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LM2622 600kHz/1.3MHz Step-up PWM DC/DC Converter
August 2000
LM2622 600kHz/1.3MHz Step-up PWM DC/DC Converter
General Description
The LM2622 is a step-up DC/DC converter with a 1.6A, 0.2 internal switch and pin selectable operating frequency. With the ability to convert 3.3V to multiple outputs of 8V, -8V, and 23V, the LM2622 is an ideal part for biasing TFT displays. The LM2622 can be operated at switching frequencies of 600kHz and 1.3MHz allowing for easy filtering and low noise. An external compensation pin gives the user flexibility in setting frequency compensation, which makes possible the use of small, low ESR ceramic capacitors at the output. The LM2622 is available in a low profile 8-lead MSOP package.
Features
n n n n n 1.6A, 0.2, internal switch Operating voltage as low as 2.0V 600kHz/1.3MHz pin selectable frequency operation Over temperature protection 8-Lead MSOP package
Applications
n TFT Bias Supplies
Typical Application Circuit
DS101273-1
600 kHz Operation
(c) 2000 National Semiconductor Corporation
DS101273
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LM2622
Connection Diagram
Top View
DS101273-4
8-Lead Plastic MSOP NS Package Number MUA08A
Ordering Information
Order Number LM2622MM-ADJ LM2622MMX-ADJ Package Type MSOP-8 MSOP-8 NSC Package Drawing MUA08A MUA08A Supplied As 1000 Units, Tape and Reel 3500 Units, Tape and Reel S18B S18B Package ID
Pin Description
Pin 1 2 3 4 5 6 7 8 Name VC FB SHDN GND SW VIN FSLCT NC Function Compensation network connection. Connected to the output of the voltage error amplifier. Output voltage feedback input. Shutdown control input, active low. Analog and power ground. Power switch input. Switch connected between SW pin and GND pin. Analog power input. Switching frequency select input. VIN = 1.3MHz. Ground = 600kHz. Connect to ground.
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LM2622
Block Diagram
DS101273-3
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LM2622
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN SW Voltage FB Voltage VC Voltage SHDN Voltage FSLCT Maximum Junction Temperature Power Dissipation(Note 2) Lead Temperature Vapor Phase (60 sec.) 12V 18V 7V 7V 7V 12V 150C Internally Limited 300C 215C
Infrared (15 sec.) ESD Susceptibility (Note 3) Human Body Model (Note 4) Machine Model
220C
2kV 200V
Operating Conditions
Operating Junction Temperature Range (Note 5) Storage Temperature Supply Voltage -40C to +125C -65C to +150C 2V to 12V
Electrical Characteristics
Specifications in standard type face are for TJ = 25C and those with boldface type apply over the full Operating Temperature Range ( TJ = -40C to +125C)Unless otherwise specified. VIN =2.0V and IL = 0A, unless otherwise specified. Symbol IQ VFB ICL(Note 7) VO/ILOAD %VFB/VIN IB VIN gm AV DMAX fS ISHDN IL RDSON ThSHDN UVP Parameter Quiescent Current Feedback Voltage Switch Current Limit Load Regulation Feedback Voltage Line Regulation FB Pin Bias Current (Note 9) Input Voltage Range Error Amp Transconductance Error Amp Voltage Gain Maximum Duty Cycle Switching Frequency Shutdown Pin Current Switch Leakage Current Switch RDSON SHDN Threshold On Threshold Off Threshold FSLCT = Ground FSLCT = VIN VSHDN = VIN VSHDN = 0V VSW = 18V VIN = 2.7V, ISW = 1A Output High Output Low 1.8 1.7 0.9 78 480 1 I = 5A 2 40 135 135 85 600 1.25 0.01 -0.5 0.01 0.2 0.6 0.6 1.92 1.82 0.3 2.0 1.9 720 1.5 0.1 -1 3 0.4 A V V V V VIN = 2.7V (Note 8) VIN = 3.3V 2.0V VIN 12.0V Conditions FB = 0V (Not Switching) VSHDN = 0V 1.2285 1.0 Min (Note 5) Typ (Note 6) 1.3 5 1.26 1.65 6.7 0.013 0.5 0.1 20 12 290 Max (Note 5) 2.0 10 1.2915 2.3 Units mA A V A mV/A %/V nA V mho V/V % kHz MHz A
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LM2622
Electrical Characteristics
(Continued)
Specifications in standard type face are for TJ = 25C and those with boldface type apply over the full Operating Temperature Range ( TJ = -40C to +125C)Unless otherwise specified. VIN =2.0V and IL = 0A, unless otherwise specified. Symbol JA Parameter Thermal Resistance Conditions Junction to Ambient(Note 10) Junction to Ambient(Note 11) Junction to Ambient(Note 12) Junction to Ambient(Note 13) Junction to Ambient(Note 14) Min (Note 5) Typ (Note 6) 235 225 220 200 195 Max (Note 5) Units C/W
