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19-1833; Rev 0; 10/00
KIT ATION EVALU BLE AVAILA
Digital Camera Step-Up Power Supply
Features
o +0.7V to +5.5V Input Voltage Range o Main DC-DC Converter 95% Efficiency +2.7V to +5.5V Adjustable Output Voltage 1.5A Load Current o Uncommitted Gain Block for Linear Regulator o Three Independent Auxiliary Step-Up Controllers Adjustable Maximum Duty Cycle o Oscillator and Reference Outputs to Drive External Slave Controllers (MAX1801) o Power-Ready Output o Up to 1MHz Switching Frequency o 1A Supply Current in Shutdown Mode o Internal Soft-Start Control o Overload Protection for all DC-DC Converters o Compact 32-Pin TQFP Package (5mm x 5mm body)
General Description
The MAX1800 provides a complete power-supply solution for digital still cameras and video cameras. The device integrates a high-efficiency main step-up DC-DC converter, three auxiliary step-up controllers, and an uncommitted gain block that drives an external P-channel MOSFET for a linear regulator. The MAX1800 is targeted for applications that use either two or three primary cells or a single lithium-ion (Li+) battery. The main DC-DC converter accepts inputs from +0.7V to +5.5V and regulates a resistor-adjustable output from 2.7V to 5.5V. It uses an internal synchronous rectifier to regulate the output with 95% efficiency. An adjustable operating frequency facilitates designs for optimum size, cost, and efficiency. The auxiliary step-up controllers can be used to power a digital camera's CCD, LCD, and backlight. The MAX1800 also features expandability by supplying power, oscillator signal, and reference to the MAX1801, a low-cost slave DC-DC controller that supports step-up, SEPIC, and flyback configurations. The MAX1800 is available in a space-saving 32-pin TQFP package (5mm x 5mm body), and the MAX1801 is available in an 8-pin SOT package. An evaluation kit (MAX1800EVKIT) featuring both devices is available to expedite designs.
MAX1800
Ordering Information
PART MAX1800EHJ TEMP. RANGE -40C to +85C PIN-PACKAGE 32 TQFP
________________________Applications
Digital Still Cameras Digital Video Cameras Hand-Held Devices Internet Access Tablets PDAs DVD Players
Pin Configuration
COMP3 DCON3
TOP VIEW
ON3 DL3 FB3
ONA
AO
26
32
31
30
29
28
27
Typical Operating Circuit
CCD INPUT 0.7V TO 5.5V TFT MAX1800 MASTER CCFL CORE MAIN OSC MAX1801 SLAVE REF
PGND DL1 ON1 FB1 COMP1 DCON1 POUT DL2
1 2 3 4
5 6
AI
25
24 ONM 23 RDYM 22 POUT 21 LX
MAX1800
20 LX 19 PGND 18 COMPM 17 FBM
7
8
9 ON2 MOTOR
10 FB2
11 COMP2
12 13 DCON2 OUT
14 REF
15 GND
16 OSC
TQFP
________________________________________________________________ Maxim Integrated Products 1
For price, delivery, and to place orders, please contact Maxim Distribution at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
Digital Camera Step-Up Power Supply MAX1800
ABSOLUTE MAXIMUM RATINGS
OUT, POUT, ON_, DCON_, FB_, RDYM to GND .....-0.3V to +6.0V PGND to GND ......................................................-0.3V to +0.3V OUT to POUT_ ......................................................-0.3V to +0.3V LX, DL_, AO to PGND .............................-0.3V to (POUT + 0.3V) REF, OSC, AI, COMP_ to GND..................-0.3V to (OUT + 0.3V) Continuous Power Dissipation (TA = +70C) 32-Pin TQFP (derate 11mW/C above +70C) ............880mW Operating Temperature Range MAX1800EHJ.................................................. -40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range. ............................-65C to +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
(Circuit of Figure 1, VOUT = VPOUT = 3.3V, PGND = GND, VONM = 3.3V, VON1 = VON2 = VON3 = VONA = 0, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 1)
PARAMETER GENERAL Input Voltage Range (Note 2) Minimum Startup Voltage Frequency in Startup Mode SUPPLY CURRENT Shutdown Supply Current Main DC/DC Converter Supply Current Main + Auxiliary 1 Supply Current Main + Auxiliary 2 Supply Current Main + Auxiliary 3 Supply Current Analog Gain Block Supply Current REFERENCE Reference Output Voltage REF Load Regulation REF Line Rejection OSCILLATOR OSC Discharge Trip Level OSC Input Bias Current OSC Discharge Resistance OSC Discharge Pulse Width Rising edge VOSC = 1.1V VOSC = 1.5V, IOSC = 3mA 1.225 1.250 0.01 37 100 1.275 100 75 V nA ns VREF IREF = 20A 10A < IREF < 200A 2.7V < VOUT < 5.5V 0.2 1.23 1.250 1.27 10 5 V mV mV VONM = 0 VFBM = 1.2V, VOSC = 0 VON1 = 3.3V, VFBM = 1.2V, VFB1 = 1.2V, VOSC = 0 VON2 = 3.3V, VFBM = 1.2V, VFB2 = 1.2V, VOSC = 0 VON3 = 3.3V, VFBM = 1.2V, VFB3 = 1.2V, VOSC = 0 VONA = 3.3V, VFBM = 1.2V, AI = REF, AO open, VOSC = 0 0.002 250 375 375 375 375 5 400 600 600 600 600 A A A A A A VIN VSTART ILOAD < 1mA, TA = +25oC VOUT = 1.5V 40 0.7 0.9 150 5.5 1.1 300 V V kHz SYMBOL CONDITIONS MIN TYP MAX UNITS
2
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Digital Camera Step-Up Power Supply
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1, VOUT = VPOUT = 3.3V, PGND = GND, VONM = 3.3V, VON1 = VON2 = VON3 = VONA = 0, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 1)
PARAMETER SYMBOL CONDITIONS 1.1V < VOUT < 1.8V (ONM only) 1.8V < VOUT < 5.5V 1.1V < VOUT < 1.8V (ONM only) 1.8V < VOUT < 5.5V Input Leakage Current MAIN DC/DC CONVERTER Main Output Voltage Adjust Range Main Undervoltage Lockout Threshold (Note 3) Main Output Maximum Duty Cycle Idle-ModeTM Threshold ERROR AMPLIFIER FBM Regulation Voltage FBM to COMPM Transconductance FBM to COMPM Maximum Voltage Gain FBM Input Leakage Current COMPM Minimum Output Voltage COMPM Maximum Output Voltage POWER SWITCHES POUT Leakage Current LX Leakage Current Switch On- Resistance N-Channel Current Limit P-Channel Turn-Off Current POWER READY RDYM Trip Level RDYM Output High Leakage RDYM Output Voltage Low VFBM rising edge, 1% typical hysteresis VRDYM = 5.5V ISINK = 1mA 1.09 1.125 0.01 1.16 1 0.4 V A V 40 RON VLX = 0, VPOUT = 5.5V VLX = VOUT = 5.5V N-channel P-channel 0.1 0.1 100 200 2 120 190 20 20 180 350 A A m A mA VFBM = 1.35V VFBM = 1.35V, COMPM open VFBM = 1.15V, COMPM open 0.1 2.00 2.15 2.30 Unity gain configuration, FBM = COMPM Unity gain configuration, FBM = COMPM, -5A < ILOAD < +5A 1.23 60 1.250 100 2000 0.01 100 1.27 140 V S V/V nA V V VOUT Rising edge Measured at LX output, VFBM = 1V VOSC = 0.625V 2.7 2.2 80 2.35 85 0.3 5.5 2.6 88 V V % A VIN = 0 or VIN = VOUT = 5.5V VOUT 0.2 1.6 0.01 1 A MIN TYP MAX 0.2 0.4 UNITS
MAX1800
LOGIC INPUTS (ONM, ON1, ON2, ON3, ONA) Input Low Level VIL V
Input High Level
VIH
V
Idle Mode is a trademark of Maxim Integrated Products.
