The LED current is
95mV/Rs. It's recommended to use a 1% or better precision resistor for the better LED current accuracy.
The formula and table 1 for Rs selection are shown as follows:
Rs=95mV/ILED
PCB Layout Guidelines
For best performance of the JTM2803, the following guidelines must be strictly followed. Input and Output capacitors should be placed close to the IC and connected to ground plane to reduce noise coupling.Keep the main current traces as possible as short and wide.SW node of DC-DC converter is with high frequency voltage swing. It should be kept at a small area.Place the feedback components as close as possible to the IC and keep away from the noisy devices.

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