Note 1: Absolute maximum ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions for which the device is intended to be functional, but device parameter specifications may not be guaranteed. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: The maximum allowable power dissipation is a function of the maximum junction temperature, TJ(MAX), the junction-to-ambient thermal resistance, JA, and the ambient temperature, TA. See the Electrical Characteristics table for the thermal resistance of various layouts. The maximum allowable power dissipation at any ambient temperature is calculated using: PD (MAX) = (TJ(MAX) - TA)/JA. Exceeding the maximum allowable power dissipation will cause excessive die temperature, and the regulator will go into thermal shutdown. Note 3: The human body model is a 100 pF capacitor discharged through a 1.5k resistor into each pin. The machine model is a 200pF capacitor discharged directly into each pin. Note 4: ESD susceptibility using the human body model is 500V for VC. Note 5: All limits guaranteed at room temperature (standard typeface) and at temperature extremes (bold typeface). All room temperature limits are 100% production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL). Note 6: Typical numbers are at 25C and represent the most likely norm. Note 7: Duty cycle affects current limit due to ramp generator. Note 8: Current limit at 0% duty cycle. See TYPICAL PERFORMANCE section for Switch Current Limit vs. VIN Note 9: Bias current flows into FB pin. Note 10: Junction to ambient thermal resistance (no external heat sink) for the MSO8 package with minimal trace widths (0.010 inches) from the pins to the circuit. See Scenario 'A' in the Power Dissipation section. Note 11: Junction to ambient thermal resistance for the MSO8 package with minimal trace widths (0.010 inches) from the pins to the circuit and approximately 0.0191 sq. in. of copper heat sinking. See Scenario 'B' in the Power Dissipation section. Note 12: Junction to ambient thermal resistance for the MSO8 package with minimal trace widths (0.010 inches) from the pins to the circuit and approximately 0.0465 sq. in. of copper heat sinking. See Scenario 'C' in the Power Dissipation section. Note 13: Junction to ambient thermal resistance for the MSO8 package with minimal trace widths (0.010 inches) from the pins to the circuit and approximately 0.2523 sq. in. of copper heat sinking. See Scenario 'D' in the Power Dissipation section. Note 14: Junction to ambient thermal resistance for the MSO8 package with minimal trace widths (0.010 inches) from the pins to the circuit and approximately 0.0098 sq. in. of copper heat sinking on the top layer and 0.0760 sq. in. of copper heat sinking on the bottom layer, with three 0.020 in. vias connecting the planes. See Scenario 'E' in the Power Dissipation section.
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LM2622
Typical Performance Characteristics
Efficiency vs. Load Current (VIN = 3.3V, fS = 600 kHz) Efficiency vs. Load Current (VIN = 3.3V, fS = 1.3 MHz)
DS101273-26
DS101273-25
Switch Current Limit vs. Temperature (VIN = 3.3V, VOUT = 8V)
Switch Current Limit vs. VIN
DS101273-22 DS101273-20
RDSON vs. VIN (ISW = 1A)
IQ vs. VIN (600 kHz, not switching)
DS101273-27
DS101273-28
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LM2622
Typical Performance Characteristics
IQ vs. VIN (600 kHz, switching)
(Continued)
IQ vs. VIN (1.3 MHz, not switching)
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IQ vs. VIN (1.3 MHz, switching)
IQ vs. VIN (In shutdown)
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DS101273-18
Frequency vs. VIN (600 kHz)
Frequency vs. VIN (1.3 MHz)
DS101273-23
DS101273-24
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LM2622
Typical Performance Characteristics
Load Transient Response (600 kHz operation)
(Continued) Load Transient Response (1.3 MHz operation)
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Test circuit is shown in Figure 4.
Test circuit is shown in Figure 5
Operation
DS101273-2
FIGURE 1. Simplified Boost Converter Diagram (a) First Cycle of Operation (b) Second Cycle Of Operation Continuous Conduction Mode The LM2622 is a current-mode, PWM boost regulator. A boost regulator steps the input voltage up to a higher output voltage. In continuous conduction mode (when the inductor current never reaches zero at steady state), the boost regulator operates in two cycles. In the first cycle of operation, shown in Figure 1 (a), the transistor is closed and the diode is reverse biased. Energy is collected in the inductor and the load current is supplied by COUT. The second cycle is shown in Figure 1 (b). During this cycle, the transistor is open and the diode is forward biased. The energy stored in the inductor is transferred to the load and output capacitor.
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The ratio of these two cycles determines the output voltage. The output voltage is defined as:
where D is the duty cycle of the switch.
