MAX8685AETD+ ,Xenon Photoflash Charger with IGBT Driver and Voltage MonitorFeatures♦ 2s to Charge 100µF to 300VThe MAX8685 family charges high-voltage photoflashcapacitors qu ..
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MAX8685AETD+
Xenon Photoflash Charger with IGBT Driver and Voltage Monitor
General DescriptionThe MAX8685 family charges high-voltage photoflash
capacitors quickly, while limiting peak drain from the
battery, through an efficient flyback switching regulator.
The internal n-channel MOSFET improves efficiency
over competing bipolar designs by lowering switch-
voltage dropout. An integrated insulated gate bipolar
transistor (IGBT) driver enables flash discharge and
reduces external component count. The device
includes an open-drain DONEoutput to indicate when
the photoflash voltage has reached regulation. The
device automatically refreshes the output voltage every
16s, thus efficiently maintains the capacitor charge
level with minimum battery drain.
The MAX8685A/MAX8685F feature an undervoltage
input (UVI) monitor. UVI monitors the supply voltage
and suspends switching if the input voltage drops
below a programmed threshold. The MAX8685A/
MAX8685F also feature a voltage-monitor output that
provides a scaled replica of the output voltage. The
voltage-monitor output is used for interfacing with a
microprocessor’s internal A/D converter to assist in
implementing red-eye reduction.
The MAX8685C/MAX8685D, with fixed peak-primary
current limits of 1A and 1.6A, respectively, are offered in
a 2mm x 3mm, 8-pin TDFN package. The MAX8685A/
MAX8685F, with resistor-programmable current limits of
up to 2A (max) and 2.6A (max), respectively, are offered
in a 3mm x 3mm, 14-pin TDFN package. All devices
operate over the -40°C to +85°C temperature range.
Features2s to Charge 100µF to 300VIntegrated IGBT DriverVoltage-Monitor Output*Short-/Open-Circuit ProtectionControlled Inrush CurrentProgrammable Input Current Limit Up to 2A
(MAX8685A) or 2.6A (MAX8685F)*Programmable Input Voltage-Overload Protection*Extended Battery Life with Input-Voltage
Monitoring*Internal 2.6A SwitchRobust Architecture Allows Use of Low-Cost
TransformersHigh Accuracy Not Dependent on Transformer
Turns RatioAutomatic Refresh Mode Draws Minimal
Quiescent CurrentCharge-Done Indicator3mm x 3mm, 14-Pin TDFN Package
(MAX8685A/MAX8685F)2mm x 3mm, 8-Pin TDFN Package
(MAX8685C/MAX8685D)
MAX8685A/MAX8685C/MAX8685D/MAX8685F
Xenon Photoflash Charger with
IGBT Driver and Voltage Monitor
Ordering Information19-0728; Rev 4; 2/09
Note:All devices are specified over the -40°C to +85°C
operating temperature range.
+Denotes a lead(Pb)-free/RoHS-compliant package.EP=Exposed paddle.
EVALUATION KITAVAILABLE
PARTPIN-PACKAGET O P M A R K
MAX8685AETD+14 TD FN - E P †
3m m x 3m m ADD
MAX8685CETA+8 TD FN - E P †
2mm x 3mmAAE
MAX8685DETA+8 TD FN - E P †
2mm x 3mmAAF
MAX8685FETD+14 TD FN - E P †
3mm x 3mmADY
MAX8685A
MAX8685F
VCC
XENON
FLASH
TUBE
1:15+300V
SEC
UVILX
VBATT
+1.5V
+10V
VCC
+2.5V TO +5.5VISET
MTR
VCCT
TRIG
GND
PGND
GATEIGBT
VCCT
+2.5V
+5.5V
ISET
Typical Operating Circuit*MAX8685A/MAX8685F only.
ApplicationsDigital Cameras
Film Cameras
Cell-Phone Cameras
Personal Media Players
MAX8685A/MAX8685C/MAX8685D/MAX8685F
Xenon Photoflash Charger with
IGBT Driver and Voltage Monitor
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(VCC= VEN= 3.3V, VCCT= 3.3V (MAX8685A/MAX8685F only); VFB= 0, RISET= 93.1kΩ(MAX8685A), RISET= 120kΩ(MAX8685F)
VUVI= 1.5V, TA= -40°C to +85°C, unless otherwise noted.) (Notes 2 and 3)
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.
