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MAX8785AETI+
Full-Bridge CCFL Controller for Piezoelectric Transformers
General DescriptionLiquid-crystal display (LCD) enclosures and cold-cathode
fluorescent lamps (CCFL) used in notebook computer
and portable electronic displays are becoming increa-
singly narrow, generating the need for a low-profile
CCFL power supply. Recent advances in piezoelectric
transformers (PZTs) have made it possible to develop
smaller, more efficient, and safer backlight inverters for
portable displays. Piezoelectric transformers have
shown better performance than magnetic transformers
with respect to efficiency, EMI requirements, human
safety, and form factor. The MAX8785A is a full-bridge
CCFL controller for piezoelectric transformer-based
backlight power supplies. The full-bridge topology pro-
vides a high-spectral purity sinusoidal drive that helps
the PZT convert electrical energy to mechanical and
back to electrical energy efficiently.
The MAX8785A employs a feed-forward control architec-
ture that provides excellent line-and-load regulation while
maintaining relatively constant switching frequency.
The MAX8785A provides protection against open-lamp,
secondary short-circuit, lamp arcing, and secondary
overvoltage with adjustable timeout periods.
The MAX8785A guarantees lamp striking by sweeping
the switching frequency from high to low until the lamp
is struck. The MAX8785A achieves 10:1 dimming range
using a digital pulse-width modulation (DPWM) method.
CCFL brightness can be set with an analog voltage on
the CNTL pin or through an external signal at LSYNC.
The maximum switching frequency and DPWM frequen-
cy can be adjusted with external resistors.
The MAX8785A gate drivers can drive large power
MOSFETs typically used in high-power CCFL applica-
tions. An internal 5.35V linear regulator powers the
MOSFET drivers and most of the internal circuitry. The
MAX8785A is available in a low-profile, 28-pin thin QFN
package and operates over the -40°C to +85°C tempe-
rature range.
ApplicationsNotebook LCDs
LCD TVs and Monitors
FeaturesResonant, All n-Channel, Full-Bridge TopologyWide Input-Voltage Range (4.5V to 28V)Frequency Sweeping Guarantees CCFL StrikingFeed-Forward Control Provides Excellent Line-
Transient ResponseProgrammable Maximum Switching FrequencyAnalog or Digital DPWMLamp-Out Detection with Adjustable TimeoutFault Protection with Adjustable TimeoutPrimary Current Limit with RDS(ON)SensingStrong Gate Drivers Support Large
External MOSFETs28-Pin Thin QFN Package
MAX8785A
Full-Bridge Controller for
Piezoelectric Transformers272524911
SHDN
LSYNC
FLT
TFLT
RATE
VDD1
PGND
GL2
BST1
VDD2
BST2
SEL
GND
VFB
IFB
COMP
PCOMP
MAX8785A
CNTLGL13
BATT
OLF
LX2
LX1
GH2
GH1
THIN QFNTOP VIEW
Pin Configuration
Ordering Information19-0600; Rev 1; 5/08
+Denotes lead-free package.
*EP = Exposed pad.
EVALUATION KIT
AVAILABLE
PARTTEMP RANGEPIN-PACKAGEMAX8785AETI+ -40°C to +85°C 28 Thin QFN*
Simplified Operating Circuit appears at end of data sheet.
MAX8785A
Full-Bridge Controller for
Piezoelectric Transformers
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(VBATT = 12V, VDD1= VDD2 = VSHDN= 5.35V, RRATE= 190kΩ,TA
= 0°C to +85°C, unless otherwise noted. See Figure 1. Typical valuesare at TA= +25°C.)
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.
