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19-1926; Rev 0; 2/01
KIT ATION EVALU ABLE AVAIL
Simple SOT23 Boost Controllers
____________________________Features
o Simple, Flexible Application Circuit o 2-Cell NiMH or Alkaline Operation (MAX1524) o Low Quiescent Current (25A typ) o Output Fault Protection and Soft-Start o High Efficiency Over 1000:1 IOUT Range o Pin-Selectable Maximum Duty Factor o Micropower Shutdown Mode o Small 6-Pin SOT23 Package o No Current-Sense Resistor
General Description
The MAX1522/MAX1523/MAX1524 are simple, compact boost controllers designed for a wide range of DC-DC conversion topologies, including step-up, SEPIC, and flyback applications. They are for applications where extremely low cost and small size are top priorities. These devices are designed specifically to provide a simple application circuit and minimize the size and number of external components, making them ideal for PDAs, digital cameras, and other low-cost consumer electronics applications. These devices use a unique fixed on-time, minimum offtime architecture, which provides excellent efficiency over a wide-range of input/output voltage combinations and load currents. The fixed on-time is pin selectable to either 0.5s (50% max duty cycle) or 3s (85% max duty cycle), permitting optimization of external component size and ease of design for a wide range of output voltages. The MAX1522/MAX1523 operate from a +2.5V to +5.5V input voltage range and are capable of generating a wide range of outputs. The MAX1524 is intended for bootstrapped operation, permitting startup with lower input voltage. All devices have internal soft-start and short-circuit protection to prevent excessive switching current during startup and under output fault conditions. The MAX1522/MAX1524 have a latched fault mode, which shuts down the controller when a shortcircuit event occurs, whereas the MAX1523 reenters soft-start mode during output fault conditions. The MAX1522/MAX1523/MAX1524 are available in a spacesaving 6-pin SOT23 package.
MAX1522/MAX1523/MAX1524
Ordering Information
PART MAX1522EUT-T MAX1523EUT-T MAX1524EUT-T TEMP. RANGE -40C to +85C -40C to +85C -40C to +85C PINPACKAGE 6 SOT23-6 6 SOT23-6 6 SOT23-6 TOP MARK AAOX AAOY AAOZ
________________________Applications
Low-Cost, High-Current, or High-Voltage Boost Conversion LCD Bias Supplies Industrial +24V and +28V Power Supplies Low-Cost, Multi-Output Flyback Converters SEPIC Converters Low-Cost BatteryPowered Applications
__________Typical Operating Circuit
INPUT
OUTPUT VCC 6V CC EXT 5 N
Pin Configurations
TOP VIEW
GND 1 6 VCC
50% 85%
FB 2
3 4
MAX1522 MAX1523 MAX1524
MAX1522 SET MAX1523 MAX1524 SHDN
FB GND
2 1
5
EXT
OFF ON
SET 3
4
SHDN
SOT23-6
________________________________________________________________ Maxim Integrated Products
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Simple SOT23 Boost Controllers MAX1522/MAX1523/MAX1524
ABSOLUTE MAXIMUM RATINGS
VCC, FB, SHDN, SET to GND ...................................-0.3V to +6V EXT to GND ................................................-0.3V to (VCC + 0.3V) Continuous Power Dissipation (TA = +70C) 6-Pin SOT23 (derate 8.7mW/C above +70C) ..........696mW 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
(VCC = SHDN = 3.3V, SET = GND , TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER VCC Operating Voltage Range VCC Minimum Startup Voltage Undervoltage Lockout Threshold VCC Supply Current VCC Shutdown Current Fixed tON Time Minimum tOFF Time Maximum Duty Factor FB Regulation Threshold (Note 2) FB Undervoltage Fault Threshold (Note 2) FB Input Bias Current EXT Resistance Soft-Start Ramp Time Logic Input High Logic Input Low Logic Input Leakage Current VCC = +2.5V to +5.5V, SET, SHDN VCC = +2.5V to +5.5V, SET, SHDN SET, SHDN = VCC or GND -1 MAX1522/MAX1523 fEXT > 100kHz, MAX1524 (Note 1), bootstrap required VCC rising VCC falling No load, nonbootstrapped SHDN = GND VFB =1.2V VFB > 0.675V VFB < 0.525V SET = GND SET = VCC VCC = +2.5V to +5.5V FB falling VFB = 1.3V IEXT = 20mA EXT high EXT low 2.2 1.6 0.4 +1 45 80 1.23 525 SET = GND SET = VCC 0.4 2.4 2.20 2.37 2.30 25 0.001 0.5 3.0 0.5 1.0 50 85 1.25 575 6 2 1.5 3.2 55 90 1.27 625 50 4 3 4.2 50 1 0.6 3.6 CONDITIONS MIN 2.5 TYP MAX 5.5 2.5 1.5 2.47 UNITS V V
V A A s s
% V mV nA ms V V A
Note 1: Actual startup voltage is dependent on the external MOSFET's VGS(TH). Note 2: Specification applies after soft-start mode is completed.