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3
Digital Camera Step-Up Power Supply MAX1800
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1, VOUT = VPOUT = 3.3V, PGND = GND, VONM = 3.3V, VON1 = VON2 = VON3 = VONA = 0, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 1)
PARAMETER ANALOG GAIN BLOCK AI Feedback Regulation Voltage AI Input Common-Mode Range AI Input Current AI to AO Voltage Gain AO Output Sink Current AO Output Source Current AO Output Low Voltage AO Output High Voltage AI to AO -3dB Bandwidth AUXILIARY DC/DC CONTROLLERS 1, 2, 3 INTERNAL CLOCK OSC Clock Low Trip Level OSC Clock High Trip Level Maximum Duty-Cycle Adjustment Range Maximum Duty Cycle Default Maximum Duty Cycle ERROR AMPLIFIER FB_ Regulation Voltage FB_ to COMP_ Transconductance FB_ to COMP_ Maximum Voltage Gain FB_ Input Leakage Current DRIVERS (DL1, DL2, DL3) DL_ Driver Resistance DL_ Drive Current SOFT-START Soft-Start Interval SHORT-CIRCUIT PROTECTION Fault Interval 1024 OSC cycles 1024 OSC cycles RON Output high or low Sourcing or sinking, VDL _ = VOUT/2 2 0.5 6 A VFB_ = 1.35V FB_ = COMP_ FB_ = COMP_, -5A < ILOAD < +5A 1.23 60 1.25 100 2000 100 1.27 140 V S V/V nA VDCON_ = 0.625V VDCON_ = 1.25V Falling edge VDCON = 0.625V VDCON = VOUT 0.2 0.575 1.00 40 50 84 0.25 0.625 1.05 0.3 0.675 1.10 90 V V % % % VAI = 1V, VAO = 2V VAI = 1.5V, VAO = 2V VAI = 1V, ISINK = 25A VAI = 1.5V or VONA = 0, ISOURCE = 25A VPOUT - 0.5 5 VAI = 1.35V 70 0.5 0.5 100 2.5 2.5 0.5 VAO = VOUT - 1.25V 1.23 -0.1 1.25 1.27 1.3 100 140 V V nA V/V mA mA V V MHz SYMBOL CONDITIONS MIN TYP MAX UNITS
Note 1: Specifications to -40C are guaranteed by design and not production tested. Note 2: Operating voltage. Since the regulator is bootstrapped to the output, once started it will operate down to +0.7V input. Note 3: The regulator is in startup mode until the voltage is reached. 4 _______________________________________________________________________________________
Digital Camera Step-Up Power Supply
Typical Operating Characteristics
(Circuit of Figure 1, VINPUT = 2.4V, TA = +25C, unless otherwise noted.)
MAIN OUTPUT EFFICIENCY vs. LOAD CURRENT
MAX1800 toc01
MAX1800
MAIN OUTPUT EFFICIENCY vs. LOAD CURRENT
90 80 EFFICIENCY (%) 70 60 50 40 30 20 VIN = 2.5V VIN = 2.0V VIN = 1.5V VIN = 3.0V
MAX1800 toc02
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 1 10 100 VOUT = 2.7V VIN = 2.0V VIN = 1.5V VIN = 2.5V
100
10 0 1 10
VOUT = 3.3V 100 1000
1000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
MAIN OUTPUT EFFICIENCY vs. LOAD CURRENT
90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 1 10 100 1000 LOAD CURRENT (mA) VOUT = 5V 0 0 VIN = 2.5V VIN = 2.0V VIN = 1.5V VIN = 3.0V
MAX1800 toc03
MINIMUM STARTUP CURRENT vs. INPUT VOLTAGE
MAX1800 toc04
100
600 500 STARTUP CURRENT (mA) 400 300 200 100
0.5
1.0
1.5
2.0
2.5
INPUT VOLTAGE (V)
REFERENCE VOLTAGE vs. TEMPERATURE
MAX1800 toc05
REFERENCE VOLTAGE vs. REFERENCE CURRENT
MAX1800 toc06
1.260
1.253 1.252 REFERENCE VOLTAGE (V) 1.251 1.250 1.249 1.248
REFERENCE VOLTAGE (V)
1.255
1.250
1.245
1.240 -40 -20 0 20 40 60 80 TEMPERATURE (C)
1.247 0 50 100 150 200 250 REFERENCE CURRENT (A)
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5
Digital Camera Step-Up Power Supply MAX1800
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VINPUT = 2.4V, TA = +25C, unless otherwise noted.)
AUXILIARY CONTROLLER MAXIMUM DUTY CYCLE vs. VDCON_
MAX1800 toc07
FB_ TO COMP_ SMALL-SIGNAL OPEN-LOOP FREQUENCY RESPONSE
MAX1800 toc08
100
60 SMALL-SIGNAL RESPONSE (dB) 50 40 30 20 10 0
80 MAXIMUM CYCLE (%)
60
40
20
0 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 VDCON_ (V)
1
10
100 FREQUENCY (kHz)
1000
10,000
AUXILIARY CONTROLLER DEFAULT MAXIMUM DUTY CYCLE vs. FREQUENCY
MAX1800 toc09
OSCILLATOR FREQUENCY vs. ROSC
COSC = 470pF OSCILLATOR FREQUENCY (kHz) 800 COSC = 220pF COSC = 100pF COSC = 47pF 400
MAX1800 toc10
100 DEFAULT MAXIMUM DUTY CYCLE (%) 80
1000
60
COSC = 470pF
600
40
20
200
0 0 200 400 600 800 1000 FREQUENCY (kHz)
0 1 10 ROSC (k) 100 1000
MAIN OUTPUT STARTUP RESPONSE
MAX1800 toc11
LDO STARTUP RESPONSE
MAX1800 toc12
VONM 2V/div 0V MAIN OUTPUT VOLTAGE 2V/div 0V
VONA 2V/div LDO OUTPUT VOLTAGE 2V/div
0V
0V
0A 1.00 ms/div MAIN LOAD = 24
INPUT CURRENT 1A/div
0A 1.00 ms/div MAIN LOAD = 24 LDO LOAD = 8
INPUT CURRENT 500mA/div
6
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Digital Camera Step-Up Power Supply
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VINPUT = 2.4V, TA = +25C, unless otherwise noted.)
