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19-4393; Rev 0; 12/08
KIT ATION EVALU E AILABL AV
High-Performance, Step-Up, DC-DC Converter
General Description
The MAX17112 is a high-performance, step-up, DC-DC converter that provides a regulated supply voltage for active-matrix thin-film transistor (TFT) liquid-crystal displays (LCDs). The MAX17112 incorporates currentmode, fixed-frequency (1MHz), pulse-width modulation (PWM) circuitry with a built-in, n-channel power MOSFET to achieve high efficiency and fast-transient response. The input overvoltage protection (OVP) function prevents damage to the MAX17112 from an input surge voltage (up to 24V). The high switching frequency (1MHz) allows the use of ultra-small inductors and low-ESR ceramic capacitors. The current-mode architecture provides fast-transient response to pulsed loads. A compensation pin (COMP) gives users flexibility in adjusting loop dynamics. The internal MOSFET can generate output voltages up to 20V from an input voltage between 2.6V and 5.5V. Soft-start slowly ramps the input current and is programmable with an external capacitor. The MAX17112 is available in a 10-pin TDFN package. o Input Overvoltage Protection o Adjustable Output from VIN to 20V o 2.6V to 5.5V Input Supply Range o Input Supply Undervoltage Lockout o 1MHz Fixed Switching Frequency o Programmable Soft-Start o Small 10-Pin, TDFN Package o Thermal-Overload Protection
Features
MAX17112
Ordering Information
PART MAX17112ETB+ TEMP RANGE -40C to +85C PIN-PACKAGE 10 TDFN-EP*
Applications
Notebook Computer Displays LCD Monitor Panels
+Denotes a lead(Pb)-free/RoHS-compliant package. *EP = Exposed pad.
Simplified Operating Circuit
VIN 2.6V TO 5.5V VOUT
Pin Configuration
TOP VIEW
6 LX 8 IN 7 LX FB 2
COMP FB
1 2 3 4 5
+
10 9
SS SHDN IN LX LX
MAX17112
9 3 10 SHDN VL SS GND GND 5 4
VL GND GND
MAX17112
8 7 *EP 6
GND EP
COMP 1
*EXPOSED PAD
TDFN 3mm x 3mm
________________________________________________________________ Maxim Integrated Products
1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
High-Performance, Step-Up, DC-DC Converter MAX17112
ABSOLUTE MAXIMUM RATINGS
LX to GND ..............................................................-0.3V to +24V IN to GND ...............................................................-0.3V to +24V SHDN, VL to GND..................................................-0.3V to +7.5V COMP, SS, FB to GND ..................................-0.3V to (VL + 0.3V) LX Switch Maximum Continuous RMS Current .....................3.2A Continuous Power Dissipation (TA = +70C) 10-Pin 3mm x 3mm Thin TDFN (derate 24.4mW/C above +70C) ............................1951mW Operating Temperature Range ...........................-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
(VL = 3V, TA = 0C to +85C, unless otherwise noted.)
PARAMETER IN Supply Range OVP Threshold OVP Switch Resistance Output Voltage Range VL Undervoltage-Lockout Threshold IN Quiescent Current IN Shutdown Supply Current Thermal Shutdown ERROR AMPLIFIER Feedback Voltage FB Input Bias Current FB Line Regulation Transconductance Voltage Gain Shutdown FB Input Voltage OSCILLATOR Frequency (fOSC) Maximum Duty Cycle n-CHANNEL MOSFET Current Limit On-Resistance Leakage Current Current-Sense Transresistance VFB = 1V, 75% duty cycle, VL = 5V VL = 5V (typ value at TA = +25C) (Note 1) VL = 3V (typ value at TA = +25C) (Note 1) VLX = 20V VL = 5V 0.09 3.9 4.6 110 135 12 0.15 5.3 170 210 25 0.25 A m A V/A 800 89 1000 92 1200 95 kHz % SHDN = GND 0.05 Level to produce VCOMP = 1.24V VFB = 1.24V Level to produce VCOMP = 1.24V, 2.6V < VIN < 5.5V 110 1.23 50 1.24 125 0.05 300 2400 0.10 0.15 1.25 225 0.15 450 V nA %/V S V/V V VL rising; typical hysteresis is 50mV; LX remains off below this level VFB = 1.3V, not switching VFB = 1.0V, switching SHDN = GND Temperature rising Hysteresis 2.30 2.45 0.3 1.5 160 160 20 VOUT < 18V 18V < V OUT < 20V VIN rising CONDITIONS MIN 2.6 4.0 6.2 8 6.6 12 TYP MAX 5.5 5.5 7 20 20 2.57 0.6 2.5 250 V V mA A C UNITS V V V
2
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High-Performance, Step-Up, DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(VVL = 3V, TA = 0C to +85C, unless otherwise noted.)