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Simple SOT23 Boost Controllers MAX1522/MAX1523/MAX1524
Typical Operating Characteristics
(TA = +25C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT (DESIGN EXAMPLE 1)
MAX1522/3/4 toc01
EFFICIENCY vs. LOAD CURRENT (DESIGN EXAMPLE 2)
MAX1522/3/4 toc02
EFFICIENCY vs. LOAD CURRENT (DESIGN EXAMPLE 3)
MAX1522/3/4 toc03
100
100 VIN = +4.2V 90 EFFICIENCY (%)
100 VIN = +2.4V VIN = +3V
90 EFFICIENCY (%)
90 EFFICIENCY (%)
80 70
80 VIN = +3.6V 70 VIN = +2.7V
80 VIN = +1.8V 70
60 VOUT = +5V VIN = 3.3V 50 0.1 1 10 100 1000 LOAD CURRENT (mA)
60 VOUT = +12V 50 0.1 1 10 100 1000 LOAD CURRENT (mA)
60 MAX1524 VOUT = +5V 50 0.1 1 10 100 1000 LOAD CURRENT (mA)
EFFICIENCY vs. LOAD CURRENT (DESIGN EXAMPLE 4)
MAX1522/3/4 toc04
EFFICIENCY vs. LOAD CURRENT (DESIGN EXAMPLE 5)
MAX1522/3/4 toc05
STARTUP INPUT VOLTAGE vs. OUTPUT CURRENT
MAX1522/3/4 toc06
100
100
1.75
90 EFFICIENCY (%)
80 70
80 70 VIN = +2.4V 60 VIN = +1.8V MAX1524 VOUT = +3.3V 0.1 1 10 100
STARTUP VOLTAGE (V)
VIN = +4.2V
90 EFFICIENCY (%)
VIN = +3.0V
1.50
1.25
VIN = +3.6V VIN = +2.7V
60 VOUT = +24V 50 0.1 1 10 100 LOAD CURRENT (mA)
1.00
VOUT = +3.3V BOOTSTRAPPED RESISTIVE LOADS 0 25 50 LOAD CURRENT (mA) 75 100
50 LOAD CURRENT (mA)
0.75
NO-LOAD INPUT CURRENT vs. INPUT VOLTAGE
BOOTSTRAPPED 1 INPUT CURRENT (mA)
MAX1522/3/4 toc07
SWITCHING WAVEFORM (CONTINUOUS CONDUCTION)
MAX1522/3/4 toc08
SWITCHING WAVEFORM (DISCONTINUOUS CONDUCTION)
MAX1522/3/4 toc09
10
A A
0.1 B 0.01 NONBOOTSTRAPPED C C B
0.001
0.0001 0 1 2 3 4 5 INPUT VOLTAGE (V) 400ns/div VIN = +3.3V, VOUT = +5V, IOUT = 350mA A : VOUT, 200mV/div, AC-COUPLED B : VLX, 5V/div C : IL, 0.5A/div 4s/div VIN = +3.3V, VOUT = +24V, IOUT = 10mA A : VOUT, 200mV/div, AC-COUPLED B : VLX, 10V/div C : IL, 0.5A/div
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Simple SOT23 Boost Controllers MAX1522/MAX1523/MAX1524
Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.)