LM2622
Operation
Compensation
(Continued)
To keep a current programmed control converter stable above duty cycles of 50%, the inductor must meet certain criteria. The inductor, along with input and output voltage, will determine the slope of the current through the inductor (see Figure 2 (a)). If the slope of the inductor current is too great, the circuit will be unstable above duty cycles of 50%. A 10H inductor is recommended for 600 kHz operation, while a 4.7H inductor may be used for 1.3 MHz operation. If the duty cycle is approaching the maximum of 85%, it may be necessary to increase the inductance by as much as 2X. The LM2622 provides a compensation pin (COMP) to customize the voltage loop feedback. It is recommended that a series combination of RC and CC be used for the compensation network, as shown in Figure 3. For any given application, there exists a unique combination of RC and CC that will optimize the performance of the LM2622 circuit in terms of its transient response. The series combination of RC and CC introduces pole-zero pair according to the following equations:
DS101273-5
FIGURE 2. (a) Inductor current. (b) Diode current. The LM2622 is a current mode PWM boost converter. The signal flow of this control scheme has two feedback loops, one that senses switch current and one that senses output voltage.
where RO is the output impedance of the error amplifier, 1Meg. For most applications, performance can be optimized by choosing values within the range 5k RC 20k and 680pF CC 4.7nF. Refer to the applications section for recommended values for specific circuits and conditions.
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LM2622
Application Information
DS101273-8
FIGURE 3. Triple Output TFT Bias (600 kHz operation) Triple Output TFT Bias The circuit in Figure 3 shows how the LM2622 can be configured to provide outputs of 8V, -8V, and 23V, convenient for biasing TFT displays. The 8V output is regulated, while the -8V and 23V outputs are unregulated. The 8V output is generated by a typical boost topology. The basic operation of the boost converter is described in the OPERATION section. The output voltage is set with RFB1 and RFB2 by: The -8V output is derived from a diode inverter. During the second cycle, when the transistor is open, D2 conducts and C1 charges to 8V minus a diode drop ()0.4V if using a Schottky). When the transistor opens in the first cycle, D3 conducts and C1's polarity is reversed with respect to the output at C2, producing -8V. The 23V output is realized with a series of capacitor charge pumps. It consists of four stages: the first stage includes C4, D4, and the LM2622 switch; the second stage uses C5, D5, and D1; the third stage includes C6, D6, and the LM2622 switch; the final stage is C7 and D7. In the first stage, C4 charges to 8V when the LM2622 switch is closed, which causes D5 to conduct when the switch is open. In the second stage, the voltage across C5 is VC4 + VD1 - VD5 = VC4 ) 8V when the switch is open. However, because C5 is referenced to the 8V output, the voltage at C5 is 16V when referenced to ground. In the third stage, the 16V at C5 appears across C6 when the switch is closed. When the switch opens, C6 is referenced to the 8V output minus a diode drop, which raises the voltage at C6 with respect to ground to about 24V. Hence, in the fourth stage, C7 is charged to 24V when the switch is open. From the first stage to the last,
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CFB is placed across RFB1 to act as a pseudo soft-start. The compensation network of RC and CC are chosen to optimally stabilize the converter. The inductor also affects the stability. When operating at 600 kHz, a 10uH inductor is recommended to insure the converter is stable at duty cycles greater than 50%. Refer to the COMPENSATION section for more information.
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LM2622
Application Information
(Continued)
there are three diode drops that make the output voltage closer to 24 - 3xVDIODE (about 22.8V if a 0.4V forward drop is assumed). TABLE 1. Components For Circuits in Figure 3 Component L COUT1 COUT2 CC CFB1 CFB2 CIN C1 C2 C4 C5 C6 C7 RFB1 RFB2 RC D1 D2 D3 D4 D5 D6 D7 600 kHz 10H 10F 10F 3.9nF 0.1F NOT USED 10F 4.7F 0.1F 1F 1F 1F 1F 40.2k 7.5k 5.1k MBRM140T3 BAT54S BAT54S BAT54S 1.3 MHz 4.7H 22F NOT USED 1.5nF 15nF 560pF 22F 4.7F 0.1F 1F 1F 1F 1F 91k 18k 10k MBRM140T3 BAT54S BAT54S BAT54S
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LM2622
Application Information
(Continued)
600 kHz Operation
DS101273-31
FIGURE 4. 600 kHz operation 1.3 MHz Operation
DS101273-30
FIGURE 5. 1.3 MHz operation
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LM2622
Application Information
Power Dissipation
(Continued)
The output power of the LM2622 is limited by its maximum power dissipation. The maximum power dissipation is determined by the formula PD = (Tjmax - TA)/JA where Tjmax is the maximum specidfied junction temperature (125C), TA is the ambient temperature, and JA is the thermal resistance of the package. JA is dependant on the layout of the board as shown below.
DS101273-13
DS101273-11
DS101273-12
DS101273-14
DS101273-15
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LM2622 600kHz/1.3MHz Step-up PWM DC/DC Converter
Physical Dimensions
inches (millimeters) unless otherwise noted
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