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LTC3413 3A, 2MHz Monolithic Synchronous Regulator for DDR/QDR Memory Termination
FEATURES
s s s s s s s s s s s s
DESCRIPTIO
High Efficiency: Up to 90% 3A Output Current Symmetrical Source and Sink Output Current Limit Low RDS(ON) Internal Switch: 85m No Schottky Diode Required 2.25V to 5.5V Input Voltage Range VOUT = VREF /2 1% Output Voltage Accuracy Programmable Switching Frequency: Up to 2MHz Power Good Output Voltage Monitor Overtemperature Protected Available in 16-Lead TSSOP Exposed Pad Package
The LTC(R)3413 is a high efficiency monolithic synchronous step-down DC/DC converter utilizing a constant frequency, current mode architecture. It operates from an input voltage range of 2.25V to 5.5V and provides a regulated output voltage equal to (0.5)VREF while sourcing or sinking up to 3A of output current. An internal voltage divider reduces component count and eliminates the need for external resistors by dividing the reference voltage in half. The internal synchronous power switch with 85m on-resistance increases efficiency and eliminates the need for an external Schottky diode. Switching frequencies up to 2MHz are set by an external resistor. Forced-continuous operation in the LTC3413 reduces noise and RF interference. Fault protection is provided by an overcurrent comparator that limits output current during both sourcing and sinking operations. Adjustable compensation allows the transient response to be optimized over a wide range of loads and output capacitors.
, LTC and LT are registered trademarks of Linear Technology Corporation. QDR RAMs and Quad Data Rate RAMs comprise a new family of products developed by Cypress Semiconductor, Hitachi, IDT, Micron Technology, Inc. and Samsung.
APPLICATIO S
s s s
Bus Termination: DDR and QDRTM Memory, SSTL, HSTL, ... Notebook Computers Distributed Power Systems
TYPICAL APPLICATIO
VIN 2.5V
100
22F SVIN VREF 4.7M 330pF LTC3413 RUN/SS 5.11k ITH 2200pF RT 309k VFB
3413 F01a
90 80
VIN = 2.5V f = 1MHz
PVIN PGOOD SW PGND SGND
COUT 100F x2
VOUT 1.25V 3A
EFFICIENCY (%)
L1 0.47H
70 60 50 40 30 20 10 0 0.01 0.1 1 LOAD CURRENT (A) 10
3413 F01b
L1: VISHAY DALE IHLP-2525CZ-01 0.47 COUT: TDK C4532X5R0J107M
Figure 1a. High Efficiency Bus Termination Supply
Figure 1b. Efficiency vs Load Current
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LTC3413
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW SVIN PGOOD ITH VFB RT VREF RUN/SS SGND 1 2 3 4 5 6 7 8 17 16 PVIN 15 SW 14 SW 13 PGND 12 PGND 11 SW 10 SW 9 PVIN
SVIN, PVIN Supply Voltages ........................ - 0.3V to 6V ITH, RUN/SS, VFB, PGOOD Voltages ........... - 0.3V to VIN VREF Voltage .............................................. - 0.3V to VIN SW Voltage .................................. - 0.3V to (VIN + 0.3V) Peak SW Sink and Source Current ........................ 7.2A Operating Ambient Temperature Range (Note 2) .............................................. - 40C to 85C Junction Temperature (Notes 5, 8) ...................... 125C Storage Temperature Range ................ - 65C to 150C Lead Temperature (Soldering, 10 sec)................. 300C
ORDER PART NUMBER LTC3413EFE
FE PART MARKING 3413EFE
FE PACKAGE 16-LEAD PLASTIC TSSOP EXPOSED PAD (PIN 17) MUST BE SOLDERED TO SGND
TJMAX = 125C, JA = 38C/ W, JC = 10C/ W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
SYMBOL VIN VFB IFB IRUN VFB VLOADREG VPGOOD RPGOOD IQ PARAMETER Input Voltage Range Feedback Voltage Accuracy Voltage Feedback Leakage Current RUN/SS Leakage Current Feedback Voltage Line Regulation Feedback Voltage Load Regulation Power Good Range Power Good Pull-Down Resistance Input DC Bias Current Active Current Shutdown Switching Frequency Switching Frequency Range RDS(ON) of P-Channel FET RDS(ON) of N-Channel FET Peak Current Limit Undervoltage Lockout Threshold SW Leakage Current RUN Threshold
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3.3V, unless otherwise noted.