SOFT-START RESPONSE
MAX1522/3/4 toc10
FAULT-DETECTION RESPONSE
MAX1522/3/4 toc11
A
A
B
B
C 400s/div 200 RESISTIVE LOAD A : VOUT, 5V/div B : VSHDN, 5V/div C : IL, 1A/div
C 400s/div A : VOUT, 10V/div B : VEXT, 5V/div C : IL, 5A/div
MAX1522
LINE-TRANSIENT RESPONSE
MAX1522/3/4 toc12
LOAD-TRANSIENT RESPONSE
MAX1522/3/4 toc13
A
A
B B
40s/div VIN = +3.5V TO +4.0V, VOUT = +12V, IOUT = 60mA A : VIN, 500mV/div, AC-COUPLED B : VOUT, 10mV/div, AC-COUPLED
100s/div VIN = +3.3V, VOUT = +12V, IOUT = 30mA TO 120mA A : IOUT, 100mA/div B : VOUT, 100mV/div, AC-COUPLED
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Simple SOT23 Boost Controllers
Pin Description
PIN 1 2 3 NAME GND FB SET Ground Feedback Input. Connect FB to external resistive voltage-divider. FB regulates to 1.25V. On-Time Control. Connect SET to VCC to set the fixed 3s on-time (85% duty cycle). Connect SET to GND to set the fixed 0.5s on-time (50% duty cycle). See On-Time SET Input section for more information. Shutdown Control Input. Drive SHDN high for normal operation. Drive SHDN low for low-power shutdown mode. Driving SHDN low clears the fault latch of the MAX1522 and MAX1524. External MOSFET Drive. EXT drives the gate of an external NMOS power FET and swings from VCC to GND. Supply Voltage to the IC. Bypass VCC to GND with a 0.1F capacitor. Connect VCC to a +2.5V to +5.5V supply, which may come from VIN (nonbootstrapped) or VOUT (bootstrapped) or from the output of another regulator. For bootstrapped operation, connect VCC to the output through a series 10 resistor. FUNCTION
MAX1522/MAX1523/MAX1524
4 5
SHDN EXT
6
VCC
Detailed Description
The MAX1522/MAX1523/MAX1524 are simple, compact boost controllers designed for a wide range of DC-DC conversion topologies including step-up, SEPIC, and flyback applications. These devices are designed specifically to provide a simple application circuit with a minimum of external components and are ideal for PDAs, digital cameras, and other low-cost consumer electronics applications. These devices use a unique fixed on-time, minimum off-time architecture, which provides excellent efficiency over a wide range of input/output voltage combinations and load currents. The fixed on-time is pin selectable to either 0.5s or 3s, permitting optimization of external component size and ease of design for a wide range of output voltages.
time, and another cycle begins when FB drops below its 1.25V regulation point.
Bootstrapped vs. Nonbootstrapped
The V CC supply voltage range of the MAX1522/ MAX1523/MAX1524 is +2.5V to +5.5V. The supply for V CC can come from the input voltage (nonbootstrapped), the output voltage (bootstrapped), or an independent regulator. The MAX1522/MAX1523 are usually utilized in a nonbootstrapped configuration, allowing for high or low output voltage operation. However, when both the input and output voltages fall within the +2.5V to +5.5V range, the MAX1522/MAX1523 may be operated in nonbootstrapped or bootstrapped mode. Bootstrapped mode provides higher gate-drive voltage to the MOSFET switch, reducing I2R losses in the switch, but will also increase the VCC supply current to charge and discharge the gate. Depending upon the MOSFET selected, there may be minor variation in efficiency vs. load vs. input voltage when comparing bootstrapped and nonbootstrapped configurations. The MAX1524 is always utilized in bootstrapped configuration for applications where the input voltage range extends down below 2.5V and the output voltage is between 2.5V and 5.5V. VCC is connected to the output (through a 10 series resistor) and receives startup voltage through the DC current path from the input through the inductor, diode, and 10 resistor. The MAX1524 features a low-voltage startup oscillator that
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Control Scheme
The MAX1522/MAX1523/MAX1524 feature a unique fixed on-time, minimum off-time architecture, which provides excellent efficiency over a wide range of input/output voltage combinations. The fixed on-time is pin selectable to either 0.5s or 3s for a maximum duty factor of either 45% or 80%, respectively. An inductor charging cycle is initiated by driving EXT high, turning on the external MOSFET. The MOSFET remains on for the fixed on-time, after which EXT turns off the MOSFET. EXT stays low for at least the minimum off-
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Simple SOT23 Boost Controllers MAX1522/MAX1523/MAX1524
guarantees startup with input voltages down to 1.5V at VCC. The startup oscillator has a fixed 25% duty cycle and will toggle the MOSFET gate and begin boosting the output voltage. Once the output voltage exceeds the UVLO threshold, the normal control circuitry is used and the startup oscillator is disabled. However, N-channel MOSFETs are rarely specified for guaranteed RDS(ON) with VGS below 2.5V; therefore, guaranteed startup down to 1.5V input will be limited by the MOSFET specifications. Nevertheless, the MAX1524 bootstrapped circuit on the MAX1524 EV kit typically starts up with input voltage below 1V and no load. The MAX1522/MAX1523 may also be utilized by connecting VCC to the output of an independent voltage regulator between 2.5V and 5.5V to allow operation with any combination of low or high input and output voltages. In this case, the independent regulator must supply enough current to satisfy the I GATE current as calculated in the Power MOSFET Selection section when considering the maximum switching frequency as calculated in the CCM or DCM design procedure. MAX1523, the fault condition is not latched, and softstart is repetitively reinitiated until a valid output voltage is realized. The MAX1524 has a latched fault detection, but when bootstrapped, the latch will be cleared when VCC falls below 2.37V.