BATT, RATE, LX1, LX2 to GND .............................-0.3V to +30V
BST1, BST2 to GND...............................................-0.3V to +36V
BST1 to VDD1, BST2 to VDD2..................................-0.3V to +30V
BST1 to LX1..............................................................-0.3V to +6V
BST2 to LX2..............................................................-0.3V to +6V
VCC, VDD1, VDD2, CNTL, LSYNC, SHDN,
FLT to GND...........................................................-0.3V to +6V
GH1 to LX...............................................-0.3V to (VBST1 + 0.3V)
GH2 to LX2.............................................-0.3V to (VBST2 + 0.3V)
GL1 to GND..............................................-0.3V to (VDD1+ 0.3V)
GL2 to GND..............................................-0.3V to (VDD2+ 0.3V)
COMP, HF, LF, PCOMP, SEL,
TFLT to GND............................................-0.3V to (VCC + 0.3V)
IFB, VFB, OLF to GND.................................................-6V to +6V
PGND to GND.......................................................-0.3V to +0.3V
Continuous Power Dissipation (TA= +70°C)
28-Pin Thin QFN (derate 21.3mW/°C above +70°C)...1702.1mW
Operating Temperature Range...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +160°C
Lead Temperature (soldering, 10s).................................+300°C
PARAMETERCONDITIONSMINTYPMAXUNITSVCC = VDD1 = VDD2 = open5.528.0BATT Input Voltage RangeVCC = VDD1 = VDD2 = BATT4.55.5V
BATT Quiescent CurrentVBATT = 28V1.56mA
BATT Quiescent Current, ShutdownVSHDN = 0, VBATT = 28V1025µA
VCC Output Voltage, Normal Operation6V < VBATT < 28V, 0 < ILOAD < 20mA5.205.355.50V
VCC Output Voltage, ShutdownVSHDN = 0, no load3.54.65.5V
VCC rising (leaving lockout)4.5VCC Undervoltage-Lockout ThresholdVCC falling (entering lockout)4.0V
VCC Undervoltage-Lockout Hysteresis150mV
GH1, GH2, GL1, and GL2
On-Resistance, Low StateITEST = 10mA13Ω
GH1, GH2, GL1, and GL2
On-Resistance, High StateITEST = 10mA48Ω
BST1, BST2 Leakage CurrentVBST1 = 24V, VLX1 = 19V; VBST2 = 24V, VLX2
= 19V, VBATT = 28V5µA
LX1, LX2 Leakage CurrentVBATT = 28V5µA
Minimum On-Time240370500ns
VBATT = 8V, RRATE = 190kΩ, RHF = 100kΩ222528
VBATT = 16V, RRATE = 190kΩ, RHF = 100kΩ10.812.5014.2
VBATT = 24V, RRATE = 190kΩ, RHF = 100kΩ7.58.59.5
VBATT = 12V, RRATE = 150kΩ, RHF = 100kΩ14
Duty Cycle
VBATT = 12V, RRATE = 300kΩ, RHF = 100kΩ25
Maximum Duty Cycle45%
Current-Limit ThresholdLX1 - PGND, LX2 - PGND370400430mV
Current-Limit Leading-Edge Blanking240370500ns
IFB Regulation Point760800840mV
0 < VIFB < +3V-2+2IFB Input Bias Current-3V < VIFB < 0-225µA
MAX8785A
Full-Bridge Controller for
Piezoelectric Transformers
PARAMETERCONDITIONSMINTYPMAXUNITSIFB Lamp-Out ThresholdReject 1µs glitches720800880mV
IFB-to-COMP Transconductance1V < VCOMP < 3V50100150µs
COMP Output Impedance10MΩ
COMP Maximum Voltage Threshold3.153.253.35V
COMP Discharge Current During OvervoltageVIFB = 800mV, VVFB = 2.5V, VCOMP = 2V0.51.02.0mA
Initial Startup COMP Charging Current7911µA
PCOMP Impedance1MΩ
VFB Input Bias Current-4V < VVFB < +4V-2+2µA
VFB Overvoltage ThresholdVFB rising2.052.252.45V