AUXILIARY CONTROLLER STARTUP RESPONSE
3.3V 0V
MAX1800 toc13
MAX1800
STARTUP SEQUENCE
MAX1800 toc14
VON_ 5V/div VOUT 2V/div
0V
VONM 5V/div MAIN OUTPUT VOLTAGE 2V/div
0V IIN 0.5A/div 0A
0V 0V
LDO OUTPUT VOLTAGE 2V/div AUXILIARY OUTPUT VOLTAGE 2V/div 1.00ms/div MAIN LOAD = 24 LDO LOAD = 8 AUXILIARY LOAD = 10 AUXILIARY OUTPUT VOLTAGE = 5V
0V 1ms/div VOUT = 5V LOAD = 10
MAIN OUTPUT LOAD-TRANSIENT RESPONSE
MAX1800 toc15
LDO OUTPUT LOAD TRANSIENT RESPONSE
MAX1800 toc16
VPOUT AC COUPLED 100mV/div
LDO OUTPUT VOLTAGE 20mV/div AC COUPLED
IOUT 200mA/div 0A COUT = 100F 400 us/div VOUT = 5V LOAD = 10
LDO LOAD CURRENT 100mA/div 0A 100s/div
AUXILIARY CONTROLLER OUTPUT LOAD-TRANSIENT RESPONSE
MAX1800 toc17
MAIN OUTPUT RESPONSE DUE TO LDO TRANSIENT
MAX1800 toc18
VPOUT AC COUPLED 100mV/div
MAIN OUTPUT VOLTAGE 20mV/div AC COUPLED
IOUT 200mA/div LDO LOAD CURRENT 100mA/div 200 s/div MAIN LOAD = 24
0A 400 us/div VOUT = 5V
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7
Digital Camera Step-Up Power Supply MAX1800
Pin Description
PIN 1, 19 NAME PGND FUNCTION Power Ground. Sources of internal N-channel MOSFET power switches. Connect both PGND pins to GND as close to the IC as possible. External MOSFET Gate Drive Output for Auxiliary Controller 1. DL1 swings between POUT and GND with typical 500mA drive current. Connect DL1 to the gate of the external switching N-channel MOSFET for auxiliary controller 1. Enable Input for Auxiliary Controller 1. Connect ON1 to POUT to automatically start auxiliary controller 1. Feedback Input for Auxiliary Controller 1. Connect a feedback resistive voltage-divider from the output to FB1 to set the output voltage. Regulation voltage is VREF (1.25V). Compensation for Auxiliary Controller 1. Output of transconductance error amplifier. Connect a series resistor and capacitor to GND to compensate the control loop. See Compensation Design. Maximum Duty-Cycle Control Input for Auxiliary Controller 1. Connect to POUT to set the default maximum duty cycle. Connect a resistive voltage-divider from REF to DCON1 to set the maximum duty cycle between 40% and 90%. Pull DCON1 below 400mV to turn the controller off. Main Power Output. Source of P-channel MOSFET synchronous rectifier switch. Connect both POUT pins together as close to the IC as possible. External MOSFET Gate Drive Output for Auxiliary Controller 2. DL2 swings between POUT and GND with typical 500mA drive current. Connect DL2 to the gate of the external switching N-channel MOSFET for auxiliary controller 2. Enable Input for Auxiliary Controller 2. Connect ON2 to POUT to automatically start auxiliary controller 2. Feedback Input for Auxiliary Controller 2. Connect a feedback resistive voltage-divider from the output to FB2 to set the output voltage. Regulation voltage is VREF (1.25V). Compensation for Auxiliary Controller 2. Output of transconductance error amplifier. Connect a series resistor and capacitor to GND to compensate the control loop. See Compensation Design. Maximum Duty-Cycle Control Input for Auxiliary Controller 2. Connect to POUT to set the default maximum duty cycle. Connect a resistive voltage-divider from REF to DCON2 to set the maximum duty cycle between 40% and 90%. Pull DCON2 below 400mV to turn the controller off. Internal Bias Supply Input. Connect to POUT through a resistor, and bypass OUT to GND with a capacitor. See Compensation Design. 1.250V Reference Output. Bypass REF to GND with a 0.1F or greater ceramic capacitor. Analog Ground. Connect GND to PGND at a single point near the IC. Oscillator Control. Connect a timing capacitor from OSC to GND and a timing resistor from OSC to POUT to set the switching frequency between 100kHz and 1MHz. See Setting the Switching Frequency. Main DC/DC Converter Feedback Input. Connect a feedback resistive voltage-divider from POUT to FBM to set the output voltage. Regulation voltage is VREF (1.25V). Compensation for Main Controller. Output of transconductance error amplifier. Connect a series resistor and capacitor to GND to compensate the control loop. See Compensation Design. Main Power Switching Node. Drains of the internal P-channel and N-channel MOSFET switches. Connect the LX pins together as close to the IC as possible. Main Converter Ready Output. An open-drain output sinks current when VFBM < 1.125V, indicating that the main output is more than 10% out of regulation.
2 3 4 5
DL1 ON1 FB1 COMP1
6
DCON1 POUT
7, 22
8 9 10 11
DL2 ON2 FB2 COMP2
12
DCON2
13 14 15 16 17 18 20, 21 23
OUT REF GND OSC FBM COMPM LX RDYM
8
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Digital Camera Step-Up Power Supply MAX1800
Pin Description (continued)
PIN 24 25 26 27 NAME ONM AI AO ONA FUNCTION Main Converter Enable Input. High level turns the main converter on. Connect ONM to POUT to automatically start the main converter. When the main converter is off, all other outputs are disabled. Analog Gain Block Input. AI is the positive input to the gain block. The negative input is internally connected to the 1.25V reference. Analog Gain Block Output. AO is a push-pull output driven between GND and POUT. The voltage gain of the block is approximately 100. Analog Gain Block Enable Input. Connect ONA to POUT to enable the gain block. When ONA is low, the AO output is driven to POUT. Maximum Duty-Cycle Control Input for Auxiliary Controller 3. Connect to POUT to set the default maximum duty cycle. Connect a resistive voltage-divider from REF to DCON3 to set the maximum duty cycle between 40% and 90%. Pull DCON3 below 400mV to turn the controller off. Compensation for Auxiliary Controller 3. Output of transconductance error amplifier. Connect a series resistor and capacitor to GND to compensate the control loop. See Compensation Design. Feedback Input for Auxiliary Controller 3. Connect a feedback resistive voltage-divider from the output to FB3 to set the output voltage. Regulation voltage is VREF (1.25V). Enable Input for Auxiliary Controller 3. Connect ON2 to POUT to automatically start auxiliary controller 3. External MOSFET Gate Drive Output for Auxiliary Controller 3. DL3 swings between POUT and GND with typical 500mA drive current. Connect DL3 to the gate of the external switching N-channel MOSFET for auxiliary controller 3.