Shutdown Mode
Drive SHDN to GND to place the MAX1522/MAX1523/ MAX1524 in shutdown mode. In shutdown, the internal reference and control circuitry turn off, EXT is driven to GND, the supply current is reduced to less than 1A, and the output drops to one diode drop below the input voltage. Connect SHDN to VCC for normal operation. When exiting shutdown mode, the 3.2ms soft-start is always initiated.
Undervoltage Lockout
The MAX1522/MAX1523 have undervoltage lockout (UVLO) circuitry, which prevents circuit operation and MOSFET switching when VCC is less than the UVLO threshold (2.37V typ). The UVLO comparator has 70mV of hysteresis to eliminate chatter due to V CC input impedance.
On-Time SET Input
The MAX1522/MAX1523/MAX1524 feature pin-selectable fixed on-time control, allowing their operation to be optimized for various input/output voltage combinations. Connect SET to VCC for the 3s fixed on-time. Connect SET to GND for the 0.5s fixed on-time. The 3s on-time setting (SET = VCC) permits higher than 80% guaranteed maximum duty factor, providing improved efficiency in applications with higher step-up ratios (such as 3.3V boosting to 12V). This setting is recommended for higher step-up ratio applications. The 0.5s on-time setting (SET = GND) permits higher frequency operation, minimizing the size of the external inductor and capacitors. The maximum duty factor is limited to 45% guaranteed, making this setting suitable for lower step-up ratios such as 3.3V to 5V converters.
Applications Information
Setting the Output Voltage
The output voltage is set by connecting FB to a resistive voltage-divider between the output and GND (Figures 1 and 2). Select feedback resistor R2 in the 30k to 100k range. R1 is then given by: V R1 = R2 OUT - 1 VFB where VFB = 1.25V.
Design Procedure
Continuous vs. Discontinuous Conduction
A switching regulator is operating in continuous conduction mode (CCM) when the inductor current is not allowed to decay to zero. This is accomplished by selecting an inductor value large enough that the inductor ripple current becomes less than one half of the input current. The advantage of this mode is that peak current is lower, reducing I2R losses and output ripple. In general, the best transient performance and most of the ripple reduction and efficiency increase of CCM are realized when the inductance is large enough to reduce the ripple current to 30% of the input current at maximum load. It is important to note that CCM circuits operate in discontinuous conduction mode (DCM)
Soft-Start
The MAX1522/MAX1523/MAX1524 have a unique softstart mode that reduces inductor current during startup, reducing battery, input capacitor, MOSFET, and inductor stresses. The soft-start period is fixed at 3.2ms and requires no external components.
Fault Detection
Once the soft-start period has expired, if the output voltage falls to, or is less than, 50% of its regulation value, a fault is detected. Under this condition, the MAX1522 disables the regulator until either SHDN is toggled low or power is removed and reapplied, after which it attempts to power up again in soft-start. For the
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Simple SOT23 Boost Controllers
under light loads. The selection of 30% ripple current causes this to happen at loads less than approximately 1/6th of maximum load. There are two common reasons not to run in CCM: 1) High output voltage. In this case, the output-toinput voltage ratio exceeds the level obtainable by the MAX1522/MAX1523/MAX1524s' maximum duty factor. Calculate the application's maximum duty cycle using the equation in the Calculate the Maximum Duty Cycle section. If this number exceeds 80%, you will have to design for DCM. 2) Small output current. If the maximum output current is very small, the inductor required for CCM may be disproportionally large and expensive. Since I2R losses are not a concern, it may make sense to use a smaller inductor and run in DCM. This typically occurs when the load current times the output-to-input voltage ratio drops below a few hundred milliamps, although this also depends on the external components. Calculate the Maximum Duty Cycle The maximum duty cycle of the application is given by: DutyCycle(MAX ) = VOUT + VD - VIN(MIN) VOUT + VD x 100% nect SET to GND for 0.5s on-time to get fast switching and a smaller inductor. For applications up to 80% duty cycle, it is necessary to connect SET to VCC for 3.0s on-time. For applications greater than 80% duty cycle, CCM operation is not guaranteed; see the Design Procedure for DCM section. Switching Frequency A benefit of CCM is that the switching frequency remains high as the load is reduced, whereas in DCM the switching frequency varies directly with load. This is important in applications where switching noise needs to stay above the audio band. The medium- and heavyload switching frequency in CCM circuits is given by: SWITCHING = V + VD - VIN x OUT t ON VOUT + VD 1
MAX1522/MAX1523/MAX1524
Note that f SWITCHING is not a function of load and varies primarily with input voltage. However, when the load is reduced, a CCM circuit drops into DCM, and the frequency becomes load dependent: SWITCHING(LIGHT-LOAD) 1 x t ON
where VD is the forward voltage drop of the Schottky diode (about 0.5V).