LX to PGND............................................................-0.3V to +40V
EN, ISET, MTR, to GND..............................-0.3V to (VCC + 0.3V)
UVI to GND.............................................................-0.3V to +12V
VCC, VCCT, FB, DONEto GND.................................-0.3V to +6V
GATE, TRIG to PGND (MAX8685A/MAX8685F)
..............................................................-0.3V to (VCCT + 0.3V)
GATE, TRIG to EP
(MAX8685C/MAX8685D).......................-0.3V to (VCC+ 0.3V)
PGND to GND (Note 1).........................................-0.3V to +0.3V
SEC Current...................................................................±200mA
Current into DONE............................................................±10mA
Continuous Power Dissipation
8-Pin TDFN (derate 16.7mW/°C above TA= +70°C)
(multilayer PCB)........................................................1333mW
14-Pin TDFN (derate 18.5mW/°C above TA= +70°C)
(single-layer PCB).....................................................1481mW
14-Pin TDFN (derate 24.4mW/°C above TA= +70°C)
(multilayer PCB)........................................................1951mW
Operating Temperature Range...........................-40°C to +85°C
Junction Temperature Range............................-40°C to +150°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s).................................+300°C
PARAMETERCONDITIONSMINTYPMAXUNITS
VCCVCC Voltage Range2.55.5V
VCC rising2.22.32.4VCC Undervoltage ThresholdVCC falling2.12.22.3V
Switching at 300kHz1.85mAVCC Supply CurrentNot switching60100µA
TA = +25°C0.11VCCT = 5.5VTA = +85°C0.1
TA = +25°C0.11
VCC Shutdown Current (MAX8685A/
MAX8685F)
VEN = 0,
VCC = 5.5V,
VTRIG = 0VCCT = 0TA = +85°C0.1
TA = +25°C0.11VCC Shutdown Current
(MAX8685C/MAX8685D)
VEN = 0, VCC = 5.5V,
VTRIG = 0TA = +85°C0.1µA
VCC = 3.3V0.180.4LX On-ResistanceILX = 190mAVCC = 2.5V0.20.5Ω
TA = +25°C0.11LX Off-LeakageVLX = 10V, VEN = 0TA = +85°C0.1µA
RISET = 93.1kΩ1.441.601.76LX Peak Current Limit
(MAX8685A Only)TA = 0°C to +85°CISET = VCC2.0A
RISET = 120kΩ1.46251.6251.7875LX Peak Current Limit
(MAX8685F Only)TA = 0°C to +85°CISET = VCC2.6A
MAX8685C1.0LX Peak Current LimitTA = 0°C to +85°CMAX8685D1.6A
LX Switching FrequencyCircuit of Figure 3 or Figure 4, output 90%300kHz
Note 1:For the MAX8685C/MAX8685D, GND and PGND are internally connected to the exposed paddle (EP). All references to GND
or PGND refer to the EP in the MAX8685C/MAX8685D.
MAX8685A/MAX8685C/MAX8685D/MAX8685F
Xenon Photoflash Charger with
IGBT Driver and Voltage Monitor
PARAMETERCONDITIONSMINTYPMAXUNITS
SECSEC Sense Resistance1.1Ω
RISET = 93.1kΩ21.4SEC Valley-Current Threshold (MAX8685A
Only)ISEC fallingISET = VCC26.7mA
