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EUP2794 White LED Driver with 1X /1.5X High-Efficiency Charge Pump
DESCRIPTION
The EUP2794 is a low noise, constant frequency charge pump that uses a dual mode load switch (1X) and fractional (1.5X) conversion to maximize efficiency for white LED applications. The EUP2794 delivers up to 80mA of load current and provides four regulated current sources, each capable of driving up to 20mA through white LEDs. All LED pin currents are regulated, constant and tightly matched to achieve uniformity of brightness across the LCD backlight. The switching frequency is 900kHz.(typ.) to keep the conducted noise spectrum away from sensitive frequencies within portable RF devices. An external resistor (RSET) sets the current in each of the LED outputs. In addition, brightness can be controlled by both linear and PWM techniques. An analog voltage between 0V and 3.0V may be applied to the BRGT pin to linearly vary the LED current. By adjusting current with BRGT pin , the brightness of the LEDs can be smoothly varied. Alternatively, a PWM digital signal can be applied to the SD pin to vary the perceived brightness of the LED. The EUP2794 is available in a 16-pin 3mm x 3mm QFN package.
FEATURES
Regulated Current Sources with 0.6% Mis-matching between any Two Outputs High Efficiency 1.5X Charge Pump Drives One, Two, Three or Four White LEDs 2.7V to 5.5V Input Voltage Up to 80mA Output Current Analog or PWM Brightness Control Soft Start Function Low Shutdown Current 900kHz Switching Frequency (typ.) Tiny 3mm x 3mm QFN-16 Package RoHS Compliant and 100% Lead(Pb)-Free
APPLICATIONS
White LED Backlighting 1-Cell Li-Ion battery-operated equipment including PDAs, hand-held PCs, cellular phones
Typical Application Circuit
Figure 1.
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Typical Application Circuit
Figure 2.
Pin Configurations
Part Number Pin Configurations
EUP2794 QFN-16
Pin Description
PIN C2P POUT SD BRGT ISET LED1-4 GND C1N VIN C2N C1P Pin 1 2 3 5 6 8,7,10,9 11 13 14 15 16 DESCRIPTION Positive terminal of C2 Charge pump output Active-low shutdown pin (LOW=shutdown, HIGH=operating). Variable voltage input controls output current Current sense input. Connect 1% resistor to ground to set constant current through LED Current source outputs. Connect directly to LED Power supply ground input Negative terminal of C1 Power supply voltage input Negative terminal of C2 Positive terminal of C1
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Ordering Information
Order Number EUP2794JIR1 EUP2794JIR0 Package Type QFN-16 Marking Operating Temperature range -40 C to 85C
xxxx
2794A
EUP2794- 1/4
1/4
1/4
1/4
Lead Free Code 1: Lead Free 0: Lead Packing R: Tape & Reel Operating temperature range I: Industry Standard Package Type J: QFN-16
Block Diagram
Figure 3.
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Absolute Maximum Ratings
VIN ------------------------------------------------------------------------------- -0.3V to 6V max SD,BRGT --------------------------------------------------------- -0.3V to (VIN+0.3V) w/ 6 max Continuous Power Dissipation ----------------------------------------------- Internally Limited QFN-16L 3 x3, JA -------------------------------------------------------------------------- 65C/W Junction Temperature (TJ ) ------------------------------------------------------- -40C to 125C Storge Temperature Range ----------------------------------------------------- -65C to 150C Lead Temp (Soldering, 5sec) ------------------------------------------------------------- 260C ESD Rating Human Body Model --------------------------------------------------------------------------- 2kV
Operating Conditions
Input Voltage (VIN) ------------------------------------------------------------------ 2.7V to 5.5V Ambient Temperature (TA) --------------------------------------------------------- -40C to 85C
Electrical Characteristics
Unless otherwise specified,C1=C2=CIN=CHOLD=1F, VIN=3.6V, BRGT pin =0V ; RSET =124[ TA= -40 to 85C. Typical values are at TA=25C. ;VLED=3.4V.