VOUT + VD - VIN ILOAD x 0.18 x ILOAD(MAX) VOUT + VD Calculate the Peak Inductor Current For CCM, the peak inductor current is given by: V + VD IPEAK = 1.15 x OUT x ILOAD(MAX) VIN(MIN)
Design Procedure for CCM
On-Time Selection For CCM to occur, the MAX1522/MAX1523/MAX1524 must be able to exceed the application's maximum duty cycle. For applications up to 45% duty cycle, con-
INPUT 2.7V TO 4.2V C1 10F 6.3V C3 0.1F 6 VCC MAX1522 MAX1523 EXT 5 L1 33H CDR74B-330 OUTPUT 12V CFF 220pF C2 33F TPSD336M020R0200
D1 MBR0530T3 Q1 R1 FDC633N 130k 1%
3 4
SET SHDN
FB GND
2 1 R1 CFB 220pF 15.0k 1%
OFF
ON
Figure 1. MAX1522/MAX1523 Standard Operating Circuit _______________________________________________________________________________________ 7
Simple SOT23 Boost Controllers MAX1522/MAX1523/MAX1524
INPUT 3.3V 10% R3 10 C1 10F 6.3V L1 33H CR43-3R3 D1 CRS01 6V CC 3 MAX1524 EXT 5 Q1 FDC633N R1 100k 1% CFF 100pF OUTPUT 5V C2 33F 10TPA33M
C3 0.1F
SET
FB
2 R2 33.2k 1%
OFF
ON
4
SHDN
GND
1
Figure 2. MAX1524 Standard Operating Circuit
Inductor Selection For CCM, the ideal inductor value is given by: LIDEAL = VIN(TYP) x t ON(TYP) 0.3 x IPEAK
COUT(MIN) =
ILOAD(MAX) x t ON 0.005 x VOUT
If LIDEAL is not a standard value, choose the next-closest value, either higher or lower. Nominal values within 50% are acceptable. Values lower than ideal will have slightly higher peak inductor current; values greater than ideal will have slightly lower peak inductor current. Due to the MAX1522/MAX1523/MAX1524s' high switching frequencies, inductors with a ferrite core or equivalent are recommended. Powdered iron cores are not recommended due to their high losses at frequencies over 50kHz. The saturation rating of the selected inductor should meet or exceed the calculated value for I PEAK , although most coil types can be operated up to 20% over their saturation rating without difficulty. In addition to the saturation criteria, the inductor should have as low a series resistance as possible. The power loss in the inductor resistance is approximately given by: I x (VOUT + VD ) PLR LOAD x RL VIN Output Capacitor Selection In CCM, to provide stable operation and to control output sag to less than 0.5%, the output bulk capacitance should be greater than:
2
To properly control peak inductor current during the 3.2ms soft-start, the output bulk capacitance should be less than: ILOAD(MAX) x t SS COUT(MAX) = VOUT where tSS = 3.2ms. Because the MAX1522/MAX1523/MAX1524 are voltage-mode devices (and therefore do not require an expensive current-sense resistor), cycle-to-cycle stability is obtained from the output capacitor's equivalent series resistance (ESR). Choose an output capacitor with actual ESR greater than: ESRCOUT > L COUT x ILOAD(MAX) VIN(MIN)
Additionally, to control peak inductor current during softstart, the output capacitor's ESR should be greater than: V ESRCOUT > 60 x 10-3 x FB IPEAK Usually, this prevents the use of ceramic capacitors in CCM applications. Alternatives include tantalum, electrolytic, and organic types such as Sanyo's POSCAP. The output capacitor must also be rated to withstand the output voltage and the output ripple current, which is equivalent to IPEAK. Since output ripple in boost DCDC designs is dominated by capacitor ESR, a capaci-
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Simple SOT23 Boost Controllers
tance value two or three times larger than COUT(MIN) is typically needed. Output ripple due to ESR is: VRIPPLE(ESR) 0.3 x IPEAK x ESRCOUT at light and medium loads, and three times as great at peak load. Continue the CCM design procedure by going to the Optional Feed-Forward Capacitor Selection section. Due to the MAX1522/MAX1523/MAX1524s' high switching frequencies, inductors with a ferrite core or equivalent are recommended. Powdered iron cores are not recommended due to their high losses at frequencies over 50kHz. Switching Frequency In DCM, the switching frequency is proportional to the load current and is approximately given by: SWITCHING 0.7IOUT x
MAX1522/MAX1523/MAX1524
Design Procedure for DCM