PARAMETER SOFT-START Reset Switch Resistance Charge Current CONTROL INPUTS SHDN Threshold SHDN Input Hysteresis SHDN Discharge Resistance SHDN Charge Current Charge Current Delay Time VL < UVLO 4.25 SHDN rising 1.1 1.16 60 20 5 80 5.75 A s 1.22 V mV VSS = 1.2V 1.5 3.5 25 5.5 A CONDITIONS MIN TYP MAX UNITS
MAX17112
ELECTRICAL CHARACTERISTICS
(VL = 3V, TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER IN Supply Range Output Voltage Range Output Switch Resistance VL Undervoltage-Lockout Threshold IN Quiescent Current IN Shutdown Supply Current ERROR AMPLIFIER Feedback Voltage FB Input Bias Current Transconductance Shutdown FB Input Voltage OSCILLATOR Frequency (fOSC) Maximum Duty Cycle 750 89 1250 96 kHz % SHDN = GND Level to produce VCOMP = 1.24V VFB = 1.24V 1.227 110 0.05 1.253 225 450 0.15 V nA S V VL rising; typical hysteresis is 50mV; LX remains off below this level VFB = 1.3V, not switching VFB = 1.0V, switching SHDN = GND 8 2.30 VOUT < 18V 18V < V OUT < 20V CONDITIONS MIN 2.6 4.0 MAX 5.5 5.5 20 20 2.57 0.6 2.5 250 V mA A UNITS V V V
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High-Performance, Step-Up, DC-DC Converter MAX17112
ELECTRICAL CHARACTERISTICS (continued)
(VVL = 3V, TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER n-CHANNEL MOSFET Current Limit On-Resistance Current-Sense Transresistance SOFT-START Reset Switch Resistance Charge Current CONTROL INPUTS SHDN Threshold SHDN Charge Current SHDN rising 1.19 4.25 1.29 5.75 V A VSS = 1.2V 1.5 25 5.5 A VFB = 1V, 75% duty cycle, VL = 5V VL = 5V VL = 3V VL = 5V 0.09 3.9 5.3 170 210 0.25 A m V/A CONDITIONS MIN MAX UNITS
Note 1: Limits are 100% production tested at TA = +25C. Maximum and minimum limits over temperature are guaranteed by design and characterization.
Typical Operating Characteristics
(Circuit of Figure 1, VIN = 5V, VMAIN = 15V, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT (VIN = 5V, VOUT = 15V)
MAX17112 toc01
EFFICIENCY vs. LOAD CURRENT (VIN = 3.3V, VOUT = 9V)
MAX17112 toc02
LOAD REGULATION (VOUT = 15V)
VIN = 5.0V LOAD REGULATION (%) 0.5 VIN = 3.3V
MAX17112 toc03
100
100
1.0
90 EFFICIENCY (%)
90 EFFICIENCY (%)
80
80
0
70
70
-0.5
60
60
50 1 10 100 1000 LOAD CURRENT (mA)
50 1 10 100 1000 LOAD CURRENT (mA)
-1.0 1 10 100 1000 LOAD CURRENT (mA)
4
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High-Performance, Step-Up, DC-DC Converter
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = 5V, VMAIN = 15V, TA = +25C, unless otherwise noted.)