VFB Short-Circuit ThresholdVFB rising200230260mV
OLF Input Bias Current-4V < VOLF < +4V-2+2µA
OLF Trip ThresholdOLF rising1.11.21.3V
RHF = 100kΩ, VCOMP = 0.0V58.060.062.0
RHF = 100kΩ, VCOMP = 3.0V50
RHF = 150kΩ, VCOMP = 0.0V40.0
RHF = 150kΩ, VCOMP = 3.0V33.3
RHF = 75kΩ, VCOMP = 0.0V80.0
Main Oscillator Frequency
RHF = 75kΩ, VCOMP = 3.0V66.6
kHz
RLF = 150kΩ202208214
RLF = 103kΩ300DPWM Chopping Frequency
RLF = 315kΩ100
DPWM Dimming ResolutionGuaranteed monotonic8Bits
CNTL Minimum Duty-Cycle Threshold0.200.230.26V
CNTL Maximum Duty-Cycle Threshold1.92.02.1V
CNTL Input Current0 < VCNTL < 2.0V-0.1+0.1µA
LSYNC Input Low Voltage0.8V
LSYNC Input High Voltage2.1V
LSYNC Input Hysteresis200mV
LSYNC Input Bias Current0 < VLSYNC < 5.5V-1+1µA
LSYNC Input Frequency RangeRLF = 150kΩ120280Hz
LSYNC Minimum Duty Cycle10%
SEL Input Low Voltage0.8V
SEL Input High Voltage2.1V
SEL Input Hysteresis200mV
SEL Input Bias Current0 < VSEL < 5.5V-1+1µA
SHDN Input Low Voltage0.8V
SHDN Input High Voltage2.1V
SHDN Input Hysteresis100mV
ELECTRICAL CHARACTERISTICS (continued)(VBATT = 12V, VDD1= VDD2 = VSHDN= 5.35V, RRATE= 190kΩ,TA
= 0°C to +85°C, unless otherwise noted. See Figure 1. Typical valuesare at TA= +25°C.)
MAX8785A
Full-Bridge Controller for
Piezoelectric Transformers
ELECTRICAL CHARACTERISTICS (continued)(VBATT = 12V, VDD1= VDD2 = VSHDN= 5.35V, RRATE= 190kΩ,TA
= 0°C to +85°C, unless otherwise noted. See Figure 1. Typical valuesare at TA= +25°C.)
PARAMETERCONDITIONSMINTYPMAXUNITSSHDN Input Bias Current0 < VSHDN < 5.5V-1+1µA
VVFB > 2.4V or VIFB < 720mV;
VOLF < 1.1V; VTFLT = 2.0V0.600.750.90
VVFB < 2.05V and VIFB > 880mV;
VOLF < 1.1V; VTFLT = 2.0V-1-0.75-0.5
TFLT Charging Current
VVFB < 200mV or VOLF > 1.3V;
VTFLT = 2.0V2.242.83.36mA
TFLT Trip ThresholdTFLT rising2.903.03.10V
TFLT Sink CurrentVTFLT = 2.0V80135160mA
FLT Sink CurrentVTFLT = 3.1V, VFLT = 0.4V1mA
FLT Leakage CurrentVTFLT = 0, VFLT = 5.5V1µA
ELECTRICAL CHARACTERISTICS(VBATT = 12V, VDD1= VDD2 = VSHDN,TA
= -40°C to +85°C, unless otherwise noted.) (Note 1)
PARAMETERCONDITIONSMINTYPMAXUNITSVCC = VDD1 = VDD2 = open5.528.0BATT Input Voltage RangeVCC = VDD1 = VDD2 = BATT4.55.5V
BATT Quiescent CurrentVSHDN = 5.5V, VBATT = 28V6mA
BATT Quiescent Current, ShutdownVSHDN = 025µA
VCC Output Voltage, Normal OperationVSHDN = 5.5V, 6V < VBATT < 28V,
0 < ILOAD < 20mA5.205.50V
VCC Output Voltage, ShutdownVSHDN = 0, no load3.55.5V
VCC rising (leaving lockout)4.5VCC Undervoltage-Lockout ThresholdVCC falling (entering lockout)3.95V
GH1, GH2, GL1, and GL2 On-Resistance,
Low StateITEST = 10mA3Ω
GH1, GH2, GL1, and GL2 On-Resistance,
High StateITEST = 10mA8Ω
BST1, BST2 Leakage CurrentVBST1 = 24V, VLX1 = 19V;
VBST2 = 24V, VLX2 = 19V; VBATT = 28V5µA
LX1, LX2 Leakage CurrentVBATT = 28V5µA
Minimum On-Time210420ns
VBATT = 8V, RRATE = 190kΩ, RHF = 100kΩ2228BATT = 16V , RRATE = 190kΩ , RH F = 100kΩ 10.814.2Duty CycleBATT = 24V , RRATE = 190kΩ , RH F = 100kΩ 7.509.50
Maximum Duty Cycle45%
Current-Limit ThresholdLX1 - PGND, LX2 - PGND370430mV
MAX8785A
Full-Bridge Controller for
Piezoelectric Transformers
Note 1:-40°C specifications are guaranteed by design, not production tested.