Symbol
Parameter
Conditions
3.0V VIN 5.5V, VDX 3.6V BRGT=50mV 2.7V VIN 3.0V, VDX 3.4V BRGT=0V VDX 3.6V ,BRGT=200mV 3.0V VIN 5.5V, VDX 3.6V BRGT=50mV
EUP2794 Min Typ Max.
15 10 20 3.6 15.57 15.57 0.6 4.1 7.64 1 3.82 50 1 -1 240 IDX/10 900 0.2 1 17.12 15.39
Unit
IDX
Available Current at Output Dx
mA
VDX IDX IDX ID-MATCH IQ ISD VCP VCPH VIH VIL ILEAK-SD RBRGT ISET fSW
Available Voltage at Output Dx
V mA mA % mA g A V mV V V g A k mA kHz
Liner Regulation of Dx Output 3.0V VIN 5.5V, VDX 3.6V Current Load Regulation of Dx Output VIN =3.6V , 3.0V VDX 3.6V Current Current Matching Between Any VIN =3.6V Two Outputs 3.0V VIN 5.5V, Active, No Load Quiescent Supply current Current RSET=OPEN Shutdown Supply Current 3.0V VIN 5.5V, shutdown Input Charge-Pump Mode To Pass Mode Threshold Input Charge-Pump Mode To Pass Mode Hysteresis SD Input Logic High 3.0V VIN 5.5V SD Input Logic Low 3.0V VIN 5.5V SD Leakage Current 0V VSD VIN BRGT Input Resistance ISET Pin Output Current Switching Frequency 3.0V VIN 3.8V
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Typical Operating Characteristics
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Application Information
Circuit Description The EUP2794 has four regulated current sources connected to the device's 1.5x charge pump output (POUT). At input voltages below 3.82V (typ.), the charge-pump provides the needed voltage to drive high forward voltage drop White LEDs. It does this by stepping up the POUT voltage 1.5 times the input voltage. The charge pump operates in Pass Mode, providing a voltage on POUT equal to the input voltage, when the input voltage is at or above 3.82V (typ.). The device can drive up to 80mA through any combination of LEDs connected to the constant current outputs D1-D4. To set the LED drive current, the device uses a resistor connected to the ISET pin to set a reference current. This reference current is then multiplied and mirrored to each constant current output. The LED brightness can then be controlled by analog and/or digital methods. Applying an analog voltage in the range of 0V to 3.0V to the Brightness pin (BRGT) adjusts the dimming profile of the LEDs. The digital technique uses a PWM (Pulse Width Modulation) signal applied to the Shutdown pin (SD). (see ISET and BRGT PINS section). Capacitor Selection The EUP2794 requires 4 external capacitors for proper operation. Surface-mount multi-layer ceramic capacitors are recommended. These capacitors are small, inexpensive and have very low equivalent series resistance (ESR, U 15m. typ.). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors are generally not recommended for use with the EUP2794 due to their high ESR, as compared to ceramic capacitors. For most applications, ceramic capacitors with X7R or X5R temperature characteristic are preferred for use with the EUP2794. These capacitors have tight capacitance tolerance (as good as 10%), hold their value over temperature (X7R: 15% over -55C to 125C; X5R: 15% over -55C to 85C), and typically have little voltage coefficient. ISET and BRGT Pins An external resistor, RSET, is connected to the ISET pin to set the current to be mirrored in each of the LED outputs. The internal current mirror sets each LED output current with a 10:1 ratio to the current through RSET. The current mirror circuitry matches the current through each LED to within 0.6%. In addition to RSET, a voltage may be applied to the VBRGT in to vary the LED current. By adjusting current with the Brightness pin (BRGT), the brightness of the LEDs can be smoothly varied. Applying a voltage on BRGT between 0 to 3 volts will linearly vary the LED current. Voltages above 3V do not increase the LED current any further. The voltage on the VBRGT pin is fed into an internal resistor network with a ratio of 0.385. The resulting voltage is then summed with a measured offset voltage of 0.188V, which comes from the reference voltage being fed through a resistor network (See Functional Block Diagram). The brightness control circuitry then uses the summed voltage to control the voltage across RSET. An equation for approximating the LED current is:
V +V *0.385 BRGT = OFFSET R LED SET
Soft-Start Soft start is implemented internally by limiting the current flow through the charge pump switches. During soft start, the input current and LED current will ramp up slowly and avoid current inrush. Shutdown Mode The shutdown pin (SD) disables the part and reduces the quiescent current less than 1A (typ.).The EUP2794 has an active-low shutdown pin (LOW = shutdown, HIGH = operating). The EUP2794 SD pin can be driven with a low-voltage CMOS logic signal (1.5V logic, 1.8V logic, etc). There is no internal pull-up or pull-down on the SD pin of the EUP2794.