SWITCHING FREQUENCY vs. INPUT VOLTAGE
MAX17112 toc04
MAX17112
SUPPLY CURRENT vs. SUPPLY VOLTAGE
3.5 SUPPLY CURRENT (mA) 3.0 2.5 SWITCHING 2.0 1.5 1.0 0.5 NONSWITCHING 0 0
MAX17112 toc05
SOFT-START (RLOAD = 30)
MAX17112 toc06
1100
4.0
SWITCHING FREQUENCY (kHz)
VOUT 5V/div
1000
INDUCTOR CURRENT 1A/div
900 2.5 3.0 3.5 4.0 4.5 5.0 5.5 INPUT VOLTAGE (V)
0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 2ms/div SUPPLY VOLTAGE (V)
LOAD-TRANSIENT RESPONSE (ILOAD = 50mA TO 550mA)
MAX17112 toc07
PULSED LOAD-TRANSIENT RESPONSE (ILOAD = 100mA TO 1.1A)
MAX17112 toc08
15V 0 VOUT 500mV/div AC-COUPLED
15V
VOUT 200mV/div AC-COUPLED IOUT 1A/div
50mA
IOUT 500mA/div
0.1A INDUCTOR CURRENT 1A/div 0 10s/div L = 2.7H RCOMP = 47k CCOMP1 = 560pF
0 100s/div L = 2.7H RCOMP = 47k CCOMP1 = 560pF
INDUCTOR CURRENT 2A/div
SWITCHING WAVEFORMS (ILOAD = 600mA)
MAX17112 toc09
VIN OVP PROTECTION
MAX17112 toc10
LX 10V/div 0 INDUCTOR CURRENT 1A/div 0
VIN 5V/div
0
VL 5V/div
0 1s/div 10ms/div
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High-Performance, Step-Up, DC-DC Converter MAX17112
Pin Description
PIN 1 2 3 4, 5 6, 7 8 NAME COMP FB VL GND LX IN FUNCTION Compensation Pin for Error Amplifier. Connect a series RC from COMP to ground. Typical values are 47k and 580pF. Feedback. The FB regulation voltage is 1.24V nominal. Connect an external resistor-divider center tap here and minimize the trace area. Set V OUT according to the Output Voltage Selection section. IC Supply. There is an internal switch between IN and VL and the switch disconnects when an overvoltage condition on IN is detected. Bypass VL to GND with a 1F capacitor. Ground Switch. LX is the drain of the internal MOSFET. Supply Voltage Input. Bypass IN with a minimum 1F ceramic capacitor directly to GND. Shutdown Control Input. Drive SHDN high to turn on the MAX17112 for normal operation. Connect a capacitor to the SHDN pin to create a delayed turn-on. The time delay is 0.25 x C (typ), C in microfarads. SHDN can be driven from a logic signal directly, in which case a resistor is required in series with SHDN. Soft-Start Control. Connect a soft-start capacitor (CSS). Leave open for no soft-start. The soft-start capacitor is charged at a rate of 4A/C SS. Exposed Pad. Connect to GND.