PARAMETERCONDITIONSMINTYPMAXUNITSCurrent-Limit Leading-Edge Blanking240500ns
IFB Regulation Point760840mV
0 < VIFB < +3V-2+2IFB Input Bias Current-3V < VIFB < 0-225µA
IFB Lamp-Out ThresholdReject 1µs glitches720880mV
IFB-to-COMP Transconductance1V < VCOMP < 3V50150µs
COMP Discharge Current During OvervoltageVIFB = 800mV, VVFB = 2.5V, VCOMP = 2V0.52.0mA
Initial Startup COMP Charging Current711µA
COMP Maximum Voltage Threshold3.153.35V
VFB Input Bias Current-4V < VVFB < +4V-2+2µA
VFB Overvoltage ThresholdVFB rising2.052.45V
VFB Short-Circuit ThresholdVFB rising200260mV
OLF Input Voltage Range-4+4V
OLF Input Bias Current-4V < VOLF < +4V-2+2µA
OLF Trip ThresholdOLF rising1.11.3V
Main Oscillator FrequencyRHF = 100kΩ, VCOMP = 0V5763kHz
DPWM Chopping FrequencyRLF = 150kΩ198218Hz
CNTL Minimum Duty-Cycle Threshold0.200.26V
CNTL Maximum Duty-Cycle Threshold1.92.1V
LSYNC Input Low Voltage0.8V
LSYNC Input High Voltage2.2V
LSYNC Input Frequency Range120280Hz
LSYNC Minimum Duty Cycle10%
SEL Input Low Voltage0.8V
SEL Input High Voltage2.2V
SHDN Input Low Voltage0.8V
SHDN Input High Voltage2.2V
VVFB > 2.4V or VIFB < 720mV;
VOLF < 1.1V; VTFLT = 2.0V0.600.90
VVFB < 2.05V and VIFB > 880mV;
VOLF < 1.1V; VTFLT = 2.0V-1.0-0.5TFLT Charging Current
VVFB < 200mV or VOLF > 1.3V;
VTFLT = 2.0V2.243.36
TFLT Sink CurrentVTFLT = 2.0V80190mA
TFLT Trip ThresholdTFLT rising2.903.10V
FLT Sink CurrentVTFLT = 3.1V; VFLT = 0.4V1mA
ELECTRICAL CHARACTERISTICS (continued)(VBATT = 12V, VDD1= VDD2 = VSHDN,TA
= -40°C to +85°C, unless otherwise noted.) (Note 1)
MAX8785A
Full-Bridge Controller for
Piezoelectric Transformers
Typical Operating Characteristics
(Circuit of Figure 1. VIN= 12V, TA = +25°C, unless otherwise noted.)