CONDITIONS (Note 3)
q
MIN 2.25
TYP
MAX 5.5 1 0.4 1
UNITS V % A A %/V % % % A A MHz MHz m m A V A V
VIN = 2.7V to 5.5V (Note 3) Measured in Servo Loop, VITH = 0.36V Measured in Servo Loop, VITH = 0.84V
q q q
0.04 0.02 - 0.02 10 120
0.2 0.2 - 0.2 12 200 330 1 1.12 2.00 110 90 2.25 1 0.8
(Note 4) VFB = 1.5V, VITH = 1.4V, VREF = 2.5V VRUN = 0V (Note 7) ROSC = 309k (Note 6) ISW = 300mA ISW = 300mA 3.8 1.75 VRUN = 0V, VIN = 5.5V (Note 7) 0.5 0.88 0.30
250 0.02 1.00 85 65 5.4 2 0.1 0.65
fOSC RPFET RNFET ILIMIT VUVLO ILSW VRUN
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LTC3413E is guaranteed to meet performance specifications from 0C to 70C. Specifications over the -40C to 85C operating temperature range are assured by design, characterization and correlation with statistical process controls. Note 3: The LTC3413E is tested in a feedback loop that adjusts VFB to achieve a specified error amplifier output voltage (ITH). Note 4: Dynamic supply current is higher due to the internal gate charge being delivered at the switching frequency.
Note 5: TJ is calculated from the ambient temperature TA and power dissipation PD as follows: LTC3413E: TJ = TA + (PD * 38C/W) Note 6: 2MHz operation is guaranteed by design and not production tested. Note 7: Shutdown current and SW leakage current are only tested during wafer sort. Note 8: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability.
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LTC3413 TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Load Current
100 VOUT = 1.25V 90 TA = 25C 80
EFFICIENCY (%)
100
EFFICIENCY (%)
60 50 40 30 20 10 0 0.01
VIN = 3.3V
70 LOAD = 100mA 60 50 40 30
VOUT/VOUT (%)
70
VIN = 2.5V
0.1 1 LOAD CURRENT (A)
Switch On-Resistance vs Temperature
120 100 VIN = 3.3V PFET ON-RESISTANCE
80 60 40 20 0 -40 -20 NFET ON-RESISTANCE
80 60 NFET ON-RESISTANCE 40 20 0
LEAKAGE CURRENT (nA)
ON-RESISTANCE (m)
ON-RESISTANCE (m)
0
20 40 60 80 TEMPERATURE (C)
Frequency vs ROSC
4500 4000 3500 VIN = 3.3V TA = 25C
FREQUENCY (kHz)
FREQUENCY (kHz)
3000 2500 2000 1500 1000
1030 1020 1010 1000
FREQUENCY (kHz)
500 0 54 154 254 354 454 554 654 754 854 954 ROSC (k)
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Efficiency vs Input Voltage
VOUT = 1.25V 90 TA = 25C 80
Load Regulation
0 TA = 25C
LOAD = 1A
-0.05
LOAD = 3A
-0.10 -0.15 -0.20 -0.25 -0.30
10
20 2.5
3.0
4.5 4.0 INPUT VOLTAGE (V)
3.5
5.0
5.5
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0
0.5
1.0 1.5 2.0 LOAD CURRENT (A)
2.5
3.0
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Switch On-Resistance vs Input Voltage
120 100 PFET ON-RESISTANCE TA = 25C
2.5
Switch Leakage vs Input Voltage
TA = 25C
2.0
1.5 PFET 1.0 NFET 0.5
0
100 120
2.5
3
3.5 4 4.5 INPUT VOLTAGE (V)
5
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2.5
3
3.5 4 4.5 INPUT VOLTAGE (V)
5
5.5
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Frequency vs Input Voltage
1050 1040 TA = 25C
1010 1008 1006 1004 1002 1000 998 996 994 992
Frequency vs Temperature
VIN = 3.3V
990
2.5
3
3.5 4 4.5 INPUT VOLTAGE (V)
5
5.5
3213 G08
990 -40 -20
0
20 40 60 80 TEMPERATURE (C)
100 120
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LTC3413 TYPICAL PERFOR A CE CHARACTERISTICS
Quiescent Current vs Input Voltage
350 300 TA = 25C
QUIESCENT CURRENT (A)
250 200 150 100 50 0 2.0
2.5
Load Step Transient
OUTPUT VOLTAGE 100mV/DIV OUTPUT VOLTAGE 500mV/DIV INDUCTOR CURRENT 1A/DIV INDUCTOR CURRENT 1A/DIV
20s/DIV VIN = 2.5V VOUT = 1.25V LOAD STEP = 0A TO -3A
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Load Step Transient
OUTPUT VOLTAGE 100mV/DIV INDUCTOR CURRENT 1A/DIV
VIN = 2.5V 20s/DIV VOUT = 1.25V LOAD STEP = 0A TO 3A
4.5 3.0 3.5 4.0 INPUT VOLTAGE (V) 5.0 5.5
3413 G11
3413 G10
Start-Up
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VIN = 2.5V VOUT = 1.25V LOAD = 0.4
1ms/DIV
3413 G13
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LTC3413
PI FU CTIO S