28
DCON3
29 30 31 32
COMP3 FB3 ON3 DL3
Detailed Description
The MAX1800 typical application circuit is shown in Figure 1. It features a main step-up DC-DC converter, three auxiliary step-up DC-DC controllers, an uncommitted gain block, a power-ready comparator, and control capability for multiple external MAX1801 slave DC-DC controllers. The uncommitted gain block can be used with an external P-Channel MOSFET to make a linear regulator. The linear regulator can be used with the main output for step-up/step-down functionality or to make a separate stand-alone output voltage. Together, these provide a complete high-efficiency power-supply solution for digital still cameras. Figure 2 shows the MAX1800 functional diagram.
Main DC-DC Converter
The MAX1800 main step-up DC-DC switching converter generates a 2.7V to 5.5V output voltage from a +0.7V to +5.5V battery input voltage. An internal switch and synchronous rectifier allow conversion efficiencies as high as 95% while reducing both circuit size and the number of external components. The converter operates in a low-noise, constant-frequency PWM mode to regulate the voltage across the load. Switching harmonics generated by fixed-frequency operation are consistent and easily filtered. The internal N-channel MOSFET switch turns on during the first part of each cycle, allowing current to ramp up in the inductor and store energy in a magnetic field. During the second part of each cycle, the MOSFET turns off and the voltage across the inductor reverses, forcing current through the internal P-channel synchronous rectifier to the output filter capacitor and load. As the energy stored in the inductor is depleted, the current ramps down. The synchronous rectifier turns off when the inductor current approaches zero or at the beginning of a cycle. The current-mode PWM controller uses the voltage at COMPM to program the inductor current and regulate
9
Master-Slave Configuration
The MAX1800 supports MAX1801 "slave" controllers that obtain input power, a voltage reference, and an oscillator signal directly from the MAX1800 "master" DC-DC converter. The master-slave configuration reduces system cost by eliminating redundant circuitry and controlling the harmonic content of noise with synchronized converter switching.
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Digital Camera Step-Up Power Supply MAX1800
INPUT 0.7V TO 5.5V POUT 4 DCON 8 DL 7 IN 1 COSC MAX1800 FB COMP GND 2 REF 3 0.1F 14 6 12 28 +7V 32 30 DL3 FB3 FB2 RCM RC1 RC2 RC3 18 COMPM 5 COMP1 11 COMP2 COMP3 29 DL2 8 REF DCON1 DCON2 DCON3 FB1 4 +18V LCD BIAS +12V POUT 20, 21 2 +15V
CCD BIAS
+5V
ROSC 16 OSC
LX DL1
-7.5V
OSC MAXIM MAX 1801
6 5
10
AO
26 LDO 1.8V
AI CCM CC1 CC2 ON OFF 27 3 POUT 9 31 CC3 24 ONM ONA ON1 ON2 ON3 FBM RDYM GND PGND 15 1, 19 OUT POUT
25
INPUT MAIN 3.3V
7, 22
COUT
13
17 23 MAIN POWER OK
Figure 1. Typical Application Circuit
10
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Digital Camera Step-Up Power Supply MAX1800
UNDERVOLTAGE LOCKOUT IC POWER 2.35V EN EN MAX1800 STARTUP OSCILLATOR MAIN PWM CONTROLLER Q D Q1 P LX ONM REF GND OSC FBM COMPM AUX1 EN OSC Q FB COMP DUTY AUX2 EN OSC Q FB COMP DUTY AUX3 EN OSC Q FB COMP DUTY 1.25V RDY ON REFERENCE REF EN EN OSCILLATOR OSC FB Q2 COMP N PGND PGND LX POUT POUT
OUT
FB1 COMP1 DCON1
DL1
FB2 COMP2 DCON2
DL2
FB3 COMP3 DCON3 AI
DL3
GAIN BLOCK ON1 ON2 ON3 ONA VREF EN N 0.9VREF
AO RDYM
Figure 2. Simplified Functional Diagram
the output voltage. The controller forces the inductor current to rise above the 300mA Idle Mode threshold to ensure pulse skipping and improved efficiency at light loads.
Auxiliary DC-DC Controllers
The MAX1800 auxiliary controllers operate in a low-noise, fixed-frequency, PWM mode, with output power limited by the external components. The controllers regulate their output voltages by modulating the pulse width of the drive signal for an external N-channel MOSFET switch. The auxiliary controllers are inactive until the main output has started.
Figure 3 shows a block diagram for a MAX1800 auxiliary PWM controller. A sawtooth oscillator signal at OSC governs the internal timing. At the beginning of each cycle, DL_ goes high to turn on the external MOSFET switch. The MOSFET switch turns off when the internally level-shifted sawtooth rises above COMP_ or when the maximum duty cycle is exceeded. The switch remains off until the beginning of the next cycle. An internal transconductance amplifier establishes an integrated error voltage at COMP_, thereby increasing the loop gain for improved regulation accuracy.
11
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Digital Camera Step-Up Power Supply MAX1800
FB
COMP R LEVEL SHIFT REFI REF DCON CLK OSC SOFTSTART* S Q DL_
*SOFT-START RAMPS REFI FROM 0V TO VREF IN 1024 CLK CYCLES.
FAULT PROTECTION
ENABLE
Figure 3. PWM Auxiliary Controller Block Diagram
Analog Gain Block
The MAX1800 analog gain block is a voltage amplifier with a gain of 100 and a push-pull output stage with 2.5mA drive capability. The analog gain block can be used with an external P-channel MOSFET pass transistor to build a low-dropout linear regulator or can function as a comparator.
Reference
The MAX1800 has an internal 1.250V, 1.6% bandgap reference. Connect a 0.1F bypass capacitor from REF to GND within 0.2in (5mm) of the REF pin. REF can source up to 200A of external load current, and it is enabled whenever ONM is high and VOUT is above the main undervoltage lockout threshold. The internal analog gain block, auxiliary controllers, and MAX1801 slave controllers each sink up to 30A REF current during startup. If multiple MAX1801 slave controllers are turned on simultaneously, ensure that the master voltage reference can provide sufficient current or buffer the reference with an appropriate unity-gain amplifier.
output voltage from zero with a time constant given by the ROSCCOSC product when the switch is open, and the comparator output becomes high when the capacitor voltage reaches VREF (1.25V). In turn, the one-shot activates the internal MOSFET switch to discharge the capacitor within a 100ns interval, and the cycle repeats. Note that the oscillation frequency changes as the main output voltage ramps upward following startup. The oscillation frequency is constant while the main output is in regulation.