LOW INPUT-VOLTAGE OPERATION
(VIN = 12V)
10μs/div
CH1: 10V/div
CH2: 2V/div
CH3: 1V/div
IFB
VFB
MAX8785A toc01
VCNTL = 2.0V
SEL = GND
HIGH INPUT-VOLTAGE OPERATION
(VIN = 24V)
10μs/div
CH1: 20V/div
CH2: 2V/div
CH3: 1V/div
IFB
VFB
MAX8785A toc02
VCNTL = 2.0V
SEL = GND
LINE-TRANSIENT RESPONSE
40μs/div
CH2: 2V/div
CH4: 10V/div
IFB
VIN
MAX8785A toc03
12V
24V
MINIMUM BRIGHTNESS OPERATION
AND STARTUP WAVEFORM
4ms/div
CH1: 2V/div
CH2: 5V/div
CH3: 5V/div
CH4: 2V/div
IFB
COMP
SHDN
VFB
MAX8785A toc04LSYNC OPERATION
1ms/div
CH1: 2V/div
CH3: 5V/div
CH4: 10V/div
IFB
LSYNC
MAX8785A toc05
LAMP-OUT VOLTAGE LIMITING
AND TIMEOUT
200ms/div
CH1: 2V/div
VFB
TFLT
MAX8785A toc06
SECONDARY SHORT-CIRCUIT
PROTECTION AND TIMEOUT
200μs/div
CH1: 2V/div
VFB
TFLT
MAX8785A toc07
400μs/div
MAXIMUM SWITCHING FREQUENCY
vs. RHF
RHF (kΩ)
MAXIMUM SWITCHING FREQUENCY (kHz)
MAX8785A toc08
DPWM FREQUENCY vs. RLF
RLF (kΩ)
DPWM FREQUENCY (Hz)
MAX8785A toc09
MAX8785A
Full-Bridge Controller for
Piezoelectric Transformers
VCC LINE REGULATION
VIN (V)
(V)18222012142410
MAX8785A toc10
VCC LOAD REGULATION
ILOAD (mA)
REGULATION (%)
MAX8785A toc11
VCC vs. TEMP
TEMPERATURE (°C)
(V)3560-1585-40
MAX8785A toc12
SWITCHING FREQUENCY vs. VIN
VIN (V)
fSW
(kHz)182022142412
MAX8785A toc13
Typical Operating Characteristics (continued)
(Circuit of Figure 1. VIN= 12V, TA = +25°C, unless otherwise noted.)
MAX8785A
Full-Bridge Controller for
Piezoelectric Transformers
Pin Description
PINNAMEFUNCTION
1 RATE Duty Ratio Ramp Adjustment Input. Connect a feed-forward network between BATT and RATE to adjust the
slope of the internal ramp that adjusts the H-bridge duty cycle with input voltage. SHDN Shutdown Control Input. The IC shuts down when SHDN is pulled to GND.
3 CNTL
Brightness Control Input. The usable brightness control range is from 0 to 2V. VCNTL = 0 represents the
minimum brightness (10% DPWM duty cycle), VCNTL = 2V represents the full brightness (100% DPWM duty
cycle). For details, see the Dimming Controlsection.
4 LSYNC
DPWM Sync Input. DPWM frequency can be synchronized with an external signal on LSYNC. When SEL is
connected to VCC, the duty cycle of the LSYNC signal determines the brightness. For details, see the
Dimming Control section.
5 FLT
Fault Status Output. FLT is an open-drain output and requires a pullup resistor between VCC and FLT. Under
normal conditions, the FLT output is high impedance. FLT pulls low when a fault condition occurs; FLT is
reset on either power or SHDN cycle.
6 TFLT Fault Time-Out Setting. Connect a capacitor from TFLT to GND to set the time out period. For details, see
the Setting the Fault Delay Time section.
7 HF Maximum Switching Frequency Adjustment Input. Connect a resistor from HF to GND to set the maximum
sweeping frequency of the main oscillator.
8 LF DPWM Frequency Adjustment Input. Connect a resistor from LF to GND to set the DPWM oscillator
9 PCOMP Phase-Lock Loop (PLL) Compensation Pin. Connect a capacitor between PCOMP and GND to compensate
the PLL.
10 COMP Transconductance Error-Amplifier Output. The COMP voltage controls the voltage-controlled oscillator to
adjust the switching frequency. Connect a capacitor from COMP to GND.
