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RT8004
3A, 4MHz, Synchronous Step-Down Regulator
General Description
The RT8004 is a high efficiency synchronous, step-down DC/DC converter. Its input voltage range is from 2.65V to 5.5V and provides an adjustable regulated output voltage from 0.8V to 5V while delivering up to 3A of output current. The internal power switch with 75m?? on-resistance increases efficiency and eliminates the need for an external Schottky diode. Switching frequency is set by an external resistor or can be synchronized to an external clock. 100% duty cycle provides low dropout operation extending battery life in portable systems. External compensation allows the transient response to be optimized over a wide range of loads and output capacitors. The RT8004 operates in Forced Continuous Mode which reduces noise and RF interference. 100% duty cycle in Low Dropout Operation further maximize battery life.
Features
High Efficiency : Up to 95% Low Quiescent Current : 100?ÌA ?Ì Low RDS(ON) Internal Switches : 75m?? ?? Programmable Frequency : 300kHz to 4MHz No Schottky Diode Required 0.8V Reference Allows Low Output Voltage Low Dropout Operation : 100% Duty Cycle Synchronizable Switching Frequency Power Good Output Voltage Monitor Over Temperature Protection Thermally Enhanced TSSOP-16 (Exposed Pad) and 16-Lead VQFN 4x4 Packages RoHS Compliant and 100% Lead (Pb)-Free
Applications
Portable Instruments Battery-Powered Equipment Notebook Computers Distributed Power Systems IP Phones Digital Cameras
Ordering Information
RT8004 Package Type CP : TSSOP-16 (Exposed Pad) QV : VQFN-16L 4x4 (V-Type) Lead Plating System P : Pb Free G : Green (Halogen Free and Pb Free)
Note : Richtek products are : RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020. Suitable for use in SnPb or Pb-free soldering processes.
Pin Configurations
(TOP VIEW)
PGOOD VDD PVDD LX 16 15 14 13 COMP FB RT SYNC 1 2 3 4 GND 17 5 6 7 8 EN/SS GND PVDD LX 12 11 10 9 LX PGND PGND LX
Marking Information
For marking information, contact our sales representative directly
or through a Richtek distributor located in your area, otherwise visit our website for detail.
VDD PGOOD COMP FB RT SYNC EN/SS GND
VQFN-16L 4x4
16 15 14 13 GND 12 17 11 10 9 PVDD LX LX PGND PGND LX LX PVDD
2 3 4 5 6 7 8
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RT8004
Typical Application Circuit
VIN 2.65V to 5.5V RPG 100k PGOOD CTH RTH 1000pF 10k R2 240k ROCS 309k External Clock RSS 4.7M CSS 470pF 47uF VDD RT8004 PGOOD COMP LX FB RT SYNC R1 510k L1 1uH C1 22pF
VOUT 2.5V/3A COUT 47uF
PVDD CIN1 10uF PVDD CIN2 10uF PGND
EN/SS
GND
Functional Pin Description
Pin No. Pin Name RT8004PCP RT8004PQP 1 2 3 4 5 6 15 16 1 2 3 4 VDD PGOOD COMP FB RT SYNC Pin Function Signal Input Supply. Decouple this pin to GND with a capacitor. Normally VDD is equal to PVDD. Power Good Indicator. Open-drain logic output that is pulled to ground when the output voltage is not within ?À12.5% of regulation point. Error Amplifier Compensation Pin. The current comparator threshold increases with this control voltage. Connect external compensation elements to this pin to stabilize the control loop. Feedback Pin. Receives the feedback voltage from a resistive divider connected across the output. Oscillator Resistor Input. Connecting a resistor to ground from this pin sets the switching frequency. External Clock Synchronization Input. The internal oscillator can be synchronized to an external clock applied to this pin. If not use, please connect this pin to VDD or GND. Enable Control and Soft-Start Input. Forcing this pin below 0.5V shuts down the RT8004. In shutdown all functions are disabled drawing < 1?ÌA of supply current. A capacitor to ground from this pin sets the ramp time to full output current. Signal Ground. All small-signal components, compensation components and the exposed pad on the bottom side of the IC should connect to this ground, which in turn connects to PGND at one point. Power Input Supply. Decouple this pin to PGND with a capacitor. Internal Power MOSFET Switches Output. Connect this pin to the inductor. Power Ground. Connect this pin close to the terminal of CIN and COUT.
