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LTC3783

3783fb

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

PWM LED Driver and Boost,

Flyback and SEPIC Controller

The LTC

3783 is a current mode LED driver and boost,

ﬂyback and SEPIC controller that drives both an N-channel

power MOSFET and an N-channel load PWM switch. When

using an external load switch, the PWMIN input not only

drives PWMOUT, but also enables controller GATE switch-

ing and error ampliﬁer operation, allowing the controller

to store load current information while PWMIN is low.

This feature (patent pending) provides extremely fast,

true PWM load switching with no transient overvoltage

or undervoltage issues; LED dimming ratios of 3000:1

can be achieved digitally, avoiding the color shift normally

associated with LED current dimming. The FBP pin allows

analog dimming of load current, further increasing the

effective dimming ratio by 100:1 over PWM alone.

In applications where output load current must be returned

to V

, optional constant current/constant voltage regula-

tion controls either output (or input) current or output

voltage and provides a limit for the other. I

LIM

provides a

10:1 analog dimming ratio.

For low- to medium-power applications, No R

SENSE

mode

can utilize the power MOSFET’s on-resistance to eliminate

the current-sense resistor, thereby maximizing efﬁciency.

The IC’s operating frequency can be set with an external

resistor over a 20kHz to 1MHz range and can be synchro-

nized to an external clock using the SYNC pin.

The LTC3783 is available in the 16-lead DFN and TSSOP

packages.

350mA PWM LED Boost Application

True Color PWM

Delivers Constant Color with

3000:1 Dimming Ratio

Fully Integrated Load FET Driver for PWM Dimming

Control of High Power LEDs

100:1 Dimming from Analog Inputs

Wide FB Voltage Range: 0V to 1.23V

Constant Current or Constant Voltage Regulation

Low Shutdown Current: I

= 20µA

1% 1.23V Internal Voltage Reference

2% RUN Pin Threshold with 100mV Hysteresis

Programmable Operating Frequency

(20kHz to 1MHz) with One External Resistor

Synchronizable to an External Clock Up to 1.3f

OSC

Internal 7V Low Dropout Voltage Regulator

Programmable Output Overvoltage Protection

Programmable Soft-Start

Can be Used in a No R

SENSE

Mode for V

< 36V

16-Lead DFN and TSSOP Packages

High Voltage LED Arrays

Telecom Power Supplies

42V Automotive Systems

24V Industrial Controls

IP Phone Power Supplies

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

True Color PWM and No R

SENSE

are trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners.

Typical Waveforms

LTC3783

RUN

PWMIN

REF

FBP

FBN

FREQ

SYNC

OV/FB

PWMOUT

LIM

GATE

SENSE

INTV

GND

LED*

STRING

6V TO 16V

(< TOTAL V

OF LEDs)

GND

M1, M2: SILICONIX Si4470EY

10µF

×2

ZETEX ZLLS1000

OUT

10µF

OUT

<25V

*LUMILEDS LHXL-BW02

3783 TA01a

0.1µF

10k 6k

105k

10µF

2.2µH

4.7µF

0.05Ω

237k

12.4k 0.3Ω

PWMIN

5V/DIV

2.5A/DIV

LED

0.5A/DIV

OUT

0.2V/DIV

AC COUPLED

1µs/DIV

3783 TA01b

Summary of content (24 pages)