SVIN (Pin 1): Signal Input Supply. Decouple this pin to SGND with a capacitor. SVIN must be greater or equal to PVIN, however, the difference between SVIN and PVIN must be less than 0.5V. PGOOD (Pin 2): Power Good Output. Open-drain logic output that is pulled to ground when the output voltage is not within 10% of regulation point. ITH (Pin 3): Error Amplifier Compensation Point. The current comparator threshold increases with this control voltage. Nominal voltage range for this pin is from 0.2V to 1.4V with 0.6V corresponding to the zero-sense voltage (zero current). VFB (Pin 4): Feedback Pin. Receives the feedback voltage from the output. RT (Pin 5): Oscillator Resistor Input. Connecting a resistor to ground from this pin sets the switching frequency. VREF (Pin 6): Reference Voltage Input. The positive input of the internal error amplifier senses one-half of the voltage at this pin through a resistor divider. RUN/SS (Pin 7): Run Control and Soft-Start Input. Forcing this pin below 0.5V shuts down the LTC3413. In shutdown all functions are disabled drawing < 1A of supply current. A capacitor to ground from this pin sets the ramp time to full output current. SGND (Pin 8): Signal Ground. All small-signal components and compensation components should connect to this ground, which in turn connects to PGND at one point. PVIN (Pins 9, 16): Power Input Supply. Decouple this pin to PGND with a capacitor. SW (Pins 10, 11, 14, 15): Switch Node Connection to Inductor. This pin connects to the drains of the internal main and synchronous power MOSFET switches. PGND (Pins 12, 13): Power Ground. Connect this pin closely to the (-) terminal of CIN and COUT. EXPOSED PAD (Pin 17): Should be connected to SGND.
FU CTIO AL DIAGRA
SVIN 1 VREF 6
SVIN SGND 8 VFB 4 SLOPE COMPENSATION RECOVERY
PGOOD 2
-
1.1VREF 2
+
0.9VREF 2
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+
+
-
-
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ITH 3
PVIN PVIN 9 16
PMOS CURRENT COMPARATOR
+ -
SLOPE COMPENSATION
ERROR AMPLIFIER
OSCILLATOR
10 SW 11 SW LOGIC
- +
14 SW 15 SW
RUN
NMOS CURRENT COMPARATOR
12 PGND 13 PGND
3413 BD
5 RT
7 RUN/SS
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LTC3413
OPERATIO
Main Control Loop The LTC3413 is a monolithic, constant frequency, current mode step-down DC/DC converter that is capable of sourcing and sinking current at the output. During normal operation, the internal top power switch (P-channel MOSFET) is turned on at the beginning of each clock cycle. Current in the inductor increases until the current comparator trips and turns off the top power MOSFET. The peak inductor current at which the current comparator shuts off the top power switch is controlled by the voltage on the ITH pin. The error amplifier adjusts the voltage on the ITH pin by comparing the feedback signal on the VFB pin with a reference voltage that is equal to one-half of the voltage on the VREF pin. When the load current increases, it causes a reduction in the feedback voltage relative to the reference. The error amplifier raises the ITH voltage until the average inductor current matches the new load current. When the top power MOSFET shuts off, the synchronous power switch (N-channel MOSFET) turns on until either the bottom current limit is reached or the beginning of the next clock cycle. The bottom current limit is set at - 7A. The operating frequency is set by an external resistor connected between the RT pin and ground. The switching frequency can range from 300kHz to 2MHz. Overvoltage and undervoltage comparators will pull the PGOOD output low if the output voltage comes out of regulation by 10%. In an overvoltage condition, the top power MOSFET is turned off and the bottom power MOSFET is switched on until either the overvoltage condition clears or the bottom MOSFET's current limit is reached. Dropout Operation When the input supply voltage decreases toward the output voltage, the duty cycle increases toward the maximum on-time. Further reduction of the supply voltage
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forces the main switch to remain on for more than one cycle until it reaches 100% duty cycle. The output voltage will then be determined by the input voltage minus the voltage drop across the internal P-channel MOSFET and the inductor. Low Supply Operation The LTC3413 is designed to operate down to an SVIN input supply voltage of 2.25V. One important consideration at low input supply voltages is that the RDS(ON) of the Pchannel and N-channel power switches increases. The user should calculate the power dissipation when the LTC3413 is used at 100% duty cycle with low input voltages to ensure that thermal limits are not exceeded. Slope