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19-3880; Rev 2; 1/10
KIT ATION EVALU ABLE AVAIL
Offline and DC-DC PWM Controllers for High-Brightness LED Drivers
General Description Features
Suitable for Buck, Boost, Flyback, SEPIC, and Other Topologies Up to 50W or Higher Output Power Universal Offline Input Voltage Range: Rectified 85VAC to 265VAC (MAX16801) IN Pin Directly Driven From 10.8VDC to 24VDC Input (MAX16802) Internal Error Amplifier with 1% Accurate Reference for Precise LED Current Regulation PWM or Linear Dimming Fixed Switching Frequency of 262kHz 12% Thermal Shutdown Digital Soft-Start Programmable Input Startup Voltage Internal Bootstrap UVLO with Large Hysteresis (MAX16801) 45A (typ) Startup Supply Current, 1.4mA (typ) Operating Supply Current 50% (MAX16801A/MAX16802A) or 75% (MAX16801B/MAX16802B) Maximum Duty Cycle Available in a Tiny 8-Pin MAX Package
MAX16801A/B/MAX16802A/B
The MAX16801A/B/MAX16802A/B high-brightness (HB) LED driver-control ICs contain all the circuitry required for the design of wide-input-voltage-range LED drivers for general lighting and display applications. The MAX16801 is well suited for universal input (rectified 85VAC to 265VAC) LED drivers, while the MAX16802 is intended for low-input-voltage (10.8VDC to 24VDC) LED drivers. When the LED current needs to be tightly regulated, an additional on-board error amplifier with 1% accurate reference can be utilized. A wide dimming range can be implemented by using low-frequency PWM dimming. The MAX16801/MAX16802 feature an input undervoltage lockout (UVLO) for programming the input-supply start voltage, and to ensure proper operation during brownout conditions. The MAX16801 has an internal-bootstrap undervoltage lockout circuit with a large hysteresis that simplifies offline LED driver designs. The MAX16802 does not have this internal bootstrap circuit and can be biased directly from a +12V rail. The 262kHz fixed switching frequency is internally trimmed, allowing for optimization of the magnetic and filter components, resulting in a compact, cost-effective LED driver. The MAX16801A/MAX16802A are offered with 50% maximum duty cycle. The MAX16801B/MAX16802B are offered with 75% maximum duty cycle. These devices are available in an 8-pin MAX(R) package and operate over the -40C to +85C temperature range.
Ordering Information
PART TEMP RANGE -40C to +85C -40C to +85C -40C to +85C -40C to +85C PINPACKAGE 8 MAX 8 MAX 8 MAX 8 MAX
Applications
Offline and DC-DC LED Drivers RGB Back Light for LCD TVs and Monitors Commercial and Industrial Lighting Decorative and Architectural Lighting
MAX16801AEUA+ MAX16801BEUA+ MAX16802AEUA+ MAX16802BEUA+
+Denotes lead-free package. MAX is a registered trademark of Maxim Integrated Products, Inc.
10.8VDC TO 24VDC ENABLE UVLO/EN DIM/FB PWM IN L1 VCC D1 C3
Typical Operating Circuit
LEDs
MAX16802B
COMP CS NDRV GND Q1
C1 GND
C2
R1
Warning: The MAX16801/MAX16802 are designed to work with high voltages. Exercise caution. ________________________________________________________________ Maxim Integrated Products 1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
Offline and DC-DC PWM Controllers for High-Brightness LED Drivers MAX16801A/B/MAX16802A/B
ABSOLUTE MAXIMUM RATINGS
IN to GND..........................................................................-0.3V to +30V VCC to GND ......................................................................-0.3V to +13V DIM/FB, COMP, UVLO, CS to GND..........................-0.3V to +6V NDRV to GND.............................................-0.3V to (VCC + 0.3V) Continuous Power Dissipation (TA = +70C) 8-Pin MAX (derate 4.5mW/C above +70C) ..............362mW Operating Temperature Range ...........................-40C to +85C Storage Temperature Range ............................-65C to +150C Junction Temperature ......................................................+150C Lead Temperature (soldering, 10s) .................................+300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VIN = +12V (MAX16801: VIN must first be brought up to +23.6V for startup), 10nF bypass capacitors at IN and VCC, CNDRV = 0F, VUVLO = +1.4V, VDIM/FB = +1.0V, COMP = unconnected, VCS = 0V, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 1)