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7
5
EN/SS
8, 6, Exposed Pad Exposed Pad GND (17) (17) 9, 16 10,11, 14, 15 12,
13
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7, 14
PVDD
8, 9, 12, 13 LX 10, 11 PGND
RT8004
Function Block Diagram
RT ISEN SYNC ExtSyn Oscillator Slope Com PVDD
COMP 0.8V FB EA Output Clamp OC Limit Driver Ext_SS Int_SS LX
EN/SS
SD
0.9V 0.7V
Control Logic
NISEN 0.4V NMOS I Limit 0.8V BG VREF OTP
PGND
PGOOD
POR
POR
VDD
GND
Operation
Main Control Loop The RT8004 is a monolithic, constant-frequency, current mode step-down DC/DC converter. During normal operation, the internal top power switch (P-MOSFET) is turned on at the beginning of each clock cycle. Current in the inductor increases until the peak inductor current reach the value defined by the voltage on the COMP pin. The error amplifier adjusts the voltage on the COMP pin by comparing the feedback signal from a resistor divider on the FB pin with an internal 0.8V reference. When the load current increases, it causes a reduction in the feedback voltage relative to the reference. The error amplifier raises the COMP voltage until the average inductor current matches the new load current. When the top power MOSFET shuts off, the synchronous power switch (N-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 ?2A. The operating frequency is set by an external resistor connected between the RT pin and ground. The practical switching frequency can range from 300kHz to 4MHz. Power Good comparators will pull the PGOOD output low if the output voltage comes out of regulation by 12.5%. In an over voltage 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. Frequency Synchronization The internal oscillator of the RT8004 can be synchronized to an external clock connected to the SYNC pin. The frequency of the external clock can be in the range
of 300kHz to 4MHz. For this application, the oscillator timing resistor should be chosen to correspond to a frequency that is about 20% lower than the synchronization frequency.
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RT8004
Dropout Operation When the input supply voltage decreases toward the output voltage, the duty cycle increases toward the maximum ontime. Further reduction of the supply voltage forces the main switch to remain on for more than one cycle eventually reaching 100% duty cycle. The output voltage will then be determined by the input voltage minus the voltage drop across the internal P-MOSFET and the inductor. Low Supply Operation The RT8004 is designed to operate down to an input supply voltage of 2.65V. One important consideration at low input supply voltages is that the RDS(ON) of the P-MOSFET and N-MOSFET power switches increases. The user should calculate the power dissipation when the RT8004 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. Normally, the maximum inductor peak current is reduced when slope compensation is added. In the RT8004, however, separated inductor current signals are used to monitor over current condition and minimum peak current. This keeps the maximum output current and minimum peak current relatively constant regardless of duty cycle. Short Circuit Protection When the output is shorted to ground, the inductor current decays very slowly during a single switching cycle. A current runaway detector is used to monitor inductor current. As current increasing beyond the control of current loop, switching cycles will be skipped to prevent current runaway from occurring.
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RT8004
Absolute Maximum Ratings
(Note 1) Supply Input Voltage
---------------------------------------------------------------------------------------------- ?0.3V to 6V LX Pin Switch Voltage -------------------------------------------------------------------------------------------- ?0.3V to (PVDD + 0.3V) Other I/O Pin Voltages
------------------------------------------------------------------------------------------- ?0.3V to (VDD + 0.3V) Power Dissipation, PD @ TA = 25?ãC TSSOP-16
----------------------------------------------------------------------------------------------------------- 2.66W VQFN-16L 4x4 ----------------------------------------------------------------------------------------------------- 2.315W Package Thermal Resistance (Note 2) TSSOP-16, ?ÈJA
----------------------------------------------------------------------------------------------------- 47?ãC/W VQFN-16L 4x4, ?ÈJA ------------------------------------------------------------------------------------------------ 54?ãC/W VQFN-16L 4x4, ?ÈJC ----------------------------------------------------------------------------------------------- 7?ãC/W Lead Temperature (Soldering, 10 sec.)