On-Time Selection The MAX1522/MAX1523/MAX1524 may operate in DCM at any duty cycle as required by the application's input and output voltages. However, best performance is achieved when the maximum duty cycle of the application is similar to the MAX1522/MAX1523/MAX1524s' maximum duty factor as set using the SET input. Connect SET to GND for applications with maximum duty cycles less than 67%. Connect SET to VCC for applications with maximum duty cycles between 67% and 99%. Inductor Selection For DCM, the ideal inductor value is given by: LIDEAL = (VIN(MIN) )2 x t ON(MIN) 3 x (VOUT + VD ) x ILOAD(MAX)
(VOUT + VD - VIN ) x 2L
t ON2 x VIN2
Note that fSWITCHING is a function of load and input voltage. Output Capacitor Selection In DCM, the MAX1522/MAX1523/MAX1524 may use either a ceramic output capacitor (with very low ESR) or other capacitors, such as tantalum or organic, with higher ESR. For less than 2% output ripple, the minimum value for ceramic output capacitors should be greater than: COUT(MIN) = t ON2 x VIN2 1 1 x x 2L (VOUT + VD - VIN ) 0.02VOUT
If LIDEAL is not a standard value, choose the next lower nominal value. The above formula already includes a factor for 30% inductor tolerance. Values higher than ideal may not supply the maximum load when the input voltage is low, while values much lower than ideal will have poorer efficiency. Calculate the Peak Inductor Current For DCM, the peak inductor current is given by: IPEAK = VIN(MAX) x t ON(MAX) L
To control inductor current during soft-start, the maximum value for any type of output capacitors should be less than: COUT(MAX) = ILOAD(MAX) x t SS VOUT
where tSS = 3.2ms. The capacitor should be chosen to provide an output ripple between 25mV minimum and 2% of VOUT maximum. The output ripple due to capacitance ripple and ESR ripple can be approximated by: 1 t ON2 x VIN2 1 VRIPPLE(COUT+ESR) x x 2L (VOUT + VD - VIN ) COUT V x t + IN ON x ESRCOUT L For output ripple close to 2% of VOUT, the optional feed-forward capacitor may not be required. For lower output ripple, a feed-forward capacitor is necessary for stability and to control inductor current during soft-start.
The saturation rating of the selected inductor should meet or exceed the calculated value for I PEAK , although most coil types can be operated up to 20% over their saturation rating without difficulty. In addition to the saturation criteria, the inductor should have as low a series resistance as possible. The power loss in the inductor resistance is approximately given by: PLR VOUT + VD 2 RL IPEAK x IOUT x VIN 3
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Simple SOT23 Boost Controllers MAX1522/MAX1523/MAX1524
Optional Feed-Forward Capacitor Selection
For proper control of peak inductor current during softstart and for stable switching, the ripple at FB should be greater than 25mV. Without a feed-forward capacitor connected between the output and FB, the output's ripple must be at least 2% of VOUT in order to meet this requirement. Alternatively, if a low-ESR output capacitor is chosen to obtain small output ripple, then a feed-forward capacitor should be used, and the output ripple may be as low as 25mV. The approximate value of the feed-forward capacitor is given by: 1 1 CFF 3 x 10-6 + R1 R2 Do not use a feed-forward capacitor that is much larger than this because line-transient performance will degrade. Do not use a feed-forward capacitor at all if the output ripple is large enough without it to provide stable switching because load regulation will degrade. falls. Since step-up DC-DC converters act as "constantpower" loads to their input supply, input current rises as input voltage falls. Consequently, in low-input-voltage designs, increasing CIN and/or lowering its ESR can add as many as five percentage points to conversion efficiency. A good starting point is to use the same capacitance value for C IN as for C OUT . The input capacitor must also meet the ripple current requirement imposed by the switching currents, which is about 30% of IPEAK in CCM designs and 100% of IPEAK in DCM designs. In addition to the bulk input capacitor, a ceramic 0.1F bypass capacitor at VCC is recommended. This capacitor should be located as close to VCC and GND as possible. In bootstrapped configuration, it is recommended to isolate the bypass capacitor from the output capacitor with a series 10 resistor between the output and VCC.