PARAMETER Bootstrap UVLO Wake-Up Level Bootstrap UVLO Shutdown Level UVLO/EN Wake-Up Threshold UVLO/EN Shutdown Threshold UVLO/EN Input Current UVLO/EN Hysteresis IN Supply Current In Undervoltage Lockout IN Voltage Range UVLO/EN Propagation Delay Bootstrap UVLO Propagation Delay INTERNAL SUPPLY VCC Regulator Set Point IN Supply Current After Startup Shutdown Supply Current GATE DRIVER Driver Output Impedance Driver Peak Sink Current Driver Peak Source Current PWM COMPARATOR Comparator Offset Voltage CS Input Bias Current Comparator Propagation Delay Minimum On-Time VOPWM ICS tPWM tON(MIN) VCOMP - VCS VCS = 0V VCS = +0.1V 1.15 -2 60 150 1.38 1.70 +2 V A ns ns RON(LOW) Measured at NDRV sinking, 100mA 2 4 1 0.65 4 12 RON(HIGH) Measured at NDRV sourcing, 20mA A A VCCSP IIN VIN = +10.8V to +24V, sinking 1A to 20mA from VCC VIN = +24V UVLO/EN = low 7 1.4 10.5 2.5 90 V mA A ISTART VIN tEXTR tEXTF tBUVR tBUVF UVLO/EN steps up from +1.1V to +1.4V UVLO/EN steps down from +1.4V to +1.1V VIN steps up from +9V to +24V VIN steps down from +24V to +9V VIN = +19V, for MAX16801 only when in bootstrap UVLO 10.8 12 1.8 5 1 SYMBOL VSUVR VSUVF VULR2 VULF2 IUVLO CONDITIONS VIN rising (MAX16801 only) VIN falling (MAX16801 only) UVLO/EN rising UVLO/EN falling TJ = +125C MIN 19.68 9.05 1.188 1.168 TYP 21.6 9.74 1.28 1.23 25 50 45 90 24 MAX 23.60 10.43 1.371 1.291 UNITS V V V V nA mV A V s s
UNDERVOLTAGE LOCKOUT/STARTUP
2
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Offline and DC-DC PWM Controllers for High-Brightness LED Drivers
ELECTRICAL CHARACTERISTICS (continued)
(VIN = +12V (MAX16801: VIN must first be brought up to +23.6V for startup), 10nF bypass capacitors at IN and VCC, CNDRV = 0F, VUVLO = +1.4V, VDIM/FB = +1.0V, COMP = unconnected, VCS = 0V, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 1)
PARAMETER CURRENT-SENSE COMPARATOR Current-Sense Trip Threshold CS Input Bias Current Propagation Delay From Comparator Input to NDRV Switching Frequency Maximum Duty Cycle IN CLAMP VOLTAGE IN Clamp Voltage ERROR AMPLIFIER Voltage Gain Unity-Gain Bandwidth Phase Margin DIM/FB Input Offset Voltage COMP Clamp Voltage Source Current Sink Current Reference Voltage Input Bias Current COMP Short-Circuit Current THERMAL SHUTDOWN Thermal-Shutdown Temperature Thermal Hysteresis DIGITAL SOFT-START Soft-Start Duration Reference Voltage Steps During Soft-Start Reference Voltage Step 15,872 31 40 Clock cycles Steps mV 130 25 C C 8 VREF (Note 2) High Low 2.2 0.4 0.5 0.5 1.218 1.230 1.242 50 RLOAD = 100k RLOAD = 100k, CLOAD = 200pF RLOAD = 100k, CLOAD = 200pF 80 2 65 3 3.5 1.1 dB MHz Degrees mV V mA mA V nA mA VINC 2mA sink current, MAX16801 only (Note 3) 24.1 26.1 29.0 V VCS ICS tPWM fSW DMAX MAX1680_A MAX1680_B VCS = 0V 50mV overdrive 230 262 -2 60 262 50 75 290 50.5 76 291 320 +2 mV A ns kHz % SYMBOL CONDITIONS MIN TYP MAX UNITS
MAX16801A/B/MAX16802A/B
Note 1: All devices are 100% tested at TA = +85C. All limits over temperature are guaranteed by characterization. Note 2: VREF is measured with DIM/FB connected to the COMP pin (see the Functional Diagram). Note 3: The MAX16801 is intended for use in universal input offline drivers. The internal clamp circuit is used to prevent the bootstrap capacitor (C1 in Figure 5) from charging to a voltage beyond the absolute maximum rating of the device when EN/UVLO is low. The maximum current to IN (hence to clamp) when UVLO is low (device in shutdown), must be externally limited to 2mA (max). Clamp currents higher than 2mA may result in clamp voltage higher than +30V, thus exceeding the absolute maximum rating for IN. For the MAX16802, do not exceed the +24V maximum operating voltage of the device.