PAGE 1
LTC3783 PWM LED Driver and Boost, Flyback and SEPIC Controller DESCRIPTION FEATURES n n n n n n n n n n n n n n n True Color PWMTM Delivers Constant Color with 3000:1 Dimming Ratio Fully Integrated Load FET Driver for PWM Dimming Control of High Power LEDs 100:1 Dimming from Analog Inputs Wide FB Voltage Range: 0V to 1.23V Constant Current or Constant Voltage Regulation Low Shutdown Current: IQ = 20µA 1% 1.
PAGE 2
LTC3783 ABSOLUTE MAXIMUM RATINGS (Note 1) VIN, SENSE, FBP, FBN Voltages.................. –0.3V to 42V INTVCC Voltage............................................. –0.3V to 9V INTVCC Output Current........................................... 75mA GATE Output Current.................................. 50mA (RMS) PWMOUT Output Current........................... 25mA (RMS) VREF Ouput Current.................................................. 1mA GATE, PWMOUT Voltages........–0.3V to (VINTVCC + 0.
PAGE 3
LTC3783 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. VIN = 12V, VRUN = 1.5V, VSYNC = 0V, VFBP = VREF , RT = 20k, unless otherwise specified. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Main Control Loop/Whole System VIN Input Voltage Range IQ Input Voltage Supply Current Continuous Mode Shutdown Mode 3 (Note 4) VOV/FB = 1.5V, VITH = 0.75V VRUN = 0V 36 1.
PAGE 4
LTC3783 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. VIN = 12V, VRUN = 1.5V, VSYNC = 0V, VFBP = VREF , RT = 20k, unless otherwise specified.
PAGE 5
LTC3783 TYPICAL PERFORMANCE CHARACTERISTICS VREF vs Temperature 1.235 VREF Line Regulation 1.235 1.20 1.233 1.233 1.15 1.231 1.231 1.229 1.10 1.00 –50 0 150 50 100 TEMPERATURE (°C) 1.225 0 10 20 30 1 0 3 2 IREF (mA) 1.4 1.9 DYNAMIC IQ (mA) IQ (mA) 0.6 1.6 1.5 1.4 1.3 0.4 1.2 0.2 CL = 3300pF IQ(TOT) = 1.3mA + QG • f 25 1.7 1.0 5 Dynamic IQ vs Frequency 30 1.8 0.8 4 3783 G03 IQ vs VIN (PWMIN Low) 2.0 1.2 IQ (mA) 1.
PAGE 6
LTC3783 TYPICAL PERFORMANCE CHARACTERISTICS 160 340 158 Maximum VSENSE vs Temperature 156 73 154 72 310 152 71 290 ISENSE (µA) 330 300 150 148 70 69 280 146 68 270 144 67 260 142 66 250 –50 140 –50 65 –50 0 150 50 100 TEMPERATURE (°C) 0 100 50 TEMPERATURE (°C) 7.05 6.95 6.90 INTVCC (V) 6.80 6.75 6.70 7.0 6.8 6.95 6.6 75°C 6.90 6.4 100°C 6.85 6.2 6.80 6.75 5.6 6.65 5.4 5.2 5.0 10 20 30 VIN (V) 40 50 INTVCC Line Regulation vs Temperature 49.4 60 49.
PAGE 7
LTC3783 PIN FUNCTIONS FBN (Pin 1): Error Amplifier Inverting Input/Negative Current Sense Pin. In voltage mode (VFBP ≤ VVREF), this pin senses feedback voltage from either the external resistor divider across VOUT for output voltage regulation, or the grounded sense resistor under the load for output current regulation. In constant current/constant voltage mode (VFBP > 2.5V), connect this pin to the negative side of the current-regulating resistor.
PAGE 8
LTC3783 PIN FUNCTIONS ITH (Pin 15): Error Amplifier Output/Compensation Pin. The current comparator input threshold increases with this control voltage, which is the output of the gm type error amplifier. Nominal voltage range for this pin is 0V to 1.40V. RUN pin threshold is nominally 1.248V and the comparator has 100mV hysteresis for noise immunity. When the RUN pin is grounded, the IC is shut down and the VIN supply current is kept to a low value (20µA typ).
PAGE 9
LTC3783 OPERATION Main Control Loop The LTC3783 is a constant frequency, current mode controller for PWM LED as well as DC/DC boost, SEPIC and flyback converter applications. In constant current LED applications, the LTC3783 provides an especially wide PWM dimming range due to its unique switching scheme, which allows PWM pulse widths as short as several converter switching periods. For voltage feedback circuit operation (defined by VFBP ≤ 1.
PAGE 10
LTC3783 OPERATION The LTC3783 can be used either by sensing the voltage drop across the power MOSFET or by connecting the SENSE pin to a conventional shunt resistor in the source of the power MOSFET, as shown in the Typical Application on the first page of this data sheet. Sensing the voltage across the power MOSFET maximizes converter efficiency and minimizes the component count, but limits the output voltage to the maximum rating for this pin (36V).
PAGE 11
LTC3783 OPERATION INTVCC Regulator Bypassing and Operation An internal, P-channel low dropout voltage regulator produces the 7V supply which powers the gate drivers and logic circuitry within the LTC3783 as shown in Figure 3. The INTVCC regulator can supply up to 50mA and must be bypassed to ground immediately adjacent to the IC pins with a minimum of 4.7µF low ESR or ceramic capacitor. Good bypassing is necessary to supply the high transient currents required by the MOSFET gate driver.
PAGE 12
LTC3783 OPERATION a maximum total gate charge of 35nC (the temperature coefficient of the gate charge is low). IQ(TOT) = 1.2mA + 35nC • 300kHz = 12mA PIC = 12V • 12mA = 144mW TJ = 70°C + 110°C/W • 144mW = 86°C A similar analysis applies to the VFBP resistive divider, if one is used: VFBP = VREF • where R3 is subject to a similar 500nA bias current. This demonstrates how significant the gate charge current can be when compared to the static quiescent current in the IC.
PAGE 13