Compensation and Inductor Peak Current Slope compensation provides stability in constant frequency architectures by preventing subharmonic oscillations at duty cycles greater than 50%. It is accomplished internally by adding a compensating ramp to the inductor current signal at duty cycles in excess of 40%. Normally, the maximum inductor peak current is reduced when slope compensation is added. In the LTC3413, however, slope compensation recovery is implemented to keep the maximum inductor peak current constant throughout the range of duty cycles. Short-Circuit Protection When the output is shorted to ground, the inductor current decays very slowly during a single switching cycle. To prevent current runaway from occurring, a secondary current limit is imposed on the inductor current. If the inductor valley current increases greater than 5A, the top power MOSFET will be held off and switching cycles will be skipped until the inductor current is reduced.
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LTC3413
APPLICATIO S I FOR ATIO
The basic LTC3413 application circuit is shown in Figure 1a. External component selection is determined by the maximum load current and begins with the selection of the inductor value and operating frequency followed by CIN and COUT. Operating Frequency Selection of the operating frequency is a tradeoff between efficiency and component size. High frequency operation allows the use of smaller inductor and capacitor values. Operation at lower frequencies improves efficiency by reducing internal gate charge losses but requires larger inductance values and/or capacitance to maintain low output ripple voltage. The operating frequency of the LTC3413 is determined by an external resistor that is connected between pin RT and ground. The value of the resistor sets the ramp current that is used to charge and discharge an internal timing capacitor within the oscillator and can be calculated by using the following equation.
ROSC =
3.23 * 1011 () - 10k f
Although frequencies as high as 2MHz are possible, the minimum on-time of the LTC3413 imposes a minimum limit on the operating duty cycle. The minimum on-time is typically 110ns. Therefore, the minimum duty cycle is equal to 100 * 110ns * f (Hz). Inductor Selection For a given input and output voltage, the inductor value and operating frequency determine the ripple current. The ripple current IL increases with higher VIN or VOUT and decreases with higher inductance.
V V IL = OUT 1- OUT VIN fL
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Having a lower ripple current reduces the core losses in the inductor, the ESR losses in the output capacitors and the output voltage ripple. Highest efficiency operation is achieved at low frequency with small ripple current. This, however, requires a large inductor. A reasonable starting point for selecting the ripple current is IL = 0.4(IMAX). The largest ripple current occurs at the highest VIN. To guarantee that the ripple current stays below a specified maximum, the inductor value should be chosen according to the following equation:
V V L = OUT 1 - OUT fIL(MAX) VIN(MAX)
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Inductor Core Selection Once the value for L is known, the type of inductor must be selected. Actual core loss is independent of core size for a fixed inductor value, but it is very dependent on the inductance selected. As the inductance increases, core losses decrease. Unfortunately, increased inductance requires more turns of wire and therefore copper losses will increase. Ferrite designs have very low core losses and are used often at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates "hard," which means that inductance collapses abruptly when the peak design current is exceeded. This results in an abrupt increase in inductor ripple current and consequent output voltage ripple. Do not allow the core to saturate! Different core materials and shapes will change the size/ current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy materials are small and don't radiate much energy, but generally cost more than powdered iron core inductors with similar characteristics. The choice of which style inductor to use mainly depends on the price versus size requirements and any radiated field/EMI requirements.