I
(
) * (Mirror Ratio )

I
0.188 + V *0.385 BRGT = R LED SET
(
) * 10 Amps
1
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ILED Current Selection Procedures The following procedures illustrate how to set and adjust output current levels. For constant brightness or analog brightness control, go to "Brightness control using BRGT". Otherwise refer to "Brightness control using PWM". Brightness Control Using PWM 1. Set the BRGT pin to 0V 2. Determine the maximum desired ILED current. Use the ILED equation to calculate RSET by setting BRGT to 0V or use Table 1 to select a value for RSET when BRGT equals 0V. 3. Brightness control can be implemented by pulsing a signal at the SD pin. LED brightness is proportional to the duty cycle (D) of the PWM signal. For linear brightness control over the full duty cycle adjustment range, the PWM frequency (f) should be limited to accommodate the turn-on time (TON = 100s) of the device.
D * (1/f ) > TON f MAX = D MIN / TON
TABLE 1. RSET Values LED Current 10mA 15mA 185.2 122.8 376.0 250.6 567.0 375.0 752.0 503.0 939.0 625.0 1.129K 756.0 1.319K 881.0
BRGT 0.0V 0.5V 1.0V 1.5V 2.0V 2.5V 3.0V
5mA 375.0 760.0 1.139K 1.526K 1.9K 2.32K 2.663K
20mA 91.9 187.8 282.2 375.0 467.0 565.0 660.0
TABLE 2. LED Current RSET Values 1.32K 881 1.52 2.20 2.92 4.31 4.36 6.47 5.73 8.57 7.20 10.72 8.64 12.81 10.02 15.00
If the PWM frequency is much less than 100Hz, flicker may be seen in the LEDs. For the EUP2794, zero duty cycle will turn off the LEDs and a 50% duty cycle will result in an average ILED being half of the programmed LED current. For example, if RSET is set to program 15mA, a 50% duty cycle will result in an average ILED of 7.5mA. Brightness Control Using BRGT 1. Choose the maximum ILED desired and determine the max voltage to be applied to the BRGT pin. For constant brightness, set BRGT to a fixed voltage between 0V to 3V. 2. Use Table 1 to determine the value of RSET required or use the ILED equation above to calculate RSET. 3. Use Table 2 as a reference for the dimming profile of the LEDs, when BRGT ranges from 0V to 3V
BRGT 0.0V 0.5V 1.0V 1.5V 2.0V 2.5V 3.0V
2.66K 0.82 1.51 2.21 2.91 3.60 4.29 5.01
660 2.88 5.71 8.59 11.39 14.22 17.07 20.00
Charge Pump Output (POUT) The EUP2794 charge pump is an unregulated switched capacitor converter with a gain of 1.5. The voltage at the output of the pump (the POUT pin) is nominally 1.5 x VIN. This rail can be used to deliver additional current to other circuitry. A ballast resistor sets the current through each LED, and LED current matching is dependent on the LED forward voltage matching. Because of this, LEDs driven by POUT are recommended for functions where brightness matching is not critical, such as keypad backlighting.