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Offline and DC-DC PWM Controllers for High-Brightness LED Drivers MAX16801A/B/MAX16802A/B
Typical Operating Characteristics
(VUVLO/EN = +1.4V, VFB = +1V, COMP = unconnected, VCS = 0V, TA = +25C, unless otherwise noted.)
BOOTSTRAP UVLO WAKE-UP LEVEL vs. TEMPERATURE
MAX16801 toc01
BOOTSTRAP UVLO SHUTDOWN LEVEL vs. TEMPERATURE
MAX16801 toc02
UVLO/EN WAKE-UP THRESHOLD vs. TEMPERATURE
UVLO/EN RISING 1.275 1.270 1.265 1.260
MAX16801 toc03 MAX16801 toc09 MAX16801 toc06
21.60 MAX16801 VIN RISING 21.55 21.50 VIN (V) 21.45 21.40
10.1 MAX16801 VIN FALLING 10.0
1.280
9.9
9.8 21.35 21.30 -40 -20 0 20 40 60 80 TEMPERATURE (C) 9.7 -40 -20 0 20 40 60 80 TEMPERATURE (C) 1.255 1.250 -40 -20 0 20 40 60 80 TEMPERATURE (C)
UVLO/EN SHUTDOWN THRESHOLD vs. TEMPERATURE
MAX16801 toc04
UVLO/EN (V)
VIN (V)
VIN SUPPLY CURRENT IN UNDERVOLTAGE LOCKOUT vs. TEMPERATURE
MAX16801 toc05
VIN SUPPLY CURRENT AFTER STARTUP vs. TEMPERATURE
1.5 VIN = 24V 1.4 IIN (mA)
1.30 UVLO/EN FALLING
52 51 50 49 VIN = 19V MAX16801 WHEN IN BOOTSTRAP UVLO MAX16802 WHEN UVLO/EN IS LOW
1.25 UVLO/EN (V) ISTART (A)
48 47 46 45 44 43
1.20
1.3
1.15
1.2
1.10 -40 -20 0 20 40 60 80 TEMPERATURE (C)
42 -40 -20 0 20 40 60 80 TEMPERATURE (C)
1.1 -40 -20 0 20 40 60 80 TEMPERATURE (C)
VCC REGULATOR SET POINT vs. TEMPERATURE
MAX16801 toc07
VCC REGULATOR SET POINT vs. TEMPERATURE
MAX116801 toc08
CURRENT-SENSE THRESHOLD vs. TEMPERATURE
310 CURRENT-SENSE THRESHOLD (V) 305 300 295 290 285 280 275 270 -3 MEAN +3 TOTAL NUMBER OF DEVICES = 100
9.8 9.7 9.6 VCC (V) 9.5 9.4 9.3 9.2 -40 -20 0 20 40 60 80 TEMPERATURE (C) NDRV OUTPUT IS SWITCHING VIN = 19V NO LOAD NDRV OUTPUT IS NOT SWITCHING, VFB = 1.5V
8.9 VIN = 10.8V 8.8 8.7 10mA LOAD 8.6 VCC (V) 8.5 8.4 20mA LOAD 8.3 8.2 8.1 -40 -20 0 20 40 60 80 TEMPERATURE (C)
-40
-20
0
20
40
60
80
TEMPERATURE (C)
4
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Offline and DC-DC PWM Controllers for High-Brightness LED Drivers MAX16801A/B/MAX16802A/B
Typical Operating Characteristics (continued)
(VUVLO/EN = +1.4V, VFB = +1V, COMP = unconnected, VCS = 0V, TA = +25C, unless otherwise noted.)