Low-Voltage Startup Oscillator
The MAX1800 internal control and reference-voltage circuitry receive power from the main output and do not function when the main output voltage is less than the main undervoltage lockout threshold. The MAX1800 main controller uses a low-voltage startup oscillator, allowing it to start from an input voltage as low as 0.9V. At startup, the low-voltage oscillator switches the internal LX-connected N-channel MOSFET until the output voltage rises to the main undervoltage lockout threshold. Above this level, the normal boost converter control circuitry takes over. Once in regulation, the MAX1800 operates with inputs as low as 0.7V since internal power for the IC is bootstrapped from the output through OUT. At low input voltages, the MAX1800 may have difficulty starting into
Oscillator
The oscillator uses a comparator, a 100ns one-shot, and an internal N-channel MOSFET switch in conjunction with an external timing resistor and capacitor to generate the oscillator signal at OSC (Figure 4). The capacitor voltage exponentially approaches the main
12
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Digital Camera Step-Up Power Supply
VPOUT
configuration, a protection device such as a fuse must be used to limit short-circuit current.
MAX1800
ROSC OSC COSC VREF (1.25V) 100ns ONE-SHOT
Ready-Main (RDYM) Output
The MAX1800 power-ready RDYM comparator opendrain output sinks up to 1mA if the main output drops 10% below its regulation voltage. When FBM exceeds the RDYM trip level, the RDYM output becomes high impedance to indicate that the main output is within the limits of regulation. The RDYM comparator has 1% hysteresis to prevent oscillations near the trip threshold. Connect RDYM to POUT with a 1M pullup resistor.
MAX1800
Shutdown
The main DC-DC converter shuts down with a low input at ONM. Auxiliary DC-DC converters 1, 2, and 3, and the uncommitted gain block shut down with low inputs at ON1, ON2, ON3, and ONA, respectively. The auxiliary converters and the gain block cannot be activated until the MAIN output reaches the RDYM trip threshold. Typical shutdown supply current is 2nA. For automatic startup, connect ON_ to POUT. When ONA is low to disable the gain block, AO is driven to POUT.
Figure 4. Master Oscillator
heavy loads (see the Startup Current vs. Input Voltage graph in the Typical Operating Characteristics).
Maximum Duty Cycle
The MAX1800 auxiliary controllers use the sawtooth oscillator signal generated at OSC, the voltage at DCON_, and an internal comparator to limit their maximum duty cycles (see Setting the Maximum Duty Cycle). Limiting the duty cycle can prevent saturation in some magnetic components. A low maximum duty cycle can also force the converter to operate in discontinuous current mode, simplifying design stability at the cost of a slight reduction in efficiency.
Design Procedure
Setting the Switching Frequency
Choose a switching frequency to optimize external component size or circuit efficiency for the particular MAX1800 application. Typically, switching frequencies between 400kHz and 500kHz offer a good balance between component size and circuit efficiency--higher frequencies generally allow smaller components, and lower frequencies give better conversion efficiency. The switching frequency is set with an external timing resistor (ROSC) and capacitor (COSC). At the beginning of a cycle, the timing capacitor charges through the resistor until it reaches VREF. The charge time t1 is: V t1 = - ROSCCOSC ln 1 - REF VPOUT and it decays to zero over time t2 = 100ns. The oscillator frequency is fOSC = 1 / (t1 + t2). Choose fOSC in the range 100kHz < fOSC < 1MHz. Choose COSC between 22pF and 470pF. Determine ROSC from the relation: 100ns - ROSC = 1 fOSC
Soft-Start
The MAX1800 gain block and auxiliary controllers feature a soft-start function that limits inrush current and prevents excessive battery loading at startup by ramping the output voltage to the regulation voltage. This is achieved by increasing the internal reference inputs to the controller transconductance amplifiers from 0 to the 1.25V reference voltage over 1024 oscillator cycles when initial power is applied or when the controller is enabled.
Overload Protection
The MAX1800 auxiliary controllers have a fault protection that prevents damage to transformer-coupled or single-ended primary inductance converter (SEPIC) circuits due to an output overload. When the output voltage drops out of regulation for 1024 oscillator clock periods, the auxiliary controller is disabled to prevent excessive output current. Restart the controller by cycling the voltage at ON_ or DCON_ to GND and back to the on state. For a step-up application, short-circuit current is not limited, due to the DC current path through the inductor and output rectifier to the short-circuit. If short-circuit protection is required in a step-up
1.25 COSC ln 1- VPOUT
See the Typical Operating Characteristics for fOSC versus ROSC using different values of COSC.
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Digital Camera Step-Up Power Supply MAX1800
Setting the Output Voltages
Set the MAX1800 output voltages by connecting a resistive voltage-divider from the output voltage to the corresponding FB_ input. The FB_ input bias current is less than 100nA, so choose RL1 (the low-side FB_-toGND resistor) to be 100k. Choose RH1 (the high-side output-to-FB_ resistor) according to the relation: V RH1 = RL1 OUT - 1 1.25 tent peak-to-peak inductor current is 0.33 IOUT / (1 - D). The maximum inductor current is 1.17 IOUT / (1 - D). Inductance values smaller than LIDEAL can be used; however, the maximum inductor current will rise as L is reduced, and a larger output capacitance will be required to maintain output ripple. The inductor current will become discontinuous if IOUT decreases by more than a factor of six from the value used to determine LIDEAL. Discontinuous Inductor Current In the discontinuous mode of operation, the MAX1800 controller regulates the output voltage by adjusting the duty cycle to allow adequate power transfer to the load. To ensure regulation under worst-case load conditions (maximum IOUT), choose V D L = OUT MAX 2 IOUT fOSC The peak inductor current is VIN DMAX / (L fOSC). The inductor's saturation current rating should meet or exceed the calculated peak inductor current.
Setting the Maximum Duty Cycle
The master oscillator signal at OSC and the voltage at DCON_ are used to generate the internal clock signals for the MAX1800 auxiliary controllers (CLK in Figure 3). The internal clock's falling edge occurs when VOSC exceeds VDCON_ (set by a resistive divider). The internal clock's rising edge occurs when VOSC falls below 0.25V (Figure 5). The adjustable maximum duty-cycle range is 40% to 90% (see the Maximum Duty Cycle vs. VDCON_ graph in the Typical Operating Characteristics.) The maximum duty cycle defaults to 84% at 100kHz if VDCON_ is at or above the voltage at V REF (1.25V) (see the Default Maximum Duty Cycle vs. Frequency graph in the Typical Operating Characteristics). The controller shuts down if VDCON_ is less than 0.3V.
