Adjustable charge pump LED energy saving driver solution

White LED energy-saving lamps in the application in portable equipment ( Such as mobile phone) As color backlit LCD screen and keyboard on the lamp. Color LED bulbs is good representation to the user calls and battery charging indicator. LED energy-saving lamp brightness is proportional to the through current and voltage must be high enough to make them and conduction. Now is the most popular battery lithium battery, its voltage is usually 3. 6 v or so, but as the battery discharge, the voltage will be landing. Based on the reason causes, LED energy-saving lamps can not directly use the battery power supply, also can raise a necessary pressure transducer. Adjustable charge pump is prevalent management plan, special is the total output current is less than 100 ma, simple and low capital applications.

1 difference current driver difference LED energy-saving lamps

output adjustable charging pump, such as Catalyst CAT3200 - of the company 5, is one of the LED bulbs drive of ambition, can realize the 5 v voltage regulator output on at the same time provide maximum load current of 100 ma. White LED energy-saving lamps for LCD backlight current usually to 20 ma, the forward voltage VF is about 3. 4V。 Each LED bulbs on the current set up by series resistance, the resistance (also known as the ballast resistance See figure 1) 。 This structure allows in parallel with current on LED bulbs. Larger resistance means that LED bulbs on the smaller current. Can be a good approximation with formula 1 draw LED bulbs on the current ( Ignoring interconnection cost) 。

formula 1: VOUT = VF + ( LED energy-saving lamp current (RS)

this, VF is the forward voltage LED energy-saving lamps, RS is series resistance.

output voltage VOUT stable at 5 v, thought to when the forward voltage VF is 3. 4 v LED bulbs on the current to 20 ma, the RS = ( 5 v - 3. 4V) / 20 ma, which is 80 ohms. In this example, accept the three white LED energy-saving lamps as reflected screen backlight, 2 color LED bulbs ( The red and blue) As the indicator. Each color LED bulbs has difference forward voltage characteristics. Red LED bulbs forward voltage is very low, through 20 ma current when the voltage is usually to 2. 8V。 Due to the indicator LED bulbs needless to lighting, with its current smaller than backlit LED bulbs.

CAT3200 - 5 charge pump is a kind of doubler, only need a large capacitor as a storage container, will charge transferred from input to output. Here Lord use 1 u F ceramic capacitors. All the required external components is the other two 1 u F capacitance, respectively in the input and output pin.

there are other LED bulbs on the market price of the drive scheme, such as inductive charging pump booster or current control. Inductance booster drive to accept an inductance will input voltage rise to drive several series of LED bulbs, it has the advantage that can provide you with the same current on these LED bulbs, but not when the application instance this structure. Is another kind of LED bulbs drive current control of the charging pump, through the use of on-chip current modulation, single LED bulbs channel can drive a LED bulbs. In this case, the LED energy-saving lamps intrusive by an external resistor to set the current. In the larger drive, through a digital interface can programming to the chip inside it. This structure is no longer necessary to ballast resistor. Now that all the current on the LED bulbs are the same, as is the management plan shall not when we discussed the application of. 2 surge current

the key title is often out of system now to electricity. When the 'enable' signal input pin from low logic to logic high, device conduction and began to large capacitor charging. The results in a very short period of time the input current increases sharply, the so-called 'surge' current generated. A criterion for the advantages of power management chip morphology is in monitoring the input current embodiment. The risks of high surge current is internal rail voltage ( VRAIL) Brake down and affect the system running. Landing is the power of rail voltage output impedance RS and RINTER interconnection impedance function.

system of rail voltage calculation formula is: VRAIL = VBAT - IIN×( RSINTER,

this, VBAT for the battery voltage, IIN for input current. For example, if a series resistance sum to 1. 5 ( The surge current is zero. 5 a, rail voltage as: VRAIL = 3. 6 v - ( 0. 5×1。 5 ohm) = 2. 85V。

but can lead to low voltage system collapse. A phase out the battery output surge current main point is increasing the input capacitance of CIN. As a result, to big capacitor charging department current is provided by capacitance CIN and can eliminate the surge current.

input current waveform is when the output voltage linearly to rated 5 v input current is rising gradually. The initial current burr as only very short 2 mu s continuous time, to, it doesn't do any harm, it will have little impact on the input voltage.

adjustable charging pump provides a good management to drive the LED energy-saving lamps, and is easy to implement on the PCB board. When choosing this device, need to consider some of the key parameters include intrusive, category of the input voltage and output current electricity order, switch frequency and low noise performance of incident.