LTC3783 OPERATION For applications where the RUN pin is only to be used as a logic input, the user should be aware of the 7V Absolute Maximum Rating for this pin! The RUN pin can be connected to the input voltage through an external 1M resistor, as shown in Figure 5c, for “always on” operation. Soft-Start Capacitor Selection For proper soft-start operation, the LTC3783 should have a sufficiently large soft-start capacitor, CSS, attached to the SS pin.
PAGE 14
LTC3783 OPERATION Figure 6 illustrates these various quantities in relation to one another. Typically, in order to avoid visible flicker, fPWM should be greater than 120Hz. Assuming inductor and capacitor sizing which is close to discontinuous operation, 2 fOSC cycles are sufficient for proper PWM operation. Thus, within the 1MHz rated maximum fOSC, a dimming ratio of 1/DPWM = 3000 is possible.
PAGE 15
LTC3783 OPERATION Boost Converter: Inductor Selection Given an operating input voltage range, and having chosen the operating frequency and ripple current in the inductor, the inductor value can be determined using the following equation:  VIN(MIN)  L= • DMAX  ∆IL • f  been shown to contribute significantly to EMI. Any attempt to damp it with a snubber will degrade the efficiency.
PAGE 16
LTC3783 OPERATION During the switch on-time, the IMAX comparator limits the absolute maximum voltage drop across the power MOSFET to a nominal 150mV, regardless of duty cycle. The peak inductor current is therefore limited to 150mV/RDS(ON). The relationship between the maximum load current, duty cycle, and the RDS(ON) of the power MOSFET is: 1–DMAX RDS(ON) < 150mV •  c  1+  •IOUT(MAX) • rT 2 The rT term accounts for the temperature coefficient of the RDS(ON) of the MOSFET, which is typically 0.4%/°C.
PAGE 17
LTC3783 OPERATION From a known power dissipated in the power MOSFET, its junction temperature can be obtained using the following formula: TJ = TA + PFET • qJA for a given output ripple voltage. The effects of these three parameters (ESR, ESL and bulk C) on the output voltage ripple waveform are illustrated in Figure 9 for a typical boost converter. The qJA to be used in this equation normally includes the qJC for the device plus the thermal resistance from the case to the ambient temperature (qCA).
PAGE 18
LTC3783 OPERATION For many designs it is possible to choose a single capacitor type that satisfies both the ESR and bulk C requirements for the design. In certain demanding applications, however, the ripple voltage can be improved significantly by connecting two or more types of capacitors in parallel. For example, using a low ESR ceramic capacitor can minimize the ESR setup, while an electrolytic capacitor can be used to supply the required bulk C.
PAGE 19
LTC3783 OPERATION And so the inductor value is: VIN(MIN) 12V L= • DMAX = • 0.53 = 11µH ∆IL • f 0.6A • 1MHz 4. RSENSE should be: RSENSE = 0.5 • VSENSE(MAX) 0.5 • 150mV = = 42mΩ IIN(PEAK) 1.8A 5. The diode for this design must handle a maximum DC output current of 0.7A and be rated for a minimum reverse voltage of VOUT , or 25V. A 1A, 40V diode from Zetex was chosen for its specifications, especially low leakage at higher temperatures, which is important for maintaining dimming range. 6.
PAGE 20
LTC3783 OPERATION 5. Check the stress on the power MOSFET by measuring its drain-to-source voltage directly across the device terminals (reference the ground of a single scope probe directly to the source pad on the PC board). Beware of inductive ringing which can exceed the maximum specified voltage rating of the MOSFET. If this ringing cannot be avoided and exceeds the maximum rating of the device, either choose a higher voltage device or specify an avalanche-rated power MOSFET. 6.
PAGE 21
LTC3783 OPERATION Similar to the boost converter, which can be dimmed via the digital PWMIN input or the analog FBP pin, the buckboost can be dimmed via the PWMIN pin or the analog ILIM pin, which adjusts the offset voltage to which the loop will drive (VFBP – VFBN). In the case of the buck-boost, however, the dimming ratio cannot be as high as in the boost converter, since there is no load switch to preserve the VOUT level while PWMIN is low.
PAGE 22
LTC3783 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. DHD Package 16-Lead Plastic DFN (5mm × 4mm) (Reference LTC DWG # 05-08-1707) 0.70 ± 0.05 4.50 ± 0.05 3.10 ± 0.05 2.44 ± 0.05 (2 SIDES) PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 4.34 ± 0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 5.00 ± 0.10 (2 SIDES) R = 0.20 TYP 4.00 ± 0.10 (2 SIDES) 9 R = 0.115 TYP 0.40 ± 0.10 16 2.44 ± 0.
PAGE 23
LTC3783 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. FE Package 16-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663) Exposed Pad Variation BC 4.90 – 5.10* (.193 – .201) 3.58 (.141) 3.58 (.141) 16 1514 13 12 1110 6.60 ± 0.10 4.50 ± 0.10 9 2.94 (.116) 6.40 2.94 (.252) (.116) BSC SEE NOTE 4 0.45 ± 0.05 1.05 ±0.10 0.65 BSC 1 2 3 4 5 6 7 8 RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.09 – 0.20 (.
PAGE 24
LTC3783 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT 1618 Monolithic 1.4MHz Boost Regulator Constant-Current/Constant-Voltage, 1A Switch LTC1871 Boost, Flyback, SEPIC Controller No RSENSE, 2.5V ≤ VIN ≤ 36V, 92% Duty Cycle LT3477 3A DC/DC LED Driver with Rail-to-Rail Current Sense 2.5V ≤ VIN ≤ 25V: Buck, Buck-Boost and Boost Topologies LTC3780 High Power Buck-Boost Controller 4-Switch, 4V ≤ VIN ≤ 36V, 0.