CURRENT-SENSE THRESHOLD
MAX16801 toc10
SWITCHING FREQUENCY vs. TEMPERATURE
MAX16801 toc11
SWITCHING FREQUENCY
TOTAL NUMBER OF DEVICES = 200
MAX16801 toc12 MAX16801 toc18 MAX16801 toc15
30 25 PERCENTAGE OF UNITS (%) 20 15 10 5 0 260 270 280 290
280 275 SWITCHING FREQUENCY (kHz) 270 265 260 255 250 245 240 -40 -20 0 20 -3 MEAN +3
TOTAL NUMBER OF DEVICES = 200
TOTAL NUMBER OF DEVICES = 100
30 25 PERCENTAGE OF UNITS (%) 20 15 10 5 0
300
310
320
40
60
80
230
240
250
260
270
280
290
CURRENT-SENSE THRESHOLD (mV)
TEMPERATURE (C)
SWITCHING FREQUENCY (kHz)
PROPAGATION DELAY FROM CURRENT-SENSE COMPARATOR INPUT TO NDRV vs. TEMPERATURE
MAX16801 toc13
UVLO/EN PROPAGATION DELAY vs. TEMPERATURE
14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 -40 -20 0 20
MAX16801 toc14
REFERENCE VOLTAGE vs. TEMPERATURE
1.230 VIN = 12V
75
tPWM (ns)
65
REFERENCE VOLTAGE (V) UVLO/EN FALLING
70
UNDERVOLTAGE LOCKOUT DELAY (s)
UVLO/EN RISING
1.229
1.228
60
1.227
55
1.226
50 -40 -20 0 20 40 60 80 TEMPERATURE (C)
1.225 40 60 80 -40 -20 0 20 40 60 80 TEMPERATURE (C) TEMPERATURE (C)
INPUT CURRENT vs. INPUT CLAMP VOLTAGE
MAX16801 toc16
INPUT CLAMP VOLTAGE vs. TEMPERATURE
26.8 INPUT CLAMP VOLTAGE (V) 26.6 26.4 RON () 26.2 26.0 25.8 25.6 25.4 25.2 25.0 IIN = 2mA
MAX16801 toc17
NDRV OUTPUT IMPEDANCE vs. TEMPERATURE
2.2 2.1 2.0 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 VIN = 24V SINKING 100mA
10 9 8 INPUT CURRENT (mA) 7 6 5 4 3 2 1 0
27.0
10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 30.0 INPUT VOLTAGE (V)
-40
-20
0
20
40
60
80
-40
-20
0
20
40
60
80
TEMPERATURE (C)
TEMPERATURE (C)
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Offline and DC-DC PWM Controllers for High-Brightness LED Drivers MAX16801A/B/MAX16802A/B
Typical Operating Characteristics (continued)
(VUVLO/EN = +1.4V, VFB = +1V, COMP = unconnected, VCS = 0V, TA = +25C, unless otherwise noted.)
NDRV OUTPUT IMPEDANCE vs. TEMPERATURE
4.8 4.6 4.4 GAIN (dB) RON () 4.2 4.0 3.8 3.6 3.4 3.2 3.0 -40 -20 0 20 40 60 80 TEMPERATURE (C) VIN = 24V SOURCING 20mA
MAX16801 toc19
ERROR-AMPLIFIER OPEN-LOOP GAIN AND PHASE vs. FREQUENCY
120 100 80 60 40 20 0 -20 -40 -60 -80 -100 0.1 1 10 100 1k PHASE GAIN
MAX16801 toc20
5.0
50 30 10 -30 -50 -70 -90 -110 -130 -150 PHASE (DEGREES) -10
-170 10k 100k 1M 10M 100M
FREQUENCY (Hz)
Pin Description
PIN 1 2 3 4 5 6 7 8 NAME UVLO/EN DIM/FB COMP CS GND NDRV VCC IN FUNCTION Externally Programmable Undervoltage Lockout. UVLO programs the input start voltage. Connect UVLO to GND to disable the device. Low-Frequency PWM Dimming Input/Error-Amplifier Inverting Input Error-Amplifier Output. Connect the compensation components between DIM/FB and COMP in highaccuracy LED current regulation. Current-Sense Connection for Current Regulation. Connect to high side of sense resistor. An RC filter may be necessary to eliminate leading-edge spikes. Power-Supply Ground External n-Channel MOSFET Gate Connection Gate-Drive Supply. Internally regulated down from IN. Decouple with a 10nF or larger capacitor to GND. IC Supply. Decouple with a 10nF or larger capacitor to GND. For bootstrapped operation (MAX16801), connect a startup resistor from the input supply line to IN. Connect the bias winding supply to this point (see Figure 5). For the MAX16802, connect IN directly to a +10.8V to +24V supply.