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LTC3413
APPLICATIO S I FOR ATIO
Table 1. Recommended Surface Mount Inductors
Manufacturer Murata Vishay/Dale Pulse Cooper Part Number LQH55DNR47M01 IHLP2525CZPJR47M01 P1166.681T SD20-R47 Value (H) 0.47 0.47 0.44 0.47
Table 1 shows some recommended surface mount inductors for LTC3413 applications.
DCR (m) 13.0 4.2 6.0 20.0
CIN and COUT Selection The input capacitance, CIN, is needed to filter the trapezoidal wave current at the source of the top MOSFET. To prevent large voltage transients from occurring, a low ESR input capacitor sized for the maximum RMS current should be used. The maximum RMS current is given by:
IRMS = IOUT (MAX)
VOUT VIN
VIN -1 VOUT
This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that ripple current ratings from capacitor manufacturers are often based on only 2000 hours of life which makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet size or height requirements in the design. The selection of COUT is determined by the effective series resistance (ESR) that is required to minimize voltage ripple and load step transients as well as the amount of bulk capacitance that is necessary to ensure that the control loop is stable. Loop stability can be checked by viewing the load transient response as described in a later section. The output ripple, VOUT, is determined by:
1 VOUT IL ESR + 8fC OUT
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The output ripple is highest at maximum input voltage since IL increases with input voltage. Multiple capacitors placed in parallel may be needed to meet the ESR and RMS current handling requirements. Dry tantalum, special polymer, aluminum electrolytic and ceramic capacitors are all available in surface mount packages. Special polymer capacitors offer very low ESR but have lower capacitance density than other types. Tantalum capacitors have the highest capacitance density but it is important to only use types that have been surge tested for use in switching power supplies. Aluminum electrolytic capacitors have significantly higher ESR, but can be used in cost-sensitive applications provided that consideration is given to ripple current ratings and long term reliability. Ceramic capacitors have excellent low ESR characteristics but can have a high voltage coefficient and audible piezoelectric effects. The high Q of ceramic capacitors with trace inductance can also lead to significant ringing. Using Ceramic Input and Output Capacitors Higher values, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple current, high voltage rating and low ESR make them ideal for switching regulator applications. However, care must be taken when these capacitors are used at the input and output. When a ceramic capacitor is used at the input and the power is supplied by a wall adapter through long wires, a load step at the output can induce ringing at the input, VIN. At best, this ringing can couple to the output and be mistaken as loop instability. At worst, a sudden inrush of current through the long wires can potentially cause a voltage spike at VIN large enough to damage the part. When choosing the input and output ceramic capacitors, choose the X5R or X7R dielectric formulations. These
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LTC3413
APPLICATIO S I FOR ATIO
dielectrics have the best temperature and voltage characteristics of all the ceramics for a given value and size. Output Voltage Programming In most applications, VOUT is connected directly to VFB. The output voltage will be equal to one-half of the voltage on the VREF pin for this case. VOUT = VREF 2
If a different output voltage relationship is desired, an external resistor divider from VOUT to VFB can be used. The output voltage will then be set according to the following equation:
VOUT R2 VFB LTC3413 SGND
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R1
Figure 2. Setting the Output Voltage
VOUT
V R2 = REF 1 + 2 R1
Soft-Start The RUN/SS pin provides a means to shut down the LTC3413 as well as a timer for soft-start. Pulling the RUN/SS pin below 0.5V places the LTC3413 in a low quiescent current shutdown state (IQ < 1A). The LTC3413 contains an internal soft-start clamp that gradually raises the clamp on ITH after the RUN/SS pin is
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pulled above 2V. The full current range becomes available on ITH after 1024 switching cycles. If a longer soft-start period is desired, the clamp on ITH can be set externally with a resistor and capacitor on the RUN/SS pin as shown in Figure 1a. The soft-start duration can be calculated by using the following formula:
VIN tSS = RSS * C SS ln (Seconds) VIN - 1.8V