9
SHDN
10 --
SS EP
VIN 4.5V TO 5.5V
L1 2.7H
D1 C7 10F 25V
VOUT +15V/600mA C8 10F 25V
C1 4.7F 10V
C2 4.7F 10V 8 C3 1F 9 C10 1F C9 1F C4 3.3nF 3 10 SHDN VL SS COMP 1 R2 47k C5 560pF LX IN
6 7 5 4 R4 221k
U1
LX GND GND FB GND
MAX17112
2 EP R3 20k
C6 OPEN
Figure 1. Typical Operating Circuit
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High-Performance, Step-Up, DC-DC Converter MAX17112
VIN VMAIN IN VL
OVP
IC SUPPLY VOLTAGE LOGIC AND DRIVER
LX
CLOCK
GND SLOPE COMPENSATION
1.2MHz OSCILLATOR
PWM COMPARATOR
CURRENT SENSE
4A
ERROR AMPLIFIER FB SHUTDOWN SOFT-START 1.24V COMP
SHDN
SS
Figure 2. Functional Diagram
Detailed Description
The MAX17112 is a highly efficient, power-management IC that employs a current-mode, fixed-frequency, PWM architecture for fast-transient response and low-noise operation. The high switching frequency (1MHz) allows the use of ultra-small inductors and low-ESR ceramic capacitors. The current-mode architecture provides fasttransient response to pulsed loads. A compensation pin (COMP) gives users flexibility in adjusting loop dynamics. The internal MOSFET can generate output voltages up to 20V from a 2.6V to 5.5V input voltage. The softstart function slowly ramps the input current and is programmable with an external capacitor. The input overvoltage protection function prevents damage to the MAX17112 from input surge voltages up to 24V.
The error amplifier compares the signal at FB to 1.24V and varies the COMP output. The voltage at COMP determines the current trip point each time the internal MOSFET turns on. As the load changes, the error amplifier sources or sinks current to the COMP output to command the inductor peak current necessary to service the load. To maintain stability at high duty cycles, a slope compensation signal is summed with the current-sense signal. At light loads, this architecture allows the device to skip cycles to prevent overcharging the output capacitors.
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7
High-Performance, Step-Up, DC-DC Converter MAX17112
Output Current Capability
The output current capability of the MAX17112 is a function of current limit, input voltage, operating frequency, and inductor value. Because of the slope compensation used to stabilize the feedback loop, the inductor current limit depends on the duty cycle. The current limit is determined by the following equation: ILIM = (1.26 - 0.35 x D) x ILIM_EC where ILIM_EC is the current limit specified at 75% duty cycle (see the Electrical Characteristics table) and D is the duty cycle. The output current capability depends on the currentlimit value and is governed by the following equation: VIN 0.5 x D x VIN IOUT(MAX) = ILIM x x fOSC x L VOUT where ILIM is the current limit calculated above, is the regulator efficiency (85% nominal), D is the duty cycle, and fOSC is switching frequency. The duty cycle when operating at the current limit is: D= VOUT - VIN + VDIODE VOUT - ILIM x RON + VDIODE
VL Undervoltage Lockout (UVLO) The undervoltage lockout (UVLO) circuit compares the voltage at VL with the UVLO (2.45V typ) to ensure that the input voltage is high enough for reliable operation. The 50mV (typ) hysteresis prevents supply transients from causing a restart. Once the VL voltage exceeds the UVLO-rising threshold, the startup begins. When the input voltage falls below the UVLO-falling threshold, the main step-up regulator turns off. Startup Using SHDN The MAX17112 can be enabled by applying high voltage on the SHDN pin. Figure 2 shows the block diagram of the internal SHDN pin function. There are two ways to apply this high voltage. When SHDN is connected to an external capacitor, an internal 5A current source charges up this capacitor and when the voltage on SHDN passes 1.24V, the IC starts up. Another way to enable the IC through the SHDN pin is to directly apply a logic-high signal to SHDN instead of connecting a capacitor.
The delay time for startup by connecting an external capacitor at SHDN can be estimated using the following equation: tDelay = 1.24V x C SHDN 0.25 x C SHDN 5A
where VDIODE is the rectifier diode forward voltage and RON is the on-resistance of the internal MOSFET.
where CSHDN is in microfarads. When enabling the IC by applying a logic-high signal to SHDN, a series resistor should be inserted between the logic signal and SHDN for protection purposes. This resistor can help limit the current drawn from the logic signal supply into the SHDN pin when SHDN is discharged to GND through the internal switch at the moment of startup when VL < UVLO. A typical value for this resistor is 10k. Figure 3 shows the application circuit for this enabling method of applying a logic-high signal to SHDN through a 10k resistor.