Detailed Description
The MAX16801/MAX16802 family of devices is intended for constant current drive of high-brightness (HB) LEDs used in general lighting and display applications. They are specifically designed for use in isolated and nonisolated circuit topologies such as buck, boost, flyback, and SEPIC, operating in continuous or discontinuous mode. Current mode control is implemented with an internally trimmed, fixed 262kHz switching frequency. A bootstrap UVLO with a large hysteresis (11.9V), very low startup current, and low operating current
6
result in an efficient universal-input LED driver. In addition to the internal bootstrap UVLO, these devices also offer programmable input startup voltage programmed through the UVLO/EN pin. The MAX16801 is well suited for universal AC input (rectified 85VAC to 265VAC) drivers. The MAX16802 is well suited for low input voltage (10.8VDC to 24VDC) applications. The MAX16801/MAX16802 regulate the LED current by monitoring current through the external MOSFET cycle by cycle.
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Offline and DC-DC PWM Controllers for High-Brightness LED Drivers
When in the bootstrapped mode with a transformer (Figure 5), the circuit is protected against most output short-circuit faults when the tertiary voltage drops below +10V, causing the UVLO to turn off the gate drive of the external MOSFET. This re-initiates a startup sequence with soft-start. When the LED current needs to be tightly regulated, an internal error amplifier with 1% accurate reference can be used (Figure 9). This additional feedback minimizes the impact of passive circuit component variations and tolerances, and can be implemented with a minimum number of additional external components. A wide dimming range can be implemented using a low-frequency PWM dimming signal fed directly to the DIM/FB pin. LED driver circuits designed with the MAX16801 use a high-value startup resistor R1 that charges a reservoir capacitor C1 (Figure 5 or Figure 9). During this initial period, while the voltage is less than the internal bootstrap UVLO threshold, the device typically consumes only 45A of quiescent current. This low startup current and the large bootstrap UVLO hysteresis help minimize the power dissipation across R1, even at the high end of the universal AC input voltage. An internal shutdown circuit protects the device whenever the junction temperature exceeds +130C (typ).
MAX16801A/B/MAX16802A/B
Dimming
Linear dimming can be implemented by creating a summing node at CS, as shown in Figures 6 and 7. Low-frequency PWM (chopped-current) dimming is possible by applying an inverted-logic PWM signal to the DIM/FB pin of the IC (Figure 8). This might be a preferred way of dimming in situations where it is critical to retain the light spectrum unchanged. It is accomplished by keeping constant the amplitude of the chopped LED current.
MAX16801/MAX16802 Biasing
Implement bootstrapping from the transformer when it is present (Figure 5). Biasing can also be realized directly from the LEDs in non-isolated topologies (Figure 1). Bias the MAX16802 directly from the input voltage of 10.8VDC to 24VDC. The MAX16802 can also be used
R1 VCC NDRV
R5 R2 Q1
IN AC IN BRIDGE RECTIFIER C1 COMP C2 C3 DIM/FB
GND
MAX16801B
CS
UVLO/EN R6
R3
R4
L1 D3
C4
TOTAL LED VOLTAGE: 11V TO 23V
Figure 1. Biasing the IC using LEDs in Nonisolated Flyback Driver
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Offline and DC-DC PWM Controllers for High-Brightness LED Drivers MAX16801A/B/MAX16802A/B
VDC R Q R IN IN VDC
D
MAX16802A
D
C
MAX16802A
(a)
(b)
Figure 2. (a) Resistor-Zener and (b) Transistor-Zener-Resistor Bias Arrangements
in applications with higher input DC voltages by implementing resistor-Zener bias (Figure 2a) or transistorZener-resistor bias (Figure 2b).
MAX16801 Bootstrap Undervoltage Lockout
In addition to the externally programmable UVLO function offered in both the MAX16801/MAX16802, the MAX16801 has an additional internal bootstrap UVLO that is very useful when designing high-voltage LED drivers (see the Functional Diagram). This allows the device to bootstrap itself during initial power-up. The MAX16801 attempts to start when V IN exceeds the bootstrap UVLO threshold of +23.6V. During startup, the UVLO circuit keeps the CPWM comparator, ILIM comparator, oscillator, and output driver shut down to reduce current consumption. Once V IN reaches +23.6V, the UVLO circuit turns on both the CPWM and ILIM comparators, as well as the oscillator, and allows the output driver to switch. If VIN drops below +9.7V, the UVLO circuit will shut down the CPWM comparator, ILIM comparator, oscillator, and output driver thereby returning the MAX16801 to the startup mode.