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LED Headroom Voltage (VHR) Four current sources are connected internally between POUT and D1-D4. The voltage across each current source, (VPOUT - VDX), is referred to as headroom voltage (VHR). The current sources require a sufficient amount of headroom voltage to be present across them in order to regulate properly. Minimum required headroom voltage is proportional to the current flowing through the current source, as dictated by the equation:
VHR - M IN = k HR * I DX
Output Current Capability The primary constraint on the total current capability is the headroom voltage requirement of the internal current sources. Combining the VPOUT and VHR equations from the previous two sections yields the basic inequality for determining the validity of an EUP2794 LED-drive application:
VPOUT = 1.5 x VIN - I TOTAL x R OUT VHR - MIN = k HR x I DX VPOUT - VDX VHR - MIN 1.5 x VIN - I TOTAL x R OUT - VDX (K HR x I DX )
The parameter kHR, typically 20mV/mA in the EUP2794, is a proportionality constant that represents the ON-resistance of the internal current mirror transistors. For worst-case design calculations, using a kHR of 25mV/Ma is recommended. (Worst-case recommendation accounts for parameter shifts from part-to-part variation and applies over the full operating temperature range). Figure 4 shows how output current of the EUP2794 varies with respect to headroom voltage.
Rearranging this inequality shows the estimated total output current capability of an application:
(1.5 x VIN - MIN ) - VDX - MAX - I TOTAL / R OUT (K HR x I DX )
Examining the equation above, the primary limiting factors on total output current capability are input and LED forward voltage. A low input voltage combined with a high LED voltage may result in insufficient headroom voltage across the current sources, causing them to fall out of regulation. When the current sources are not regulated, LED currents will be below desired levels and brightness matching will be highly dependent on LED forward voltage matching. Typical EUP2794 output resistance is 3.0. For worst-case design calculations, using an output resistance of 3.5 is recommended . EUP2794 has a typical kHR constant of 20mV/mA. For worst-case design calculations, use kHR =25mV/mA. (Worst-case recommendations account for parameter shifts from part-to-part variation and apply over the full operating temperature range). ROUT and kHR increase slightly with temperature, but losses are typically offset by the negative temperature coefficient properties of LED forward voltages. Power dissipation and internal self-heating may also limit output current capability but is discussed in a later section.
Figure 4. ILED VS VHR 4 LEDs, VIN=3.0V On the flat part of the graph, the currents regulate properly as there is sufficient headroom voltage for regulation. On the sloping part of the graph the headroom voltage is too small, the current sources are squeezed, and their current drive capability is limited Changes in headroom voltage from one output to the next, possible with LED forward voltage mismatch, will result in different output currents and LED brightness mismatch. Thus, operating the EUP2794 with insufficient headroom voltage across the current sources should be avoided.
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Parallel Dx Outputs for Increased Current Drive Outputs D1 through D4 may be connected together in any combination to drive higher currents through fewer LEDs. For example in Figure 5, outputs D1 and D2 are connected together to drive one LED while D3 and D4 are connected together to drive a second LED. Connecting outputs in parallel does not affect internal operation of the EUP2794 and has no impact on the Electrical Characteristics and limits previously presented. The available diode output current, maximum diode voltage, and all other specifications provided in the Electrical Characteristics table apply to parallel output configurations, just as they do to the standard 4-LED application circuit.
Thermal Protection When the junction temperature exceeds 150C (typ.), the EUP2794 internal thermal protection circuitry disables the part. This feature protects the device from damage due to excessive power dissipation. The device will recover and operate normally when the junction temperature falls below 140C (typ.). It is important to have good thermal conduction with a proper layout to reduce thermal resistance.