Soft-Start The MAX17112 can be programmed for soft-start upon power-up with an external capacitor. When the shutdown pin is taken high, the soft-start capacitor (CSS) is immediately charged to 0.4V. Then the capacitor is charged at a constant current of 4A (typ). During this time, the SS voltage directly controls the peak inductor current period. Full current limit is readied at VSS = 1.5V. The maximum load current is available after the softstart is completed. When SHDN is low, SS is discharged to ground. Overvoltage Protection (OVP) To prevent damage due to an input surge voltage, the MAX17112 integrates an OVP circuit. There is an internal switch between IN and VL, which is on when the IN voltage is less than 6.6V (typ). The switch is off when the IN exceeds 6.6V (typ). Since VL supplies the IC, the switch protects the IC from damage when excessively high voltage is applied to IN.
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High-Performance, Step-Up, DC-DC Converter MAX17112
VIN 4.5V TO 5.5V L1 2.7H VOUT +15V/600mA C7 10F 25V 8 C3 1F LOGIC INPUT 10k SHDN PROTECTION RESISTOR C9 1F C4 3.3nF 9 3 10 SHDN VL SS COMP 1 R2 47k C5 560pF LX IN 6 7 5 4 R4 221k C8 10F 25V
D1
C1 4.7F 10V
C2 4.7F 10V
U1
LX GND GND FB GND
MAX17112
2 EP R3 20k
C6 OPEN
Figure 3. Application Circuit Using Logic Input at SHDN
Table 1. Component List
DESIGNATION C1, C2 DESCRIPTION 4.7F 10%, 10V X5R ceramic capacitors (0603) TDK C1608X5R1A475K 10F 10%, 25V X5R ceramic capacitors (1210) Murata GRM32DR61E106K 2.7H 20% power inductor TOKO FDV0630-2R7 (27m , 4.4A) Sumida CDRH5D18BHPNP-2R7M (65m , 3.9A)
Table 2. Component Suppliers
SUPPLIER Murata Sumida TDK PHONE 770-436-1300 408-321-9660 516-535-2600 WEBSITE www.murata.com www.sumida.com www.component.tdk.com
C7, C8
L1
The choice of external components is primarily dictated by output voltage, maximum load current, and maximum and minimum input voltages. Begin by selecting an inductor value. Once the inductance is known, choose the diode and capacitors.
Inductor Selection
Applications Information
Step-up regulators using the MAX17112 can be designed by performing simple calculations for a first iteration. All designs should be prototyped and tested prior to production. Table 1 provides a list of power components for the typical applications circuit. Table 2 lists component suppliers.
The minimum inductance value, peak current rating, and series resistance are factors to consider when selecting the inductor. These factors influence the converter's efficiency, maximum output load capability, transient response time, and output voltage ripple. Physical size and cost are also important factors to be considered.