MAX16801/MAX16802 Undervoltage Lockout
The MAX16801/MAX16802 have an input voltage UVLO/EN pin. The threshold of this UVLO is +1.28V. Before any operation can commence, the voltage on this pin has to exceed +1.28V. The UVLO circuit keeps the CPWM comparator, ILIM comparator, oscillator, and output driver in shutdown to reduce current consumption (see the Functional Diagram). Use this UVLO function to program the input start voltage. Calculate the divider resistor values, R2 and R3 (Figure 5), by using the following formulas: R3 VULR2 x VIN 500 x IUVLO (VIN - VULR2 )
The value of R3 is calculated to minimize the voltagedrop error across R2 as a result of the input bias current of the UVLO/EN pin. V ULR2 = +1.28V, I UVLO = 50nA (max), VIN is the value of the input-supply voltage where the power supply must start. V - VULR2 R2 = IN x R3 VULR2 where IUVLO is the UVLO/EN pin input current, and VULR2 is the UVLO/EN wake-up threshold.
MAX16801 Startup Operation
In isolated LED driver applications, VIN can be derived from a tertiary winding of a transformer. However, at startup there is no energy delivered through the transformer. Therefore, a special bootstrap sequence is required. Figure 3 shows the voltages on IN and VCC during startup. Initially, both VIN and VCC are 0V. After the line voltage is applied, C1 charges through the startup resistor R1 to an intermediate voltage. At this point, the internal regulator begins charging C2 (see Figure 5). The MAX16801 uses only 45A of the current supplied by R1, and the remaining input current charges C1 and C2. The charging of C2 stops when the VCC voltage reaches approximately +9.5V, while the voltage across C1 continues rising until it reaches
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Offline and DC-DC PWM Controllers for High-Brightness LED Drivers
Soft-Start
VCC 2V/div
MAX16801A/B/MAX16802A/B
MAX16801 VIN PIN 5V/div
0
The MAX16801/MAX16802 soft-start feature allows the LED current to ramp up in a controlled manner. Softstart begins after UVLO deasserts. The voltage applied to the noninverting node of the amplifier ramps from 0 to +1.23V over a 60ms soft-start timeout period. Figure 4 shows a typical 0.5A output current during startup. Note the staircase increase of the LED current. This is a result of the digital soft-starting technique used. Unlike other devices, the reference voltage to the internal amplifier is soft-started. This method results in superior control of the LED current.
n-Channel MOSFET Switch Driver
100ms/div
Figure 3. VIN and VCC During Startup when Using the MAX16801 in Bootstrapped Mode
the wake-up level of +23.6V. Once VIN exceeds the bootstrap UVLO threshold, NDRV begins switching the MOSFET and transfers energy to the secondary and tertiary outputs. If the voltage on the tertiary output builds to a value higher than +9.7V (the bootstrap UVLO lower threshold), then startup has been accomplished and sustained operation commences. If VIN drops below +9.7V before startup is complete, the device goes back to low-current UVLO. In this case, increase C1 in order to store enough energy to allow for the voltage at the tertiary winding to build up.
The NDRV pin drives an external n-channel MOSFET. The NDRV output is supplied by the internal regulator (VCC), which is internally set to approximately +9.5V. For the universal input voltage and applications with a transformer, the MOSFET used must be able to withstand the DC level of the high-line input voltage plus the reflected voltage at the primary of the transformer. For most offline applications that use the discontinuous flyback topology, this requires a MOSFET rated at 600V. NDRV can source/sink in excess of the 650mA/1000mA peak current. Select a MOSFET that yields acceptable conduction and switching losses.
Internal Error Amplifier
The MAX16801/MAX16802 include an internal error amplifier that can be used to regulate the LED current very accurately. For example, see the nonisolated power supply in Figure 5. Calculate the LED current using the following equation: V ILED = REF R7
100mA/div
where V REF = +1.23V. The amplifier's noninverting input is internally connected to a digital soft-start circuit that gradually increases the reference voltage during startup and is applied to this pin. This forces the LED current to come up in an orderly and well-defined manner under all conditions.