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Efficiency Considerations The efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce the most improvement. Efficiency can be expressed as: Efficiency = 100% - (L1 + L2 + L3 + ...) where L1, L2, etc. are the individual losses as a percentage of input power. Although all dissipative elements in the circuit produce losses, two main sources usually account for most of the losses: VIN quiescent current and I2R losses. The VIN quiescent current loss dominates the efficiency loss at very low load currents whereas the I2R loss dominates the efficiency loss at medium to high load currents. In a typical efficiency plot, the efficiency curve at very low load currents can be misleading since the actual power lost is of no consequence. 1. The VIN quiescent current is due to two components: the DC bias current as given in the Electrical Characteristics and the internal main switch and synchronous switch gate charge currents. The gate charge current results from switching the gate capacitance of the internal power MOSFET switches. Each time the gate is switched from
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LTC3413
APPLICATIO S I FOR ATIO
high to low to high again, a packet of charge dQ moves from VIN to ground. The resulting dQ/dt is the current out of VIN that is typically larger than the DC bias current. In continuous mode, IGATECHG = f(QT + QB) where QT and QB are the gate charges of the internal top and bottom switches. Both the DC bias and gate charge losses are proportional to VIN and thus their effects will be more pronounced at higher supply voltages. 2. I2R losses are calculated from the resistances of the internal switches, RSW, and external inductor RL. In continuous mode the average output current flowing through inductor L is "chopped" between the main switch and the synchronous switch. Thus, the series resistance looking into the SW pin is a function of both top and bottom MOSFET RDS(ON) and the duty cycle (DC) as follows: RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 - DC) The RDS(ON) for both the top and bottom MOSFETs can be obtained from the Typical Performance Characteristics curves. Thus, to obtain I2R losses, simply add RSW to RL and multiply the result by the square of the average output current. Other losses including CIN and COUT ESR dissipative losses and inductor core losses generally account for less than 2% of the total loss. Thermal Considerations In most applications, the LTC3413 does not dissipate much heat due to its high efficiency. But, in applications where the LTC3413 is running at high ambient temperature with low supply voltage and high duty cycles, such as in dropout, the heat dissipated may
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exceed the maximum junction temperature of the part. If the junction temperature reaches approximately 150C, both power switches will be turned off and the SW node will become high impedance. To avoid the LTC3413 from exceeding the maximum junction temperature, the user will need to do some thermal analysis. The goal of the thermal analysis is to determine whether the power dissipated exceeds the maximum junction temperature of the part. The temperature rise is given by: TR = (PD)(JA) where PD is the power dissipated by the regulator and JA is the thermal resistance from the junction of the die to the ambient temperature. The junction temperature, TJ, is given by: TJ = TA + TR where TA is the ambient temperature. As an example, consider the LTC3413 in dropout at an input voltage of 3.3V, a load current of 3A and an ambient temperature of 70C. From the Typical Performance graph of switch resistance, the RDS(ON) of the P-channel switch at 70C is approximately 97m. Therefore, power dissipated by the part is: PD = (ILOAD2)(RDS(ON)) = (3A)2(97m) = 0.87W For the TSSOP package, the JA is 38C/W. Thus the junction temperature of the regulator is: TJ = 70C + (0.87W)(38C/W) = 103C which is below the maximum junction temperature of
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LTC3413
APPLICATIO S I FOR ATIO
125C.
Note that at higher supply voltages, the junction temperature is lower due to reduced switch resistance (RDS(ON)). Checking Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately shifts by an amount equal to ILOAD(ESR), where ESR is the effective series resistance of COUT. ILOAD also begins to charge or discharge COUT generating a feedback error signal used by the regulator to return VOUT to its steady-state value. During this recovery time, VOUT can be monitored for overshoot or ringing that would indicate a stability problem. The ITH pin external components and output capacitor shown in Figure 1a will provide adequate compensation for most applications. Output Voltage Tracking of VREF For applications in which the VREF pin is connected to the VIN pin, the output voltage will be equal to one-half of the voltage on the VIN pin. Because the output voltage will track the input voltage, any disturbance on VIN will appear on VOUT. For example, a load step transient could cause the input voltage to drop if there is insufficient bulk capacitance at the VIN pin. The corresponding drop in the output voltage during the load step transient is caused by the VOUT tracking of VIN and should not be confused with poor load regulation.