11 IFB Lamp Current-Feedback Input. IFB regulates the average lamp-current feedback to 800mV (typ) by
controlling the switching frequency. For details, see the Lamp-Current Regulation section.
12 VFB
Secondary Voltage-Feedback Input. A resistive voltage-divider between the high-voltage terminal of the
transformer and GND sets the maximum peak lamp voltage during striking and lamp-out fault. Add 1nF
capacitor from VFB to GND for noise rejection. For details, see the Lamp-Out Detection and Overvoltage
Protection section.
13 LX2 GH2 Gate-Driver Return. LX2 is the input to the primary current-limit comparator. The controller senses the
voltage across the low-side MOSFET (LX2 - PGND) for primary overcurrent condition.
14 GH2 High-Side MOSFET Gate-Driver Output
15 BST2 GH2 Gate-Driver Supply Input. Connect a 0.1μF capacitor from LX2 to BST2.
16 VDD2 GL2 Low-Side Gate-Driver Supply Input. Connect VDD2 to VCC. Bypass VDD2 with a 1μF capacitor to PGND.
17 GL2 Low-Side MOSFET Gate-Driver Output
18 PGND Power Ground. PGND is the return of GL1 and GL2 gate drivers.
19 GL1 Low-Side MOSFET Gate-Driver Output
20 VDD1 GL1 Low-Side Gate-Driver Supply Input. Connect VDD1 to VCC. Bypass VDD1 with a 1μF capacitor to PGND.
21 BST1 GH1 Gate-Driver Supply Input. Connect a 0.1μF capacitor from LX1 to BST1.
22 GH1 High-Side MOSFET Gate-Driver Output
23 LX1 GH1 Gate-Driver Return. LX1 is the input to the primary current-limit comparator. The controller senses the
voltage across the low-side MOSFET (LX1 - PGND) for primary overcurrent condition.
MAX8785A
Full-Bridge Controller for
Piezoelectric Transformers
Pin Description (continued)
PINNAMEFUNCTION
24 OLF
Arc Fault-Detection Input. When the peak voltage on OLF rises above the internal threshold of 1.2V (typ), an
internal current source starts charging the TFLT capacitor. The MAX8785A sets the fault latch and disables the
gate drivers after the TFLT voltage reaches 3V. For details, see Setting the Arc Protection Threshold section.
25 SEL
Brightness Control Select Input. Brightness can be adjusted with an analog voltage on CNTL or with an
external signal at LSYNC. Connect SEL to GND to enable analog control. Connect SEL to VCC to enable
external synchronization control.
26 VCC5.35V/20mA Linear-Regulator Output. Supply voltage for the device. Bypass VCC with a 1μF ceramic
capacitor to GND.
27 GND System Ground
28 BATT Supply Input. Input to the internal 5.35V linear regulator that powers the device. Bypass BATT to ground
with a 0.22μF ceramic capacitor.
29 PAD Backside Exposed Pad. PAD is internally connected to GND. Connect the exposed pad to a ground plane
through a thermally enhanced via.
MAX8785A
Figure 1. Typical Operating Circuit
Full-Bridge Controller for
Piezoelectric Transformers
BATT
GND
VCC
SEL
RATE
SHDN
CNTLDIMMING
LSYNC
COMP
PCOMP
TFLT
VDD1
VDD2
VCC
0.22μF
3.6V
1.0μFR10
280kΩ
R11
90kΩ
47nF
10nF
0.22μF
R12
150kΩ
FLT
VCC
GH1
BST1
LX1
LX2
BST2
GL1
PGND
GL2
GH2
OLF
VFB
R14
100kΩ
6.8nF
C12
1.0nF
1.56kΩ
200kΩ
200kΩ
200kΩ
200kΩ
200kΩ
200kΩ
200kΩ
150kΩ
IFB
VIN
GND
ON/OFF
LSYNC
NH1NH2
NL1NL2
L1, 27μH
C11
10μF, 25V
1.0μF
0.1μF
C10
0.1μF
R13
100kΩ
1.0μF
CCFL
MAX8785A
Typical Operating Circuit