0 10ms/div
Figure 4. Typical Current Soft-Start During Initial Startup
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Offline and DC-DC PWM Controllers for High-Brightness LED Drivers MAX16801A/B/MAX16802A/B
Applications Information
Startup Time Considerations for HighBrightness LED Drivers Using MAX16801
The IN bypass capacitor C1 supplies current immediately after wake-up (Figure 5). The size of C1 and the connection configuration of the tertiary winding determine the number of cycles available for startup. Large values of C1 increase the startup time but also supply gate charge for more cycles during initial startup. If the value of C1 is too small, V IN drops below +9.7V because NDRV does not have enough time to switch and build up sufficient voltage across the tertiary winding that powers the device. The device goes back into UVLO and does not start. Use low-leakage capacitors for C1 and C2. Assuming that offline LED drivers keep typical startup times to less than 500ms even in low-line conditions (85VAC input for universal offline applications), size the startup resistor R1 to supply both the maximum startup bias of the device (90A, worst case) and the charging current for C1 and C2. The bypass capacitor C2 must charge to +9.5V and C1 to +24V, all within the desired time period of 500ms. Because of the internal 60ms soft-start time of the MAX16801, C1 must store enough charge to deliver current to the device for at least this much time. To calculate the approximate amount of capacitance required, use the following formula: Ig = Qgtot x fSW C1 = Assuming C1 > C2, calculate the value of R1 as follows: x C1 V IC1 = SUVR (500ms) R1 = VIN(MIN) - VSUVR IC1 + ISTART
where VIN(MIN) is the minimum input supply voltage for the application, VSUVR is the bootstrap UVLO wake-up level (+23.6V, max), and ISTART is the IN supply current at startup (90A, max). For example, for the minimum AC input of 85V: IC1 = R1 =
( 24V ) x (15F ) = 0.72mA ( 500ms)
120V - 24V = 119k (0.72mA + (90A))
(IIN + Ig ) (tSS )
VHYST
Choose the 120k standard value. Choose a higher value for R1 than the one calculated above if longer startup time can be tolerated in order to minimize power loss on this resistor. The above startup method is applicable to a circuit similar to the one shown in Figure 5. In this circuit, the tertiary winding has the same phase as the output windings. Thus, the voltage on the tertiary winding at any given time is proportional to the output voltage and goes through the same soft-start period as the output voltage. The minimum discharge voltage of C1 from +22V to +10V must be greater than the soft-start time of 60ms. Another method of bootstrapping the circuit is to have a separate bias winding than the one used for regulating the output voltage and to connect the bias winding so that it is in phase with the MOSFET ON time (see Figure 9). In this case, the amount of capacitance required is much smaller. However, in this mode, the input voltage range has to be less than 2:1. Another consideration is whether the bias winding is in phase with the output. If so, the LED driver circuit hiccups and soft-starts under output shortcircuit conditions. However, this property is lost if the bias winding is in phase with the MOSFET ON time.
where IIN is the MAX16801's internal supply current after startup (1.4mA), Qgtot is the total gate charge for Q1, f SW is the MAX16801's switching frequency (262kHz), V HYST is the bootstrap UVLO hysteresis (11.9V) and tSS is the internal soft-start time (60ms). For example: Ig = (8nC) x (262kHz) = 2.1mA C1 =
(1.4mA + 2.1mA) x (60ms) = 17.5F (12V)
Choose the 15F standard value.
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Offline and DC-DC PWM Controllers for High-Brightness LED Drivers
Application Circuits
Figure 5 shows an offline application of an HB LED driver using the MAX16801. The use of transformer T1 allows significant design flexibility. Use the internal error amplifier for a very accurate LED current control. Figure 6 shows a discontinuous flyback LED driver with linear dimming capability. The total LED voltage can be lower or higher than the input voltage. Figure 7 shows a continuous-conduction-mode HB LED buck driver with linear dimming and just a few external components. Figure 8 shows an offline isolated flyback HB LED driver with low-frequency PWM using MAX16801. The PWM signal needs to be inverted (see the Functional Diagram). Transformer T1 provides full safety isolation and operation from universal AC line (85VAC to 265VAC).