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9
High-Performance, Step-Up, DC-DC Converter
The maximum output current, input voltage, output voltage, and switching frequency determine the inductor value. Very high inductance values minimize the current ripple, and therefore, reduce the peak current, which decreases core losses in the inductor and I2R losses in the entire power path. However, large inductor values also require more energy storage and more turns of wire, which increase physical size and can increase I2R losses in the inductor. Low inductance values decrease the physical size, but increase the current ripple and peak current. Finding the best inductor involves choosing the best compromise between circuit efficiency, inductor size, and cost. The equations used here include a constant called LIR, which is the ratio of the inductor peak-to-peak ripple current to the average DC inductor current at the full load current. The best trade-off between inductor size and circuit efficiency for step-up regulators generally has an LIR between 0.3 and 0.5. However, depending on the AC characteristics of the inductor core material and ratio of inductor resistance to other power-path resistances, the best LIR can shift up or down. If the inductor resistance is relatively high, more ripple is acceptable to reduce the number of turns required, and to increase the wire diameter. If the inductor resistance is relatively low, increasing inductance to lower the peak current can decrease losses through the power path. If extremely thin high-resistance inductors are used, as is common for LCD panel applications, the best LIR can increase to between 0.5 and 1.0. Once a physical inductor is chosen, higher and lower values of the inductor should be evaluated for efficiency improvements in typical operating regions. Calculate the approximate inductor value using the typical input voltage (VIN), the maximum output current (IMAIN(EFF)), the expected efficiency (TYP) taken from an appropriate curve in the Typical Operating Characteristics, and an estimate of LIR based on the above discussion:
2 V VMAIN - VIN TYP L = IN VOUT IMAIN(EFF) x fOSC LIR
MAX17112
Calculate the ripple current at that operating point and the peak current required for the inductor: IRIPPLE = VIN(MIN) x VMAIN - VIN(MIN) L x VMAIN x fOSC
(
)
I IPEAK = IIN(DC,MAX) + RIPPLE 2 The inductor's saturation current rating and the MAX17112's LX current limit (ILIM) should exceed IPEAK and the inductor's DC current rating should exceed IIN(DC,MAX). For good efficiency, choose an inductor with less than 0.1 series resistance. Considering the typical operating circuit, the maximum load current (IMAIN(MAX)) is 600mA with a 15V output and a typical input voltage of 5V. Choosing an LIR of 0.5 and estimating 85% efficiency at this operating point: 5V 15V - 5V 0.85 L= 2.7H 15V 0.6 A x 1.2MHz 0.5 5 Using the circuit's minimum input voltage (4.5V) and estimating 85% efficiency at this operating point: IIN(DC,MAX) = 0.6 A x 15V 2.35A 4.5V x 0.85
2
The ripple current and the peak current at that input voltage are: IRIPPLE = 4.5V x (15V - 4.5V ) 0.97A 2.7H x 15V x 1.2MHz 0.97A = 2.84 A 2
IPEAK = 2.35A +
Output Capacitor Selection
The total output voltage ripple has two components: the capacitive ripple caused by the charging and discharging of the output capacitance, and the ohmic ripple due to the capacitor's equivalent series resistance (ESR): VRIPPLE = VRIPPLE(C) + VRIPPLE(ESR) V I -V VRIPPLE(C) MAIN MAIN IN COUT VMAINfOSC
Choose an available inductor value from an appropriate inductor family. Calculate the maximum DC input current at the minimum input voltage, VIN(MIN), using conservation of energy and the expected efficiency at that operating point (MIN) taken from an appropriate curve in the Typical Operating Characteristics: IIN(DC,MAX) = IMAIN(EFF) x VOUT VIN(MIN) x MIN
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High-Performance, Step-Up, DC-DC Converter
and: VRIPPLE(ESR) IPEAKRESR(COUT) where I PEAK is the peak inductor current (see the Inductor Selection section). For ceramic capacitors, the output voltage ripple is typically dominated by VRIPPLE(C). The voltage rating and temperature characteristics of the output capacitor must also be considered. For low-ESR output capacitors, use the following equations to obtain stable performance and good transient response: 253 x VIN x VOUT x COUT RCOMP L x IOUT CCOMP VOUT x COUT 10 x IOUT x RCOMP
MAX17112
Input Capacitor Selection
The input capacitor (CIN) reduces the current peaks drawn from the input supply and reduces noise injection into the IC. Two 4.7F ceramic capacitors are used in the typical operating circuit in Figure 1 because of the high source impedance seen in typical lab setups. Actual applications usually have much lower source impedance since the step-up regulator often runs directly from the output of another regulated supply. Typically, CIN can be reduced below the values used in Figure 1. Ensure a low-noise supply at IN by using adequate CIN. Alternatively, greater voltage variation can be tolerated on CIN if IN is decoupled from CIN using an RC lowpass filter (see Figure 1).
To further optimize transient response, vary RCOMP in 20% steps and CCOMP in 50% steps while observing transient response waveforms.