RISET = 120kΩ10SEC Valley-Current Threshold (MAX8685F
Only)ISEC fallingISET = VCC16mA
MAX8685C16SEC Valley-Current ThresholdISEC fallingMAX8685D16mA
TA = +25°C1.241.251.26FB Trip ThresholdVFB risingTA = -40°C to +85°C1.2371.2501.263V
TA = +25°C0.11FB Input CurrentVFB = 1.25VTA = +85°C0.1µA
Output Refresh RateFrom VFB > 1.25V to LX switching16s
UVI (MAX8685A/MAX8685F only)UVI Trip Threshold Falling0.981.001.02V
UVI Trip Threshold Rising1.051.071.09V
TA = +25°C0.11UVI Input CurrentVEN = 0,
VUVI = VCC = 5.5VTA = +85°C0.1µA
VEN rising1.01.4EN Input ThresholdVEN falling0.40.9V
TA = +25°C5.510EN Input Leakage CurrentVEN = 5.5V
VCC = 5.5VTA = +85°C5.5µA
VOLTAGE MONITOR (MAX8685A/MAX8685F only)VFB = 1.25V1.9422.06MTR Output AccuracyVFB = 0.833V1.2801.3331.387V
MTR Output CurrentVFB = 1.25V100µA
THERMAL SHUTDOWNThermal-Shutdown Threshold170°C
Thermal-Shutdown Hysteresis15°C
DONE
DONE Output Voltage, LowI DONE = 5mA100400mV
TA = +25°C0.11DONE Output Current, HighV DONE = 5.5VTA = +85°C0.1µA
ELECTRICAL CHARACTERISTICS (continued)(VCC= VEN= 3.3V, VCCT= 3.3V (MAX8685A/MAX8685F only); VFB= 0, RISET= 93.1kΩ(MAX8685A), RISET= 120kΩ(MAX8685F)
VUVI= 1.5V, TA= -40°C to +85°C, unless otherwise noted.) (Notes 2 and 3)
MAX8685A/MAX8685C/MAX8685D/MAX8685F
Xenon Photoflash Charger with
IGBT Driver and Voltage Monitor
ELECTRICAL CHARACTERISTICS (continued)(VCC= VEN= 3.3V, VCCT= 3.3V (MAX8685A/MAX8685F only); VFB= 0, RISET= 93.1kΩ(MAX8685A), RISET= 120kΩ(MAX8685F)
VUVI= 1.5V, TA= -40°C to +85°C, unless otherwise noted.) (Notes 2 and 3)
PARAMETERCONDITIONSMINTYPMAXUNITS
IGBT DRIVERVTRIG rising1.4TRIG Input ThresholdsVTRIG falling0.4V
TA = +25°C5.510TRIG Input CurrentVTRIG = 5.5VTA = +85°C5.5µA
GATE Source CurrentVTRIG = 3.3V250mA
GATE Sink CurrentVTRIG = 050mA
VCCT (MAX8685A/MAX8685F only)VCCT Voltage Range2.55.5V
TA = +25°C0.11VCCT Shutdown CurrentVTRIG = 0,
VCCT = 5.5VTA = +85°C0.1µA
Note 2: For the MAX8685C/MAX8685D, GND and PGND are internally connected to the exposed paddle (EP). All references to GNDor PGND refer to the EP in the MAX8685C/MAX8685D.
Note 3:Devices are 100% production tested at TA= +25°C. Limits over the operating temperature range are guaranteed by design
and characterization.
CHARGE TIME (FROM 30V TO 300V)
vs. VBATTMAX8685A/C/D/F toc01
VBATT (V)
CHARGE TIME (s)8642
VCC = 5.5V
IPEAK = 2A
150μF
100μF
50μF26810
CHARGE TIME (FROM 30V to 300V)
vs. VBATT (MAX8685F)MAX8685A/C/D/F toc02
VBATT (V)
CHARGE TIME (s)
150μF
100μF
50μF
VCC = 5.5V
IPEAK = 2.6A
CHARGE TIME (FROM 30V TO 300V)
vs. VBATTMAX8685A/C/D/F toc03
VBATT (V)
CHARGE TIME (s)7546321
VCC = 2.5V
VCC = 3.6V
VCC = 5V
COUT = 100μF
IPEAK = 2A
Typical Operating Characteristics(VEN= VBATT= VISET= VCC= 3.3V, VCCT= 5.5V, circuits of Figure 3, TA= +25°C, unless otherwise noted.)