Power MOSFET Selection
The MAX1522/MAX1523/MAX1524 drive a wide variety of N-channel power MOSFETs (NFETs). Since the output gate drive is limited to VCC, a logic-level NFET is required. Best performance, especially when VCC is less than 4.5V, is achieved with low-threshold NFETs that specify on-resistance with a gate-source voltage (VGS) of 2.7V or less. When selecting an NFET, key parameters include: 1) Total gate charge (Qg) 2) Reverse transfer capacitance or charge (CRSS) 3) On-resistance (RDS(ON)) 4) Maximum drain-to-source voltage (VDS(MAX)) 5) Minimum threshold voltage (VTH(MIN)) At high switching rates, dynamic characteristics (parameters 1 and 2 above) that predict switching losses may have more impact on efficiency than R DS(ON), which predicts I2R losses. Qg includes all capacitances associated with charging the gate. In addition, this parameter helps predict the current needed to drive the gate when switching at high frequency. The continuous VCC current due to gate drive is: IGATE = Qg x SWITCHING Use the FET manufacturer's typical value for Qg (see manufacturer's graph of Qg vs. Vgs) in the above equation since a maximum value (if supplied) is usually too conservative to be of any use in estimating IGATE.
Optional Feedback Capacitor Selection
When using a feed-forward capacitor, it is possible to achieve too much ripple at FB. The symptoms of this include excessive line and load regulation and possibly high output ripple at light loads in the form of pulse groupings or "bursts." Fortunately, this is easy to correct by either choosing a lower-ESR output capacitor or by adding a feedback capacitor between FB and ground. This feedback capacitor (CFB), along with the feed-forward capacitor, form an AC-coupled ripple voltage-divider from the output to FB: CFF RippleFB = RippleOUTPUTx CFB + CFF It is relatively simple to determine a good value for CFB experimentally. Start with CFB = CFF to cut the FB ripple in half; then increase or decrease CFB as needed. The ideal ripple at FB is from 25mV to 40mV, which will provide stable switching, low output ripple at light and medium loads, and reasonable line and load regulation. Never use a feedback capacitor without also using a feed-forward capacitor.
Input Capacitor Selection
The input capacitor (CIN) in boost designs reduces the current peaks drawn from the input supply, increases efficiency, and reduces noise injection. The source impedance of the input supply largely determines the value of CIN. High source impedance requires high input capacitance, particularly as the input voltage
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Simple SOT23 Boost Controllers
Diode Selection
The MAX1522/MAX1523/MAX1524s' 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. Ensure that the diode's current rating is adequate to withstand the diode's RMS current: IDIODE(RMS) < IOUT x IPEAK Also, the diode reverse breakdown voltage must exceed VOUT. For high output voltages (50V or above), Schottky diodes may not be practical because of this voltage requirement. In these cases, use a high-speed silicon rectifier with adequate reverse voltage. Another consideration for high input voltages is reverse leakage of the diode. This should be considered using the manufacturer's specification due to its direct influence on system efficiency. paths and voltage gradients in the ground plane, both of which can result in instability or regulation errors. Connect the inductor, input filter capacitor, and output filter capacitor as close together as possible, and keep their traces short, direct, and wide. Connect their ground pins at a single common node in a star-ground configuration. The external voltage-feedback network should be very close to the FB pin, within 0.2in (5mm). Keep noisy traces (such as the trace from the junction of the inductor and MOSFET) away from the voltagefeedback network; also keep them separate, using grounded copper. The MAX1522/MAX1523/ MAX1524 evaluation kit manual shows an example PC board layout and routing scheme.
MAX1522/MAX1523/MAX1524
Generating Resistance with PC Board Traces
If the output capacitor's ESR is too low for proper regulation, it can be increased artificially directly on the PC board. For example, an additional 50m of ESR added to the output capacitor provides best regulation. The resistivity of a 10mil trace using 1oz copper is about 50m per inch. Therefore, a 10mil trace 1in long generates the required resistance.