Input and Output Filter Capacitors
The input capacitor (CIN) in step-up designs reduces the current peaks drawn from the battery or input power source and reduces switching noise in the controller. The impedance of the input capacitor at the switching frequency should be less than that of the
Inductor Selection
Select the inductor for either continuous or discontinuous current. Continuous conduction generally offers the best efficiency. Use discontinuous current if the stepup ratio (VOUT / VIN ) is greater than 1 / ( 1 - DMAX). Continuous Inductor Current A reasonable inductor value (LIDEAL) can be derived from the following equation, which sets continuous peak-to-peak inductor current at one-third the DC inductor current: LIDEAL = 3 (VIN(MAX) - VSW ) D (1 - D) IOUT fOSC
VOSC (V) 1.25 VDCON_
DMAX =
tH tL + tH
0.25 0
where D, the duty cycle, is given by: D 1- VIN VOUT + VD
CLK
In these equations, VSW is the voltage drop across the N-channel MOSFET switch, and VD is the forward voltage drop across the rectifier. Given LIDEAL, the consis-
tL
tH
Figure 5. Auxiliary Controller Internal Clock Signal Generation
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Digital Camera Step-Up Power Supply
input source so that high-frequency switching currents do not pass through the input source. The output capacitor is required to keep the output voltage ripple small and to ensure stability of the regulation control loop. The output capacitor must have low impedance at the switching frequency. Tantalum and ceramic capacitors are good choices. Tantalum capacitors typically have high capacitance and medium-tolow equivalent series resistance (ESR) so that ESR dominates the impedance at the switching frequency. In turn, the output ripple is approximately: VRIPPLE IL(PEAK) ESR where IL(PEAK) is the peak inductor current. Ceramic capacitors typically have lower ESR than tantalum capacitors, but with relatively small capacitance that dominates the impedance at the switching frequency. In turn, the output ripple is approximately: VRIPPLE IL(PEAK) ZC where IL(PEAK) is the peak inductor current, and ZC 1 / (2 fOSC COUT ). See the Compensation Design section for a discussion of the influence of output capacitance and ESR on regulation control loop stability. The capacitor voltage rating must exceed the maximum applied capacitor voltage. For most tantalum capacitors, manufacturers suggest derating the capacitor by applying no more than 70% of the rated voltage to the capacitor. Ceramic capacitors are typically used up to the voltage rating of the capacitor. Consult the manufacturer's specifications for proper capacitor derating. P1 D IL2 RDS(ON) where D is the duty cycle, IL is the average inductor current, and RDS(ON) is the on-resistance of the MOSFET. The transition loss is approximately: V If t P2 OUT L OSC T 3 where VOUT is the output voltage, IL is the average inductor current, fOSC is the converter switching frequency, and tT is the transition time. The transition time is approximately Qg / IG , where Qg is the total gate charge, and IG is the gate drive current (typically 0.5A). The total power dissipation in the MOSFET is: PMOSFET = P1 + P2
MAX1800
Diode Selection
For low-output-voltage applications, use a Schottky diode to rectify the output voltage because of the diode's low forward voltage and fast recovery time. Schottky diodes exhibit significant leakage current at high reverse voltages and high temperatures. Thus, for high-voltage, high-temperature applications, use ultrafast junction rectifiers.
Compensation Design
Each DC/DC converter has an internal transconductance error amplifier whose output is used to compensate the control loop. Typically, a series resistor and capacitor are inserted from COMP_ to GND to form a pole-zero pair. The external inductor, the output capacitor, the compensation resistor and capacitor, and the POUT-to-OUT RC filter govern control-loop stability. The inductor and output capacitor are usually chosen in consideration of performance, size, and cost, but the compensation resistor and capacitor and the POUT-toOUT RC filter are chosen to optimize control-loop stability. The component values in the circuit of Figure 1 yield stable operation over a broad range of input/output voltages and converter switching frequencies. Follow the procedures below for optimal compensation. Main Controller The main converter uses current mode to regulate the main output voltage, thereby simplifying the controlloop compensation. When the converter operates with continuous inductor current, a right-half-plane zero appears in the loop-gain frequency response. To ensure stability, the control loop must cross over (drop below unity gain) at a frequency much less than that of the right-half-plane zero.
MOSFET Selection
The MAX1800 auxiliary controllers drive an external logic-level N-channel MOSFET as the circuit switch element. The key selection parameters are: * On-resistance (RDS(ON)) * Maximum drain-to-source voltage (VDS(MAX)) * Total gate charge (Qg) * Reverse transfer capacitance (CRSS) Since the external gate drive swings between POUT and GND, use a MOSFET whose "on" resistance is specified at or below the main output voltage. The gate charge, Qg, includes all capacitance associated with gate charging and helps to predict the transition time required to drive the MOSFET between on and off states. The power dissipated in the MOSFET is due to on-resistance and transition losses. The on-resistance loss is:
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Digital Camera Step-Up Power Supply MAX1800
To determine the compensation components: 1) Find the frequency of the right-half-plane zero: V (1 - DM )2 fRHPZ = POUT 2 ILOAD(MAX) L where VPOUT is the output voltage, ILOAD(MAX) is the maximum load current, L is the inductor value, and D M is the duty-cycle under maximum load, specifically: DM = VPOUT - VIN + ILIM (RPCH + ESRL ) VPOUT + ILIM (RPCH + RNCH ) CC = 50 x 10-9 A VLOOP 2 fCROSS
5) Determine the pole due to the output capacitor (fOUT), and set the compensation zero (fCOMPZ) at the same frequency. The pole occurs at: ILOAD(MAX) fOUT = 2 COUT VPOUT where COUT is the total output capacitance at POUT, and the zero occurs at: fCOMPZ = 1 2RCCC
[
]
where I LIM is the average inductor current under maximum load, ESRL is the equivalent series resistance of the inductor, RPCH and RNCH are the onstate drain-source resistance of the P-channel switch (200m typ) and N-channel switch (100m typ), respectively. 2) Specify the control-loop crossover frequency (the frequency at which the loop gain drops to unity) at one-fifth the frequency of the right-half-plane zero: fCROSS = fRHPZ / 5 3) Find the DC open-loop voltage gain: V (1 - DM )A VCOMP A VLOOP = REF A VCS ILOAD where VREF is the 1.25V reference voltage, AVCOMP is the DC voltage gain of the internal error amplifier (2000), A VCS is the transresistance gain of the internal current-sense amplifier (0.375), and DM is the maximum duty cycle determined in step 1 above. With these parameter values, the open-loop voltage gain is: A VLOOP = 6666 (1 - DM ) ILOAD
Thus, setting fOUT to fCOMZ: ILOAD(MAX) COUT VPOUT = 1 RC C C
The compensation resistor RC (positioned in series with the compensation capacitor) is: RC = COUT VPOUT CC ILOAD(MAX)
6) Find the frequency of the zero (fESRZ) due to the output capacitance equivalent series resistance (ESR), and set the POUT-to-OUT RC filter pole (fFILTER) at the same frequency. The zero occurs at: fESRZ = 1 2 COUT ESR 1 2 RF CF
and the pole occurs at: fFILTER =
where RF and CF are the filter resistor and capacitor, respectively. Thus: COUT ESR = RF CF To ensure that noise is reduced at OUT, choose CF 1F. Then determine RF: RF = COUTESR CF