MAX16801A/B/MAX16802A/B
D1
T1
VSUPPLY
D2
VOUT
R1
R2 Q1
C4 LEDs
IN
NDRV
C1 VCC CS
C2 C3 COMP
R4
R7
MAX16801
GND
R6
DIM/FB
UVLO/EN
R3 R5 GND
Figure 5. Offline, Nonisolated, Flyback LED Driver with Programmable Input-Supply Start Voltage
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11
Offline and DC-DC PWM Controllers for High-Brightness LED Drivers MAX16801A/B/MAX16802A/B
VIN 10.8V TO 24V LED(s) C4
R1
UVLO/EN DIM/FB
1 2
8 7
IN VCC NDRV GND
L1
COMP 3 CS R2 4
MAX16802B
6 5
Q1
D1
R4 R3 DIMMING C1 GND C2 C3 R5
Figure 6. MAX16802 Flyback HB LED Driver with Dimming Capability, 10.8V to 24V Input Voltage Range
VIN 10.8V TO 24V LED(s) UVLO/EN DIM/FB 1 2 8 7 IN VCC NDRV GND Q1 L1 D1 C4
R1
COMP 3 CS R2 4
MAX16802B
6 5
R3 DIMMING C1 GND R4 C2 C3 R5
Figure 7. MAX16802 Buck HB LED Driver with Dimming Capability, 10.8V to 24V Input Voltage Range
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Offline and DC-DC PWM Controllers for High-Brightness LED Drivers MAX16801A/B/MAX16802A/B
OPTIONAL ONLY WHEN PWM DIMMING IS USED
D3 C4
D1
T1
D2 R2
R1
VCC
NDRV
Q1 LEDs
UNIVERSAL AC INPUT
BRIDGE RECTIFIER
C3 IN GND C6
MAX16801B
CS C1 C2
DIM/FB *PWM
UVLO/EN R3
R4
C5
*WARNING: PWM DIMMING SIGNAL IS SHOWN AT THE PRIMARY SIDE. USE AN OPTOCOUPLER FOR SAFETY ISOLATION OF THE PWM SIGNAL.
Figure 8. Universal AC Input, Offline, Isolated Flyback HB LED Driver with Low-Frequency PWM Dimming
D1
T1
+VIN
D3
VOUT U2 OPTO LED
R1
R2 Q1
IN C1 VCC
NDRV
R8
CS C4 R4
R11 C3
U2 OPTO TRANS
R7 COMP
MAX16801
GND U3 TLV431
R9 Z1
R5 DIM/FB UVLO/EN
R6 C2 GND
R3 C5 R10
Figure 9. Universal Input, Offline, High-Accuracy Current Regulation in an Isolated Flyback HB LED Driver ______________________________________________________________________________________ 13
Offline and DC-DC PWM Controllers for High-Brightness LED Drivers MAX16801A/B/MAX16802A/B
Functional Diagram
IN IN CLAMP 26.1V BOOTSTRAP UVLO**
IN
VCC
VCC
REGULATOR REG_OK DIGITAL SOFT-START REFERENCE 1.23V 21.6V 9.74V VL
UVLO COMP 1.28V 1.23V
UVLO
(INTERNAL 5.25V SUPPLY)
FB ERROR AMP CPWM CS 1.38V VOPWM *OSCILLATOR 264kHz THERMAL SHUTDOWN VCS 0.3V S Q R
DRIVER NDRV
GND
LIM
MAX16801 MAX16802
*MAX16801A/MAX16802A: 50% MAXIMUM DUTY CYCLE MAX16801B/MAX16802B: 75% MAXIMUM DUTY CYCLE **MAX16801 ONLY
Selector Guide
PART MAX16801A MAX16801B MAX16802A MAX16802B BOOTSTRAP UVLO Yes Yes No No STARTUP VOLTAGE (V) 22 22 10.8* 10.8* MAX DUTY CYCLE (%) 50 75 50 75
CS 4 TOP VIEW
Pin Configuration
UVLO/EN 1 DIM/FB 2 COMP 3
8 IN 7 VCC
MAX16801 MAX16802
6 NDRV 5 GND
*The MAX16802 does not have an internal bootstrap UVLO. The MAX16802 starts operation as long as the VCC pin is higher than +7V, (the guaranteed output with an IN pin voltage of +10.8V), and the UVLO/EN pin is high.
MAX
14
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Offline and DC-DC PWM Controllers for High-Brightness LED Drivers
Package Information
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. Note that a "+", "#", or "-" in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status.
PACKAGE TYPE 8 MAX PACKAGE CODE -- DOCUMENT NO. 21-0036
MAX16801A/B/MAX16802A/B
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Offline and DC-DC PWM Controllers for High-Brightness LED Drivers MAX16801A/B/MAX16802A/B
Revision History
REVISION NUMBER 0 1 2 REVISION DATE 10/05 1/06 1/10 Initial release MAX16802AEUA+ parts are available Corrected formulas, updated subscripts, and removed package outline DESCRIPTION PAGES CHANGED -- 1 1, 2, 3, 6-14
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2010 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.

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