Richtek RT8015AGQW Schematic [ru]
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RT8015A
3A, 2MHz, Synchronous Step-Down Converter
General Description
The RT8015A is a high efficiency synchronous, step-down DC/DC converter. Its input voltage range is from 2.6V 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 synchronous low on-resistance power switches increase efficiency and eliminate the need for an external Schottky diode. The switching frequency is set by an external resistor or can be synchronized to an external clock. The 100% duty cycle provides low dropout operation extending battery life in portable systems. Current mode operation with external compensation allows the transient response to be optimized over a wide range of loads and output capacitors.
The RT8015A is operated in forced continuous PWM Mode which minimizes ripple voltage and reduces the noise and RF interference.
The 100% duty cycle in Low Dropout Operation further maximize battery life.
The RT8015A is available in the WDFN-10L 3x3 package.
Ordering Information
RT8015A 
Package Type
QW : WDFN-10L 3x3
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.
Features
zHigh Efficiency : Up to 95%
zLow RDS(ON) Internal Switches : 110mΩ
zProgrammable Frequency : 300kHz to 2MHz
zNo Schottky Diode Required
z0.8V Reference Allows for Low Output Voltage
zForced Continuous Mode Operation
zLow Dropout Operation : 100% Duty Cycle
zRoHS Compliant and 100% Lead (Pb)-Free
Applications
zPortable Instruments
zBattery-Powered Equipment
zNotebook Computers
zDistributed Power Systems
zIP Phones
zDigital Cameras
Pin Configurations
(TOP VIEW)
SHDN/RT 1 |
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10 |
COMP |
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GND 2 |
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9 |
FB |
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LX 3 |
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8 |
VDD |
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LX |
4 |
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7 |
PVDD |
PGND |
5 |
11 |
9 |
PVDD |
WDFN-10L 3x3
Marking Information
For marking information, contact our sales representative directly or through a Richtek distributor located in your area.
DS8015A-04 March 2011 |
www.richtek.com |
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RT8015A
Typical Application Circuit
VIN |
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5V |
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ROSC |
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RT8015A |
RCOMP |
CCOMP |
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1000pF |
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332k |
1 |
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COMP 10 |
30k |
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SHDN/RT |
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2 |
GND |
FB 9 |
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R2 |
240k |
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3, 4 |
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LX |
VDD 8 |
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5 |
PGND |
PVDD 6, 7 |
C1 |
R1 |
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CIN |
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22pF |
510k |
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22µF |
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L1 |
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2µH |
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VOUT |
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COUT |
2.5V/3A |
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22µFx2 |
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Note : Using all Ceramic Capacitors
Functional Pin Description
Pin No. |
Pin Name |
Pin Function |
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1 |
SHDN/RT |
Oscillator Resistor Input. Connecting a resistor to ground from this pin sets the |
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switching frequency. Forcing this pin to VDD causes the device to be shut down. |
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2 |
GND |
Signal Ground. All small-signal components and compensation components should |
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connect to this ground, which in turn connects to PGND at one point. |
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3, 4 |
LX |
Internal Power MOSFET Switches Output. Connect this pin to the inductor. |
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5 |
PGND |
Power Ground. Connect this pin close to the negative terminal of CIN and COUT. |
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6, 7 |
PVDD |
Power Input Supply. Decouple this pin to PGND with a capacitor. |
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8 |
VDD |
Signal Input Supply. Decouple this pin to GND with a capacitor. Normally VDD is |
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equal to PVDD. |
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9 |
FB |
Feedback Pin. This pin Receives the feedback voltage from a resistive divider |
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connected across the output. |
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Error Amplifier Compensation Point. The current comparator threshold increases |
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10 |
COMP |
with this control voltage. Connect external compensation elements to this pin to |
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stabilize the control loop. |
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11 (Exposed Pad) |
NC |
No Internal Connection. The exposed pad must be soldered to a large PCB and |
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connected to GND for maximum power dissipation. |
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www.richtek.com |
DS8015A-04 March 2011 |
2
RT8015A
Function Block Diagram
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SHDN/RT |
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SD |
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ISEN |
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PVDD |
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OSC |
Slope |
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Com |
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COMP |
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0.8V |
EA |
Output |
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OC |
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FB |
Clamp |
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Limit |
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Int-SS |
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Driver |
LX |
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0.9V |
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Control |
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Logic |
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0.7V |
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NISEN |
PGND |
POR |
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NMOS I Limit |
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0.4V |
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VREF |
OTP |
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GND |
VDD |
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Layout Guide
CIN must be placed between VDD |
Output capacitor must be |
and GND as closer as possible |
near RT8015A |
VIN
CF R1
VOUT
GND |
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COUT |
CIN |
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RT8015A |
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PVDD 6 |
5 |
PGND |
PVDD 7 |
4 |
LX |
VDD 8 |
3 |
LX |
FB 9 |
2 |
GND |
COMP 10 |
1 |
SHDN/RT |
R2 RCOMP |
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CCOMP |
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GND |
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VOUT |
LX should be |
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connected to Inductor |
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by wide and short |
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trace, keep sensitive |
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L1 |
components away |
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from this trace |
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ROSC
Connect the FB pin directly to feedback resistors. The resistor divider must be connected between VOUT and GND.