Soft-Start Capacitor
The soft-start capacitor should be large enough so that it does not reach final value before the output has reached regulation. Calculate CSS to be: VOUT2 - VIN x VOUT CSS > 21 x 10-6 x COUT x VIN x IINRUSH - IOUT x VOUT where COUT is the total output capacitance including any bypass capacitor on the output bus, VOUT is the maximum output voltage, IINRUSH is the peak inrush current allowed, IOUT is the maximum output current during power-up, and VIN is the minimum input voltage. The load must wait for the soft-start cycle to finish before drawing a significant amount of load current. The soft-start duration after which the load can begin to draw maximum load current is: tMAX = 2.4 x 105 x CSS
Rectifier Diode Selection
The MAX17112 high switching frequency demands a high-speed rectifier. Schottky diodes are recommended for most applications because of their fast recovery time and low forward voltage. The diode should be rated to handle the output voltage and the peak switch current. Make sure that the diode's peak current rating is at least IPEAK calculated in the Inductor Selection section and that its breakdown voltage exceeds the output voltage.
Output Voltage Selection
The MAX17112 operates with an adjustable output from VIN to 20V. Connect a resistive voltage-divider from the output (VMAIN) to GND with the center tap connected to FB (see Figure 1). Select R3 in the 10k to 50k range. Calculate R4 with the following equation: V R4 = R3 x MAIN - 1 VFB where VFB, the step-up regulator's feedback set point, is 1.24V (typ). Place R3 and R4 as close as possible to the IC.
PCB Layout and Grounding
Careful PCB layout is important for proper operation. Use the following guidelines for good PCB layout: 1) Minimize the area of high-current loops by placing the inductor, output diode, and output capacitors near the input capacitors and near the LX and GND pins. The high-current input loop goes from the positive terminal of the input capacitor to the inductor, to the IC's LX pin, out of GND, and to the input capacitor's negative terminal. The high-current output loop is from the positive terminal of the input capacitor to the inductor, to the output diode (D1), to the positive terminal of the output capacitors, reconnecting between the output capacitor and input capacitor ground terminals. Connect these loop components with short, wide connections. Avoid using vias in the high-current paths. If vias are unavoidable, use many vias in parallel to reduce resistance and inductance.
11
Loop Compensation
Choose RCOMP to set the high-frequency integrator gain for fast-transient response. Choose CCOMP to set the integrator zero to maintain loop stability.
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High-Performance, Step-Up, DC-DC Converter
2) Create a power ground island (PGND) consisting of the input and output capacitor grounds and GND pins. Connect all of these together with short, wide traces or a small ground plane. Maximizing the width of the power ground traces improves efficiency and reduces output voltage ripple and noise spikes. Create an analog ground plane (AGND) consisting of the feedback-divider ground connection, the COMP and SS capacitor ground connections, and the device's exposed backside pad. Connect the AGND and PGND islands by connecting the GND pins directly to the exposed backside pad. Make no other connections between these separate ground planes. 3) Place the feedback-voltage-divider resistors as close as possible to the feedback pin. The divider's center trace should be kept short. Placing the resistors far away causes the FB trace to become an antenna that can pick up switching noise. Care should be taken to avoid running the feedback trace near LX or the switching nodes in the charge pumps.
MAX17112
4) Place IN and VL pin bypass capacitors as close as possible to the device. The ground connections of the IN and VL bypass capacitor should be connected directly to the AGND with a wide trace. 5) Minimize the length and maximize the width of the traces between the output capacitors and the load for best transient responses. 6) Minimize the size of the LX node while keeping it wide and short. Keep the LX node away from the feedback node and analog ground. Use DC traces as a shield if necessary. Refer to the MAX17112 evaluation kit for an example of proper board layout.
Chip Information
TRANSISTOR COUNT: 4624 PROCESS: BiCMOS
Package Information
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. PACKAGE TYPE 10 TDFN-EP PACKAGE CODE T1033+2 DOCUMENT NO. 21-0137
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.
12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2008 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.

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