Figure 5. Two Parallel Connected LEDs With this configuration, two parallel current sources of equal value provide current to each LED. RSET and VBRGT should therefore be chosen so that the current through each output is programmed to 50% of the desired current through the parallel connected LEDs. For example, if 30mA is the desired drive current for 2 parallel connected LEDs , RSET and VBRGT should be selected so that the current through each of the outputs is 15mA. Other combinations of parallel outputs may be implemented in similar fashions, such as in Figure 6.
Power Efficiency Figure 7 shows the efficiency of the EUP2794. The change in efficiency shown by the graph comes from the transition from Pass Mode to a gain of 1.5. Efficiency (E) of the EUP2794 is defined here as the ratio of the power consumed by LEDs (PLED) to the power drawn from the input source (PIN). In the equations below, IQ is the quiescent current of the EUP2794, ILED is the current flowing through one LED, VLED is the forward voltage at that LED current, and N is the number of LEDs connected to the regulated current outputs. In the input power calculation, the 1.5 represents the switched capacitor gain configuration of the EUP2794.
PLED = N x VLED x I LED PIN = VIN x I IN PIN = VIN x 1.5 x N x I LED + I Q E = (PLED / PIN )
(
)
Figure 6. One Parallel Connected LED
Efficiency, as defined here, is in part dependent on LED voltage. Variation in LED voltage does not affect power consumed by the circuit and typically does not relate to the brightness of the LED. For an advanced analysis, it is recommended that power consumed by the circuit (VIN x IIN) be evaluated rather than power efficiency. Figure 8 shows the power consumption of the EUP2794 Typical Application Circuit.
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Power Dissipation The power dissipation (PDISSIPATION) and junction temperature (TJ) can be approximated with the equations below. PIN is the power generated by the 1.5x charge pump, PLED is the power consumed by the LEDs, PPOUT is the power provided through the POUT pin, TA is the ambient temperature, and JA is the junction-to-ambient thermal resistance for the QFN-16 package. VIN is the input voltage to the EUP2794, VDX is the LED forward voltage, IDX is the programmed LED current, and IPOUT is the current drawn through POUT.
PDISSIPATION = PIN - PLED - PPOUT = [1.5 x VIN x (41DX + I POUT )] - (1.5 x VIN x I POUT ) TJ = TA + (PDISSIPATION x JA )
Figure 7. Efficiency vs VIN 4 LEDS, VLED=3.4V,ILED=15mA
The junction temperature rating takes precedence over the ambient temperature rating. The EUP2794 may be operated outside the ambient temperature rating, so long as the junction temperature of the device does not exceed the maximum operating rating of 100C. The maximum ambient temperature rating must be derated in applications where high power dissipation and/or poor thermal resistance causes the junction temperature to exceed 100C.
Figure 8. PIN vs VIN 4 LEDS, 2.5 VDX 3.4V, IDX=15mA
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Packaging Information
QFN-16
Note 1. All dimensions are in millmeters. K is in Degrees. 2. M: The maximum allowable corner on the molded plastic body corners. 3. Dimension D does not include mold protrusions or gate burrs. Mold 4. Dimension E does not include interterminal mold protrusions or terminal protrusions. Interminal mold protrusions and/or terminal protrusions shall not exceed 0.20 mm per side. 5. Dimension b applies to plated terminals. Dimension a1 is primarily Y terminal plating ,but may or may not include A small protrusion of terminal below the bottom surface of the package.
SYMBOLS A A1 A3 b D D1 E E1 e L
c
aaa bbb ccc M Burr
MIN. 0.80 0 ---0.18 2.85 ---2.85 ------0.30 -12 ------------0
DIMENSIONS IN MILLIMETERS NOM. 0.90 0.015 0.20REF. 0.23 3.00BSC 1.48 BSC 3.00BSC 1.48 BSC 0.50BSC 0.40 ---0.25 0.10 0.10 ---0.030
MAX. 1.00 0.030 ---0.30 3.15 ---3.15 ------0.50 0 ---------0.05 0.060
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