4) Set the dominant pole so that the loop crossover occurs at the frequency specified in step 2 above: f GM fDOM = CROSS = A VLOOP (2A VCOMP CC ) where GM is the transconductance of the error amplifier (typically 100S), and CC is the compensation capacitor. Subject to this condition, the compensation capacitor is:
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Digital Camera Step-Up Power Supply
Auxiliary Controllers The auxiliary controllers use voltage mode to regulate their output voltages, so the control-loop compensation is slightly more complex than that for the main converter. Use one of the two following procedures: Discontinuous Inductor Current For discontinuous inductor current, the PWM converter has a single pole. The pole frequency and DC gain of the PWM controller are dependent on the operating duty cycle, which is: 2Lf 2 D = OSC RE
1
MAX1800
PC =
1 GEA = 4000 CC 4 x 107 CC ZC = 1 2 RC CC
The equivalent series resistance (ESR) of the output filter capacitor causes a zero in the loop response at the frequency (in Hz): ZO = 1 2 COUT ESR
where RE is the equivalent load resistance, or: RE = VIN2 RLOAD VOUT (VOUT - VIN )
The DC gain, and the poles and zeros are shown in the Bode plot of Figure 6. To achieve a stable circuit with the Bode plot of Figure 6, perform the following procedure: 1) Choose the compensation resistor RC that is equivalent to the inverse of the transconductance of the error amplifier, 1/ RC = GEA = 100S, or RC = 10k. This sets the high-frequency voltage gain of the error amplifier to 0dB. 2) Determine the maximum output pole frequency: PO(MAX) = where: RLOAD(MIN) = VOUT IOUT(MAX) 2(VOUT - VIN) 2(VOUT - VIN ) RLOAD(MIN) COUT
The frequency of the single pole due to the PWM converter is: PO = 2 (VOUT - VIN ) RLOAD COUT
(2 VOUT - VIN )
The DC gain of the PWM controller is: AVO = 2VOUT (VOUT - VIN ) 2( VOUT - VIN ) RLOAD COUT
Note that the pole frequency decreases and the DC gain increases proportionally as the load resistance (RLOAD) is increased. Since the crossover frequency is the product of the pole frequency and the DC gain, it remains independent of the load. The gain through the voltage-divider is: V A VDV = REF VOUT The DC gain of the error amplifier is AVEA = 2000V/V. Thus, the DC loop gain is: AVDC = AVDV AVEA AVO The compensation resistor-capacitor pair at COMP cause a pole and zero at frequencies (in Hz):
O 80 AVDC 60
180 PC PHASE
40 GAIN (dB) 20 GAIN ZC = PO
90 PHASE
0 ZO FREQUENCY
-20
Figure 6. MAX1800 Discontinuous-Current, Voltage-Mode, Step-Up Converter Bode Plot ______________________________________________________________________________________ 17
Digital Camera Step-Up Power Supply MAX1800
4) Place the compensation zero at the same frequency as the maximum output pole frequency (in Hz): 1 2 RCCC 2(VOUT - VIN) 2(VOUT - VIN ) RLOAD(MIN) COUT The response of the control loop includes a right-halfplane zero and a complex pole pair due to the inductor and output capacitor. For stable operation, the controller loop gain must drop below unity (0dB) at a much lower frequency than the right-half-plane zero frequency. The zero arising from the ESR of the output capacitor is typically used to compensate the control circuit by increasing the phase near the crossover frequency, increasing the phase margin. If a low-value, low-ESR output capacitor (such as a ceramic capacitor) is used, the ESR-related zero occurs at too high a frequency and does not increase the phase margin. In this case, use a lower value inductor so that it operates with discontinuous current (see the Discontinuous Inductor Current section). For continuous inductor current, the gain of the voltage divider is AVDV = VREF / VOUT, and the DC gain of the error amplifier is AVEA = 2000. The gain through the PWM controller in continuous current is: A VO = VOUT 2 VIN VREF
ZC =
=
Solving for CC: VOUT - VIN CC = COUT VOUT RC IOUT(MAX) 2(VOUT - VIN ) Use values of CC less than 10nF. If the above calculation determines that the capacitor should be greater than 10nF, use CC = 10nF, skip step 4 , and proceed to step 5. 4) Determine the crossover frequency (in Hz): fC = VREF D COUT
To maintain at least a 10dB gain margin, make sure that the crossover frequency is less than or equal to 1/3 of the output capacitor ESR zero frequency, or: 3fC ZO or: ESR D D 6 VERF
Thus, the total DC loop gain is: A VDC = 2000 VOUT VIN
The complex pole pair due to the inductor and output capacitor occurs at the frequency (in Hz): PO = VOUT 2 VIN LCOUT
If this is not the case, go to step 5 to reduce the error amplifier high-frequency gain to decrease the crossover frequency. 5) The high-frequency gain may be reduced, thus reducing the crossover frequency, as long as the zero due to the compensation network remains at or below the crossover frequency. In this case: ESR and: V GR 1 fC = REF EA C 1 D COUT 2 RC CC Choose COUT, RC, and CC to simultaneously satisfy both equations. Continuous Inductor Current For continuous inductor current, there are two conditions that change, requiring different compensation.
18
The pole and zero due to the compensation network at COMP occur at the frequencies (in Hz): PC = 1 GEA = 7 4000 CC 4 x 10 CC ZC = 1 2 RC CC
D GEA RC 6 VERF
The frequency (in Hz) of the zero due to the ESR of the output capacitor is: ZO = 1 2 COUT ESR
The right-half-plane zero frequency (in Hz) is: ZRHP = (1 - D)2 RLOAD 2L
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Digital Camera Step-Up Power Supply MAX1800
INPUT 1-CELL LITHIUM-ION MAIN L1 L2
PC AVDC PHASE ZC=PO GAIN (dB) PHASE MARGIN ZO GAIN
180
90 PHASE
IN
DCON DL
C2 Q1
D1
OUTPUT 3.3V
PART OF MAX1800 GAIN MARGIN ZrRHP 0 COMP RC CC FB
R1
O dB
R2
FREQUENCY
Figure 7. MAX1800 Continuous-Current, Voltage-Mode, Step-Up Converter Bode Plot
The Bode plot of the loop gain of this control circuit is shown in Figure 7. To configure the compensation network for a stable control loop, set the crossover frequency at that of the zero due to the output capacitor ESR. Use the following procedure: 1) Determine the frequency of the right-half-plane zero: ZRHP = (1 - D) RLOAD 2L
2
Figure 8. Auxiliary SEPIC Configuration
5) Determine the desired compensation pole. Since the response between the compensation pole and the complex pole pair is 1st order (-20dB per decade), the ratio of the frequencies is equal to the ratio of the amplitudes at those frequencies. Thus: PO A = DC PC A(PO ) Solving this equation for CC: CC = VOUT (COUT ) 2 ESR2 20M VIN (L)
1 2 3
2) Find the DC loop gain: AVDC = 2000 VOUT VIN 3) Determine the frequency of the complex pole pair due to the inductor and output capacitor: fO = VOUT 2 VIN LCOUT
6) Determine that the compensation resistor, RC for the compensation zero frequency, is equal to the complex pole-pair frequency: ZC = PO Solving for RC: RC = VIN LCOUT VOUT CC
4) Since response is 2nd order (-40dB per decade) between the complex pole pair and the ESR zero, determine the desired amplitude at the complex pole pair to force the crossover frequency equal to the ESR zero frequency. Thus: Z A PO = O PO
()
2
=
LVIN
2 2
COUT ESR2 VOUT
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19
Digital Camera Step-Up Power Supply MAX1800
Applications Information
Using the MAX1801 with the MAX1800 Step-Up Master
The MAX1801 is a slave DC-DC controller that can be used with the MAX1800 to generate additional output voltages. It does not generate its own reference or oscillator. Instead, it uses the reference and oscillator of the MAX1800 step-up master converter controller (Figure 1). The MAX1801 controller operation and design are similar to that for the MAX1800 auxiliary controllers. For more details, consult the MAX1801 data sheet.