MAX8685A/MAX8685C/MAX8685D/MAX8685F
Xenon Photoflash Charger with
IGBT Driver and Voltage Monitor
EFFICIENCY
vs. OUTPUT VOLTAGEMAX8685A/C/D/F toc04
OUTPUT VOLTAGE (V)
EFFICIENCY (%)
VBATT = 5V
VBATT = 3.3VVBATT = 2.7V
OUTPUT VOLTAGE
vs. VCCMAX8685A/C/D/F toc05
VCC (V)
OUTPUT VOLTAGE (V)
VBATT = 5V
OUTPUT VOLTAGE
vs. TEMPERATUREMAX8685A/C/D/F toc06
TEMPERATURE (°C)
OUTPUT VOLTAGE (V)1035-15
VBATT = 5V
VBATT = 3.3VVBATT = 2.7V
PRIMARY CURRENT LIMIT
vs. RISETMAX8685A/C/D/F toc07
RISET (kΩ)
LIM
(A)
PRIMARY CURRENT LIMIT
vs. RISET (MAX8685F)
MAX8685A/C/D/F toc08
RISET (kΩ)
LIM
(A)
PEAK PRIMARY CURRENT
vs. TEMPERATURE
MAX8685A/C/D/F toc09
TEMPERATURE (°C)
PEAK PRIMARY CURRENT (A)3560-15
ISET = VCC
PEAK PRIMARY CURRENT
vs. VCCMAX8685A/C/D/F toc10
VCC (V)
PEAK PRIMARY CURRENT (A)
ISET = VCC
RISET = 150kΩ
PEAK PRIMARY CURRENT
vs. VCC (MAX8685F)MAX8685A/C/D/F toc11
VCC (V)
PEAK PRIMARY CURRENT (A)
ISET = VCC
RISET = 100kΩ
RISET = 150kΩ
TDK LDT565630T-041
UVI RISING THRESHOLD
vs. TEMPERATUREMAX8685A/C/D/F toc12
TEMPERATURE (°C)
UVI THRESHOLD (V)1035-15
VCC = 2.7V
VCC = 3.3V
VCC = 5V
Typical Operating Characteristics (continued)(VEN= VBATT= VISET= VCC= 3.3V, VCCT= 5.5V, circuits of Figure 3, TA= +25°C, unless otherwise noted.)
MAX8685A/MAX8685C/MAX8685D/MAX8685F
Xenon Photoflash Charger with
IGBT Driver and Voltage Monitor
Typical Operating Characteristics (continued)(VEN= VBATT= VISET= VCC= 3.3V, VCCT= 5.5V, circuits of Figure 3, TA= +25°C, unless otherwise noted.)
QUIESCENT CURRENT
vs. VCCMAX8685A/C/D/F toc13
VCC (V)
EN = VCC
EN = GND
VCCT = VBATT = 3.3V
SWITCHING WAVEFORMSMAX8685A/C/D/F toc14
1μs/div
ILX2A/div
VLX20V/div
ISEC100mA/div
2μs/div
SWITCHING WAVEFORMS (MAX8685F)ILX
ISEC200mA/div
1A/div
20V/div
MAX8685A/C/D/F toc15
VLX
CHARGE PROFILE
COUT = 100μFMAX8685A/C/D/F toc17
1s/div
VDONE
VOUT
VEN
10V/div
100V/div
5V/div
5V/div
VLX
VCC = VBATT = 5V
LINE STEP WITH UVI
VOUT = 300V; RUVI = 75kΩMAX8685A/C/D/F toc16
20ms/div
VBATT200mV/div
ILX1A/div
200ms/div
CHARGE PROFILE (MAX8685F)
COUT = 35μFVDONE
VEN
VLX20V/div
5V/div
5V/div
100V/div
MAX8685A/C/D/F toc18
VOUT
MAX8685A/MAX8685C/MAX8685D/MAX8685F
Xenon Photoflash Charger with
IGBT Driver and Voltage Monitor
Typical Operating Characteristics (continued)(VEN= VBATT= VISET= VCC= 3.3V, VCCT= 5.5V, circuits of Figure 3, TA= +25°C, unless otherwise noted.)