Layout Considerations
High switching frequencies and large peak currents make PC board layout a very important part of design. Good design minimizes excessive EMI on the feedback
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Simple SOT23 Boost Controllers MAX1522/MAX1523/MAX1524
Table 1. Design Examples Using CCM
PARAMETER VIN VOUT IOUT(MAX) R1, R2 Duty Cycle (max) tON fSWITCHING IPEAK LIDEAL LACTUAL PLR COUT(MIN) to COUT(MAX) COUT ESRCOUT(MIN) COUT(ACTUAL) VRIPPLE(ESR) CFF CFB CIN MOSFET Qg IGATE IDIODE(RMS) Diode 5V 700mA 274k, 90.9k 45.5% 0.5s (SET = GND) 691kHz to 909kHz when IOUT > 120mA 1.48A 3.73H Sumida CR43-3R3 3.3H, 86m, 1.44A 29mW at IOUT = 350mA 14F to 448F 33F 23m for stability, 51m for soft-start Sanyo POSCAP 10TPA33M 33F, 10V, 60m, 100m max 27mVp-p at light loads, 81mVp-p at full load 100pF 100pF 10F, 6.3V ceramic Fairchild FDC633N 8nC at Vgs = 3V 12nC at Vgs = 5V 7.3mA nonbootstrapped, 10.9mA bootstrapped 0.96A Nihon EP10QY03, 1A EXAMPLE 1 3.3V 10% 12V 200mA 866k, 100k 78.4% 3s (SET = VCC) 221kHz to 261kHz when IOUT > 35mA 1.06A 33.8H Sumida CDR74B-330 33H, 180m, 0.97A 22mW at IOUT = 100mA 10F to 53F 33F 74m for stability, 70m for soft-start AVX TPSD336M020R0200 33F, 20V, 150m, 200m max 48mVp-p at light loads, 144mVp-p at full load 100pF 330pF 10F, 6.3V ceramic Fairchild FDC633N 9nC at Vgs = 3.6V 2.4mA nonbootstrapped 0.49A Nihon EP10QY03, 1A EXAMPLE 2 2.7V to 4.2V 5V 1.0A 274k, 90.9k 67.3% 3s (SET = VCC) 152kHz to 224kHz when IOUT > 167mA 3.51A 6.83H Sumida CDRH125-5R8 5.8H, 17m, 4.4A 22mW at IOUT = 500mA 120F to 640F 150F 21m for stability, 21m for soft-start Sanyo POSCAP 6TPB150M 150F, 6.3V, 40m, 55m max 42mVp-p at light loads, 126mVp-p at full load 100pF 220pF 10F, 6.3V ceramic Vishay Si3446DV 10nC at Vgs = 5V 2.2mA bootstrapped 1.84A Nihon EC21QS03L, 2A EXAMPLE 3 1.8V to 3.0V
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Simple SOT23 Boost Controllers MAX1522/MAX1523/MAX1524
Table 2. Design Examples Using DCM
PARAMETER VIN VOUT IOUT(MAX) R1, R2 Duty Cycle (max) tON LIDEAL EXAMPLE 4 2.7V to 4.2V 24V 30mA 909k, 49.9k 89.0% 3s (SET = VCC) 11.9H Sumida CDRH5D28-100 10H, 65m, 1.3A 1.51A 4.5mW at IOUT = 10mA 208kHz when IOUT = 20mA 0.8F to 2.7F Taiyo Yuden GMK325BJ225K 2.2F, X5R, 35V, 1210 10m 126mVp-p 100pF 220pF 10F, 6.3V Fairchild FDC633N 8nC at Vgs = 3V 1.7mA nonbootstrapped 0.17A Nihon EP10QY03, 1A EXAMPLE 5 1.8V to 3.0V 3.3V 100mA 150k, 93.1k 52.6% 0.5s (SET = GND) 1.14H Sumida CDRH4D18-1R0 1H, 45m, 1.72A 1.80A 5.7mW IOUT = 50mA 737kHz when IOUT = 100mA 3F to 97F Taiyo Yuden TMK316BT106ML 10F, X7R, 6.3V, 1206 10m 40mVp-p 220pF 100pF optional 10F, 6.3V Vishay Si2302DS 5nC at Vgs = 3.3V 3.7mA bootstrapped 0.42A Nihon EP10QY03, 1A
Table 3. Component Manufacturers
MANUFACTURER Coilcraft Fairchild International Rectifier Kemet NIC Components Panasonic Sumida Taiyo Yuden PHONE 847-639-6400 800-341-0392 310-322-3331 408-986-0424 408-954-8470 847-468-5624 847-956-0666 408-573-4150 WEB www.coilcraft.com www.fairchildsemi.com www.irf.com www.kemet.com www.niccomp.com www.panasonic.com www.sumida.com www.t-yuden.com
LACTUAL
IPEAK PLR fSWITCHING COUT(MIN) to COUT(MAX)
Chip Information
TRANSISTOR COUNT: 1302
COUT(ACTUAL)
ESRCOUT(ACTUAL) VRIPPLE(COUT+ESR) CFF CFB CIN MOSFET Qg IGATE IDIODE(RMS) Diode
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Simple SOT23 Boost Controllers MAX1522/MAX1523/MAX1524
Package Information
6LSOT.EPS
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.
14 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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