----------------------------------------------------------------------- 260?ãC Junction Temperature
--------------------------------------------------------------------------------------------- 150?ãC Storage Temperature Range ------------------------------------------------------------------------------------ ?65?ãC to +150?ãC ESD Susceptibility (Note 3) HBM (Human Body Mode)
-------------------------------------------------------------------------------------- 2kV MM (Machine Mode)
---------------------------------------------------------------------------------------------- 200V
Recommended Operating Conditions
(Note 4)
Supply Input Voltage
---------------------------------------------------------------------------------------------- 2.65V to 5.5V Ambient Temperature Range ------------------------------------------------------------------------------------ ?40?ãC to 85?ãC Junction Temperature Range ------------------------------------------------------------------------------------ ?40?ãC to 125?ãC
Electrical Characteristics
(VDD = 3.3V, TA = 25?ãC, unless otherwise specified)
Parameter Input Voltage Range Feedback Voltage Feedback Leakage Current Input DC Bias Current Reference Voltage Line Regulation Output Voltage Load Regulation Power Good Power Good Range Power Good Pull-Down Resistance Switching Frequency Sync Frequency Range
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Symbol VDD VFB IFB (Note 5)
Test Conditions
Min 2.65 0.784 --
Typ -0.8 -400 -0.04 0.05
Max 5.5 0.816 0.4 520 1 0.5 +/-0.2
Unit V V ?ÌA ?ÌA ?ÌA
Active, VFB = 0.78V, Not switching Shutdown, VEN < 0.1V VIN = 2.7V to 5.5V Measured in Servo Loop, VCOMP = 1.2V to 1.6V (Note 5) (Note 5) (Note 5)
180 ----
%/V %
--fOSC ROSC = 309k Switching Frequency Range (Note 6) 0.8 0.3 0.3
?À12.5 -1 ---
?À15 120 1.2 4 4
% ?? MHz MHz MHz
To be continued
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RT8004
Parameter Switch On Resistance, High Switch On Resistance, Low Peak Current Limit Under Voltage Lockout Threshold SW Leakage Current EN/SS Leakage Current Enable Threshold VEN Symbol RPFET RNFET ILIM VDD Rising Hysteresis VEN = 0V, VIN = 5.5V Test Conditions I SW = 1A I SW = 1A Min 45 45 4 2.25 ---0.65 Typ 75 69 5.2 2.52 0.15 ---Max 110 100 -2.7 -1 1 0.95 Unit m?? m?? A V V ?ÌA ?ÌA V
Note 1. Stresses listed as the above ??Absolute Maximum Ratings?? may cause permanent damage to the device. These are for stress ratings. 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 remain possibility to affect device reliability. Note 2. ?ÈJA is measured in the natural convection at TA = 25?ãC on 4-layers high effective thermal conductivity test board of JEDEC 51-7 thermal measurement standard. The measurement case position of ?ÈJC is on the exposed pad of the package. Note 3. Devices are ESD sensitive. Handling precaution recommended. Note 4. The device is not guaranteed to function outside its operating conditions. Note 5. The specifications over the -40?ãC to 85?ãC operation ambient temperature range are assured by design, characterization and correlation with statistical process controls. Note 6. The external synchronous frequency must be equal to 1 to 1.3 times of the internal setting frequency. The switching frequency range is guaranteed by design but not production tested.
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RT8004
Typical Operating Characteristics
Efficiency vs. Output Current
100% 100
0.30% 0.30
Load Regulation
VIN = 3.3V, VOUT = 2.5V
90 90% 80 80%
VIN = 5V
Efficiency (%)
70% 70 60% 60
VOUT Deviation (%)
1000 1500 2000 2500 3000
VIN = 3.3V
0.20% 0.20
0.10% 0.10
50 50% 40 40% 30 30%
20% 20
0.00% 0
-0.10% -0.10
10 10% 0 0%
0
VOUT = 2.5V, 1M Continuous Mode Operation
500
-0.20% -0.20 50 550 1050 1550 2050 2550 3050
Output Current (mA)
Output Current (mA)
Soft-Start Power On
VIN = 3.3V, VOUT = 2.5V RLOAD = 0.83??