CHARGE-UP WAVEFORM OF MTRMAX8685A/C/D/F toc19
1s/div
500mV/divVMTR
100V/div
VOUT
MTR OUTPUT IMPEDANCE = 12kΩ
200ms/div
CHARGE-UP WAVEFORM OF MTR
(MAX8685F)VOUT100V/div
500V/div
MAX8685A/C/D/F toc20
VMTR
IGBT DRAIN AND GATE WAVEFORMSMAX8685A/C/D/F toc21
10ms/div
VGATE
2V/div
100V/div
VDG
STARTUP INTO
SHORT-CIRCUIT WAVEFORMSMAX8685A/C/D/F toc22
4s/div
VLX
ILX500mA/div
2V/div
100mV/divVMTR
OUTPUT OPEN-CIRCUIT WAVEFORMSMAX8685A/C/D/F toc23
1μs/div
VLX
ILX1A/div
10V/div
FIRST CHARGE CYCLEMAX8685A/C/D/F toc24
2μs/div
VLX
ILX1A/div
10V/div
MAX8685A/MAX8685C/MAX8685D/MAX8685F
Xenon Photoflash Charger with
IGBT Driver and Voltage Monitor
Pin Description
PIN
MAX8685A/
MAX8685F
MAX8685C/
MAX8685D
NAMEFUNCTIONENEnable Input. Drive EN high to turn on the charger or low to turn it off. EN is internally pulled
to GND through a 1MΩ resistor.—GNDAnalog Ground. Connect GND directly to PGND and the exposed paddle in a star ground
configuration.2TRIGIGBT Driver Trigger Input. Drive TRIG to VCCT (VCC) to trigger GATE. TRIG is internally
pulled to PGND through a 1MΩ resistor.VCCTSupply Voltage Connection for the IGBT Driver. Bypass VCCT to PGND with a 1µF ceramic
capacitor. Connect VCCT to VCC or an external supply up to 5.5V maximum.3GATE
IGBT Driver Output. Connect GATE to the IGBT gate. The GATE output voltage swings
between VCCT and PGND (MAX8685A/MAX8685F only). For the MAX8685C/MAX8685D,
the GATE output voltage swings between VCC and PGND (EP).—PGNDPower Ground. Connect PGND directly to GND and the exposed paddle in a star ground
configuration.DONE
Charge Done Indicator. DONE is an open-drain output that is internally pulled low when EN
is driven high and the output capacitor is charged. DONE is high impedance when EN is
driven low (shutdown mode) and while the output capacitor is charging.LX
Transformer Primary Connection. Connect LX to the transformer primary as shown in Figure
3 or Figure 4. In shutdown mode, the internal switch is off and LX is connected to the
battery voltage through the primary side of the transformer. An internal clamp limits the LX
voltage to 34V.6SECSecondary Current-Sense Input. Connect SEC to the return of the secondary winding to
measure current.—UVI
Battery Input Undervoltage Detect. Connect a resistor from UVI to the battery to make a
resistor-divider with an internal 75kΩ resistor to GND. The input current from the battery is
reduced when VUVI drops below 1V. Connect UVI to VCC when this feature is not in use.
UVI is high impedance when EN is driven low (shutdown mode).—ISET
Current-Limit Set. Connect a resistor from ISET to GND to set the peak current limit through
the primary winding. For the MAX8685A, RISET = 2A x 75kΩ / IPEAK. For the MAX8685F,
RISET = 2.6A x 75kΩ / IPEAK. Connect ISET to VCC to set the current limit to 2A (MAX8685A)
or to 2.6A (MAX8685F).7VCCSupply Voltage for the IC. Bypass VCC to GND (EP) with a 1µF ceramic capacitor.8FBOutp ut Feed b ack. C onnect FB to the center of a r esi stor - d i vi d er connected b etw een the
tr ansfor m er ’ s second ar y w i nd i ng and GN D to set the outp ut vol tag e. V FB r eg ul ates to 1.25V .—MTR
Voltage-Monitor Output. The sample-and-hold monitor circuitry provides a voltage
proportional to the output voltage. MTR provides a 2V output when VFB equals 1.25V. The
voltage monitor output is only valid when the part is charging. In shutdown mode, MTR is
internally grounded. See the Output-Voltage Monitoring (MAX8685A/MAX8685F Only)
section.EPExposed Paddle. Connect the exposed paddle, GND, and PGND together. Note that the
exposed paddle is the only ground connection for the MAX8685C/MAX8685D.
MAX8685A/MAX8685C/MAX8685D/MAX8685F
Xenon Photoflash Charger with
IGBT Driver and Voltage MonitorMAX8685A
MAX8685F
COMP
COMP
SAMPLE-AND-HOLD
CIRCUITRY
1.0V
DONE
MTR
75kΩ
VCC
1.0V
ISET
UVI
TRIG
GATE
IGBTDRV
100kΩ
VCCT
VCCT
+2.5V
+5.5V
16s
23μs
250ns
DRV
VCC
VCC
+2.5V TO +5.5V
VBATT
+1.5V TO +10V
SEC
PGND
1.25V
1.1Ω
+300V
GND
COMP
COMP
Figure 1. MAX8685A/MAX8685F Functional Diagram
Detailed DescriptionThe MAX8685 family of devices utilizes a flyback DC-DC
converter topology with current-limited continuous-mode
(CCM) control scheme and internal switch to charge
photoflash capacitors quickly and efficiently. Low-
battery-detection circuitry monitors the input voltage on a
cycle-by-cycle basis and reduces peak primary current
if the input voltage decreases below the UVI threshold.