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Design Example As a design example, consider using the LTC3413 in an application with the following specifications: VIN = 2.5V, VOUT = 1.25V, IOUT(MAX) = 3A, f = 1MHz. First, calculate the timing resistor: ROSC = 3.23 * 1011 1* 106 - 10k = 313k Use a standard value of 309k. Next, calculate the inductor value for about 40% ripple current:
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1.25V 1.25V L= 1- = 0.47H 2.5V 1MHz * 1.2A
Using a 0.47H inductor results in a maximum ripple current of:
1.25V 1.25V IL = 1- = 1.33A 2.5V 1MHz * 0.47H
COUT will be selected based on the ESR that is required to satisfy the output voltage ripple requirement and the bulk capacitance needed for loop stability. For this design, two 100F ceramic capacitors will be used. CIN should be sized for a maximum current rating of:
1.25V 2.5V IRMS = 3A - 1 = 1.5ARMS 2.5V 1.25V
Decoupling the PVIN pins with two 100F capacitors is
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LTC3413
APPLICATIO S I FOR ATIO
RPG 100k PGOOD RITH 5.11k CITH CC 2200pF 100pF X7R
ROSC 309k RSS 4.7M CSS 330pF X7R
*VISHAY DALE IHLP-2525CZ-01 0.47H **TDK C4532X5R0J107M
Figure 3. One-Half VREF, 3A DDR Memory Termination Supply at 1MHz (Efficiency Curve is Shown in Figure 1b)
adequate for most applications. Connect the VREF pin directly to SVIN. Connecting the VFB pin directly to VOUT will set the output voltage equal to one-half of the voltage on the VREF pin. The complete circuit for this design example is illustrated in Figure 3. PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3413. Check the following in your layout. 1. A ground plane is recommended. If a ground plane layer is not used, the signal and power grounds should be segregated with all small-signal components returning to
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VIN 2.5V 1 2 3 4 5 6 7 8 SVIN PGOOD ITH VFB RT VREF RUN/SS SGND PVIN SW SWVFB PGND LTC3413 PGND SW SW PVIN 12 11 10 9 CIN2** 100F
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16 15 14 13
CIN1** 100F
L1* 0.47H
VOUT 1.25V 3A COUT** 100F x2
GND
the SGND pin at one point which is then connected to the PGND pin close to the LTC3413. 2. Connect the (+) terminal of the input capacitor(s), CIN, as close as possible to the PVIN pin. This capacitor provides the AC current into the internal power MOSFETs. 3. Keep the switching node, SW, away from all sensitive small-signal nodes. 4. Flood all unused areas on all layers with copper. Flooding with copper will reduce the temperature rise of power components. You can connect the copper areas to any DC net (PVIN, SVIN, VOUT, PGND, SGND or any other DC rail in your system). 5. Connect the VFB pin directly to the VOUT pin.