Power Off
VIN = 3.3V, VOUT = 2.5V, IOUT = 0A
VOUT (1V/Div) VSS (1V/Div) IL (1A/Div) Time (2.5ms/Div)
IL (2A/Div) VOUT (2V/Div) VLX (5V/Div) VIN (2V/Div) Time (25ms/Div)
Ripple Voltage
VIN = 3.3V, VOUT = 2.5V, IOUT = 3A
Load Transient Response
VIN = 3.3V, VOUT = 2.5V ILOAD = No Load to 3A
VOUT (10mV/Div) VLX (2V/Div)
VOUT (50mV/Div)
IL (2A/Div) Time (500ns/Div)
IL (1A/Div) Time (25?Ìs/Div)
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RT8004
VREF Deviation vs. Temperature
1.50% 1.00%
Frequency vs. RRT
4500 4000 3500
VIN = 3.3V
VIN = 3.3V
VREF Deviation (%)
0.50% 0.00% -0.50% -1.00% -1.50% -50 -25 0 25 50 75 100 125
Frequency (kHz)
3000 2500 2000 1500 1000 500 0 0 200 400 600 800 1000 1200 1400
Temperature (?ãC)
RRT (k ) (k??)
Frequency vs. Input Voltage
1.04 1.032
Frequency vs. Temperature
5 5 4 4 VIN = 3.3V
R = 309k
Frequency Deviation (%)1
3 3.5 4 4.5 5 5.5
3 3
2 2
Frequency (MHz)
1.024 1.016 1.008 1 0.992
1 1 0 0 -1 -1 -2 -2 -3 -3 -4 -4 -5 -5
-50 -25 0 25 50 75 100 125
Input Voltage (V)
Temperature (?ãC)
Quiescent Current vs. Input Voltage
500 450
5.5
Peak Current Limited vs. Input Voltage
VOUT = 2.5V
Quiescent Current (?ÌA)
350 300 250 200 150 100 50 0 3 3.5 4 4.5 5 5.5
Peak Current Limited (A)
400
5.3
5.1
4.9
4.7
4.5 3 3.5 4 4.5 5 5.5
Input Voltage (V)
Input Voltage (V)
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DS8004-07 March 2011
RT8004
Application Information
The basic RT8004 application circuit is shown in Typical Application
Circuit. 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 and switching losses but requires larger inductance values and/or capacitance to maintain low output ripple voltage. The operating frequency of the RT8004 is determined by an external resistor that is connected between the RT pin 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. The RT resistor value can be determined by examining the frequency vs. RRT curve. Although frequencies as high as 4MHz are possible, the minimum on-time of the RT8004 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 x 110ns x 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 and decreases with higher inductance. ? ?V ?? V ??IL = ? OUT ? ?1 ? OUT ? VIN ? ? f ?Á L ?? Having a lower ripple current reduces 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 :
VOUT ? ? VOUT ? ? L=? ? ? ?1 ? V ? f ?Á ??IL(MAX) ? ? IN(MAX) ? ? ?
Inductor Core Selection Once the value for L is known, the type of inductor must be selected. High efficiency converters generally cannot afford the core loss found in low cost powdered iron cores, forcing the use of more expensive ferrite or mollypermalloy cores. 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 preferred 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 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 vs size requirements and any radiated field/EMI requirements. CIN and COUT Selection The input capacitance, C IN, is needed to filter the trapezoidal current at the source of the top MOSFET. To prevent large ripple voltage, a low ESR input capacitor sized for the maximum RMS current should be used. RMS current is given by : IRMS = IOUT(MAX) VOUT VIN VIN ?1 VOUT
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RT8004
This formula has a maximum at VIN = 2VOUT, where IRMS = I OUT/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 ?Ü ??IL ?ESR + 8fCOUT ? ? ?
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. Output Voltage Programming The output voltage is set by an external resistive divider according to the following equation :
VOUT = 0.8V(1 +
R2 ) R1
The resistive divider allows the VFB pin to sense a fraction of the output voltage as shown in Figure 1.
VOUT R2 VFB RT8004 GND R1
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.