An integrated IGBT driver with internal pullup and
pulldown resistors safely drives IGBTs for discharging the
MAX8685A/MAX8685C/MAX8685D/MAX8685F
Xenon Photoflash Charger with
IGBT Driver and Voltage MonitorMAX8685C
MAX8685DOP
COMP
COMP
COMP
1.0V
DONE
16s
23μs
250ns
DRV
VCC
VCC
+2.5V TO +5.5VVBATT
+1.5V TO +10V
SEC
1.25V
1.1Ω
R16
R18
+300V
TRIG
GATE
IGBTDRV
100kΩ
VCC
Figure 2. MAX8685C/MAX8685D Functional Diagram
output capacitor through a xenon flash bulb. A voltage-
monitor output generates a sampled replica of the output
voltage to interface with the microprocessor’s internal A/D
converter to assist in implementing red-eye reduction.
Figure 1 shows the MAX8685A/MAX8685F functional
diagram; Figure 2 is the functional diagram for the
MAX8685C/MAX8685D.
MAX8685A/MAX8685C/MAX8685D/MAX8685F
Xenon Photoflash Charger with
IGBT Driver and Voltage MonitorMAX8685A
MAX8685F
ISET
VCC
FT1
XENON
FLASH
TUBE
R7A
1MΩ
100μF
330V
1kΩ
248kΩ
10μF
1:15
1:43.5
0.022μF
630V
+300V
SECUVI
VBATT
1.5V TO 10V
VCC
+2.5V TO +5.5V
DONEDONE
MTR
VCCTVCCT
TRIGTRIG
MTREN
GATE
1μF
1nF
402Ω
R14
1kΩ
75kΩT1
GND
PGNDEPC4
1μF
Figure 3. MAX8685A/MAX8685F Typical Application Circuit
Control SchemeThe MAX8685 family uses a constant peak-and-valley
current-control scheme to precisely control the
photoflash capacitor charging current. The MAX8685A/
MAX8685F current limit is set by the ISET resistor (see
the Choosing a Resistor for Lowering the Charge
Current (MAX8685A/MAX8685F Only) section) or by
connecting ISET to VCCfor a maximum limit of 2A
(MAX8685A) or 2.6A (MAX8685F). The resistor at ISET
(MAX8685A/MAX8685F) and the transformer turns ratio
set the peak charge current.
The MAX8685C/MAX8685D offer fixed peak primary cur-
rent limits of 1A and 1.6A, respectively. Drive EN high to
turn on the LX switch and initiate charging. After the LX
switch turns on, the current in the transformer primary
winding increases to the peak current limit. When the LX
switch turns off, the energy stored in the transformer is
deliveredto the photoflash capacitor through the trans-
former secondary and rectifying diode. As the sec-
ondary current ramps down, it is monitored through
SEC. When the current drops to 1.67% of the primary
peak current limit, the LX switch turns on after a 50ns
delay (MAX8685C/MAX8685D/MAX8685F) and a new
charge cycle begins. There is no delay in the
MAX8685A. This cycle repeats itself, adding energy to
the photoflash capacitor until the target output voltage is
reached. The switching frequencyis determined by the
time required to ramp the primary-side inductance to
the LX peak current limit and the discharge time of the
secondary current. The switching frequency increases
as the output capacitor charges to the targeted output
voltage. Once the target output voltage is reached, the
IC automatically refreshes the output every 16s, effi-
ciently maintaining the capacitor charge level with mini-
mum battery drain. The MAX8685 family draws only
60µA (typ) in automatic refresh mode. Automatic
refresh can be overridden by driving EN low.
Secondary-Side SensingOutput regulation is accomplished using a resistor-
divider connected to the anode of the output rectifying
diode (see Figure 3 or Figure 4). This connection elimi-
nates DC current drain on the output capacitor while
still providing direct output sensing for optimum voltage
accuracy that is not dependent on the transformer turns
ratio. The MAX8685 samples VFBduring the flyback