INPUT 1-CELL LITHIUM-ION + OUTPUT
MAIN
- OUTPUT IN DCON DL PART OF MAX1800 FB COMP RC CC R2 Q1 R1
Using an Auxiliary Controller in a SEPIC Configuration
Where the battery voltage may be above or below the required output voltage, a step-up converter or stepdown converter will not be suitable; use a step-up /step-down converter. One type of step-up/step-down converter is the single-ended primary inductance converter (or SEPIC) shown in Figure 8. Inductors L1 and L2 can be separate inductors or can be wound on a single core and coupled like a transformer. Typically, using a coupled inductor will improve efficiency since some power is transferred through the coupling so that less power passes through coupling capacitor C2. Likewise, C2 should be a low-ESR type capacitor to improve efficiency. The ripple current rating must be greater than the larger of the input and output currents. The MOSFET (Q1) drain-source voltage rating and the rectifier (D1) reverse voltage rating must exceed the sum of the input and output voltages. Other types of step-up/step-down circuits are a flyback converter and a step-up converter followed by a linear regulator.
Figure 9. Auxiliary Flyback Configuration
Using a Charge Pump For Negative Output Voltages
Negative output voltages can be produced without a transformer, using a charge-pump circuit with an auxiliary controller as shown in Figure 10. When MOSFET Q1 turns off, the voltage at its drain rises to supply current to VOUT+. At the same time, C1 charges to the voltage at VOUT+ through D1. When the MOSFET turns on, C1 discharges through D3, thereby charging C3 to VOUT- minus the drop across D3 to create roughly the same voltage as V OUT+ at V OUT- but with inverted polarity. If different magnitudes are required for the positive and negative voltages, a linear regulator can be used at one of the outputs to achieve the desired voltages.
Using an Auxiliary Controller for a Multi-Output Flyback Circuit
Some applications require multiple voltages from a single converter that features a flyback transformer. Figure 9 shows a MAX1800 auxiliary controller in a two-output flyback configuration. The controller drives an external MOSFET that switches the transformer primary, and the two secondaries generate the output voltages. Only a single positive output voltage can be regulated using the feedback resistive voltage-divider, so the other voltages are set by the turns ratio of the transformer secondaries. The regulation of the other secondary voltages degrades due to transformer leakage inductance and winding resistance. Voltage regulation is best when the load current is limited to a small range. Consult the transformer manufacturer for the proper design for a given application.
20
Using the Gain Block as a Linear Regulator
The gain block at AO can be used with an external Pchannel MOSFET to make a low-dropout linear regulator. The gain block output has push-pull drive, which makes it suitable to drive a MOSFET gate. The circuit for this application is shown in Figure 11. The output voltage is set by the resistive voltage-divider of R1 and R2. Use 100k for R2. R1 is:
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Digital Camera Step-Up Power Supply MAX1800
D3 VOUTINPUT 1-CELL LITHIUM-ION MAIN L D2 IN DCON DL PART OF MAX1800 FB COMP RC CC R2 Q1 C2 R1 VOUT+ C1 C3 D1
INPUT VOLTAGE (FROM MAIN OUTPUT)
PART OF MAX 1800 Q1 AI LOAD RESISTOR (RL)
COUT
R1
AI VREF ONA ON OFF R2
Figure 11. Linear Regulator Figure 10. Auxiliary Charge-Pump Configuration
V R1 = R2 OUT - 1 VREF Choose the MOSFET for low dropout. The maximum acceptable on-resistance of the MOSFET is determined by the maximum load current to achieve the required dropout voltage (minimum input voltage minus output voltage): RDS - ON VDROPOUT ILOAD(MAX)
where RAO is the output resistance of the gain block at AO, and C(GATE-Q1) is the total gate capacitance of the MOSFET, Q1. The control loop DC gain is: V A VLOOP = REF A (V -GB)G(FS-Q1)RLOAD VOUT where AV-GB is the voltage gain from AI to AO (typically 100), and G(FS-Q1) is the forward transconductance gain of Q1. Choose the output capacitance so that the second pole occurs at or above the loop-gain crossover frequency: V COUT REF A (V -GB)G(FS-Q1)RAOC(GATE-Q1) VOUT Since VREF is 1.25V, A(V-GB) is typically 100, and RAO is typically 800, then: 12,500G(FS-Q1)C(GATE-Q1) COUT VOUT
Determine the minimum output capacitance as follows. The output capacitor and load resistance set the dominant pole (fP1): fP1 = 1 2RLOAD COUT
The second pole (fP2) occurs due to the AO output resistance and the gate capacitance of the external MOSFET: fP2 = 1 2RAO C(GATE - Q1)
Using the Linear Regulator to Make a Step-Up/Step-Down Circuit
Some applications have a battery voltage that can be either greater than or less than the desired output voltage. In this case, a step-up or step-down converter will not be able to generate the required output voltage under all conditions. To avoid this limitation, use a step21
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Digital Camera Step-Up Power Supply MAX1800
up/step-down DC/DC converter. One way of making such a circuit is to add a low-dropout linear regulator after a step-up converter. When the battery voltage is below the output voltage, the step-up converter generates the higher voltage, and the LDO regulator is in dropout. When the battery voltage is greater than the output voltage, the LDO drops the voltage to the required output voltage. ground plane at one point to minimize the effects of power-ground currents. Keep the voltage feedback network very close to the IC, preferably within 0.2in (5mm) of the FB_ pin. Nodes with high dV/dt (switching nodes) should be kept as small as possible and should stay away from highimpedance nodes such as FB_. Consult the MAX1800EVKIT evaluation kit data sheet for a full PC board example.
Designing a PC Board
A good PC board layout is important to achieve optimal performance from the MAX1800. Poor design can cause excessive conducted and/or radiated noise, both of which are undesirable. Conductors carrying discontinuous currents should be kept as short as possible. Conductors carrying high currents should be made as wide as possible. A separate low-noise ground plane containing the reference and signal grounds should only connect to the power-
Chip Information
TRANSISTOR COUNT: 5641
22
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Digital Camera Step-Up Power Supply MAX1800
Package Information
32L TQFP, 5x5x01.0.EPS
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23
Digital Camera Step-Up Power Supply MAX1800
Package Information (continued)
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.
24 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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