sn3413 3413fs
LTC3413
APPLICATIO S I FOR ATIO
(4a) Top Layer
Figure 4. LTC3413 Layout Diagram
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(4b) Bottom Layer (4c) PCB Photo
sn3413 3413fs
W
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13
LTC3413
TYPICAL APPLICATIO
PGOOD RITH 5.11k CITH CC 2200pF 100pF X7R
2.5V RSS 4.7M CSS 330pF X7R
EFFICIENCY (%)
14
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1.25V, 3A DDR Memory Termination Supply at 1MHz
VIN 3.3V CIN1** 100F 1 RPG 100k 2 3 4 5 ROSC 309k 6 7 8 SVIN PGOOD ITH VFB RT VREF RUN/SS SGND PVIN SW SWVFB PGND LTC3413 PGND SW SW PVIN 12 11 10 9 CIN2** 100F
3413 TA01
16 15 14 13
L1* 0.47H
VOUT 1.25V 3A COUT** 100F x2
GND
*VISHAY DALE IHLP-2525CZ-01 0.47H **TDK C4532X5R0J107M
Efficiency vs Load Current, VIN = 3.3V, VOUT = 1.25V, f = 1MHz
100 90 80 70 60 50 40 30 20 10 0 0.01 0.1 1 LOAD CURRENT (A) 10
3413 TA01b
sn3413 3413fs
LTC3413
PACKAGE DESCRIPTIO
2.74 (.108)
6.60 0.10 4.50 0.10 SEE NOTE 4
0.65 BSC RECOMMENDED SOLDER PAD LAYOUT 4.30 - 4.50* (.169 - .177) 0 - 8 12345678 1.10 (.0433) MAX
0.09 - 0.20 (.0036 - .0079)
0.45 - 0.75 (.018 - .030)
NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2. DIMENSIONS ARE IN (INCHES) 3. DRAWING NOT TO SCALE
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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FE Package 16-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1663, Exposed Pad Variation BA)
4.90 - 5.10* (.193 - .201) 2.74 (.108) 16 1514 13 12 1110 9 2.74 (.108) 0.45 0.05 1.05 0.10 2.74 6.40 (.108) BSC 0.65 (.0256) BSC 0.195 - 0.30 (.0077 - .0118) 0.05 - 0.15 (.002 - .006)
FE16 (BA) TSSOP 0203
4. RECOMMENDED MINIMUM PCB METAL SIZE FOR EXPOSED PAD ATTACHMENT *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.150mm (.006") PER SIDE
sn3413 3413fs
15
LTC3413
TYPICAL APPLICATIO
PGOOD RITH 10k CITH CC 2200pF 100pF X7R
1.5V RSS 4.7M CSS 330pF X7R
RELATED PARTS
PART NUMBER LTC3406 LTC3407 LTC3411 LTC3412 LTC3414 LTC3713 LTC3717 LTC3718 DESCRIPTION 600mA, (IOUT) 1.5MHz Synchronous Step-Down Regulator Dual 600mA, (IOUT) 1.5MHz, Synchronous Step-Down Regulator 1.25A, (IOUT) 4MHz, Monolithic Synchronous Step-Down Regulator 2.5A, (IOUT) 4MHz, Monolithic Synchronous Step-Down Regulator 4A, (IOUT) 4MHz, Monolithic Synchronous Step-Down Regulator Low Input Voltage, No RSENSE Synchronous Controller No RSENSE Controller for DDR Memory Termination Low Input Voltage, No RSENSE Controller for DDR Memory Termination
TM
No RSENSE is a trademark of Linear Technology Corporation.
16
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 q FAX: (408) 434-0507
q
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3.3V to 0.75V, 3A HSTL Application
VIN 3.3V CIN1** 100F 1 RPG 100k 2 3 4 5 ROSC 309k 6 7 8 SVIN PGOOD ITH VFB RT VREF RUN/SS SGND PVIN SW SWVFB PGND LTC3413 PGND SW SW PVIN 12 11 10 9 CIN2** 100F
3413 TA02
16 15 14 13
L1* 0.47H
VOUT 0.75V 3A
COUT1 22F
+
COUT2 470F
GND
*VISHAY DALE IHLP-2525CZ-01 0.47H **TDK C4532X5R0J107M TAIYO YUDEN JMK325BJ226MM SANYO POSCAP 4TPD470M
COMMENTS VIN: 2.5V to 5.5V, VOUT(MIN): 0.6V, IQ: 20A, ThinSOT VIN: 2.5V to 5.5V, VOUT(MIN): 0.6V, IQ: 40A, MS10E VIN: 2.5V to 5.5V, VOUT(MIN): 0.8V, IQ: 60A, MS, DFN-10 VIN: 2.5V to 5.5V, VOUT(MIN): 0.8V, IQ: 60A, TSSOP-16 VIN: 2.25V to 5.5V, VOUT(MIN): 0.8V, IQ: 64A, TSSOP-20E VIN: 1.5V to 10V, VOUT(MIN): 0.8V, SSOP-24 VIN: 5V to 36V, VOUT(MIN): 0.8V, SSOP-24 VIN: 1.5V to 10V,VOUT(MIN): 0.8V, SSOP-24
sn3413 3413fs LT/TP 0703 1K * PRINTED IN USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2002

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