DS8015A-04 March 2011 |
www.richtek.com |
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3 |
RT8015A
Operation
Main Control Loop
The RT8015A is a monolithic, constant-frequency, current mode step-down DC/DC converter. 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 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 operating frequency is set by an external resistor connected between the RT pin and ground. The practical switching frequency can range from 300kHz to 2MHz. In an over-voltage condition, the top power MOSFET is turned off and the bottom power MOSFET is switched on until either the over voltage 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 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-Channel MOSFET and the inductor.
Low Supply Operation
The RT8015A is designed to operate down to an input supply voltage of 2.6V. One important consideration at low input supply voltages is that the RDS(ON) of the P-Channel and N-Channel power switches increases. The user should calculate the power dissipation when the
RT8015A 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 sub-harmonic 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 RT8015A, however, separated inductor current signals are used to monitor over current condition. This keeps the maximum output 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.
www.richtek.com |
DS8015A-04 March 2011 |
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RT8015A |
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Absolute Maximum Ratings |
(Note 1) |
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z Supply Input Voltage, VDD, PVDD ---------------------------------------------------------------------------- |
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−0.3V to 6V |
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z LX Pin Switch Voltage -------------------------------------------------------------------------------------------- |
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−0.3V to (PVDD + 0.3V) |
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<200ns --------------------------------------------------------------------------------------------------------------- |
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−5V to 7.5V |
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z Other I/O Pin Voltages ------------------------------------------------------------------------------------------- |
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−0.3V to (VDD + 0.3V) |
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z LX Pin Switch Current -------------------------------------------------------------------------------------------- |
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4A |
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z Power Dissipation, PD @ TA = 25°C |
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WDFN-10L 3x3 ----------------------------------------------------------------------------------------------------- |
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909mW |
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z Package Thermal Resistance (Note 2) |
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WDFN-10L 3x3, θJA ----------------------------------------------------------------------------------------------- |
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110°C/W |
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z Junction Temperature --------------------------------------------------------------------------------------------- |
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150°C |
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z Lead Temperature (Soldering, 10 sec.) ----------------------------------------------------------------------- |
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260°C |
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z Storage Temperature Range ------------------------------------------------------------------------------------ |
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−65°C to 150°C |
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z ESD Susceptibility (Note 3) |
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HBM (Human Body Mode) -------------------------------------------------------------------------------------- |
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2kV |
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MM (Machine Mode) ---------------------------------------------------------------------------------------------- |
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200V |
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Recommended Operating Conditions (Note 4) |
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z Supply Input Voltage ---------------------------------------------------------------------------------------------- |
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2.6V to 5.5V |
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z Junction Temperature Range ------------------------------------------------------------------------------------ |
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−40°C to 125°C |
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z Ambient Temperature Range ------------------------------------------------------------------------------------ |
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−40°C to 85°C |
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Electrical Characteristics |
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(VDD = 3.3V, TA = 25°C, unless otherwise specified) |
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Parameter |
Symbol |
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Test Conditions |
Min |
Typ |
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Max |
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Unit |
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Input Voltage Range |
VDD |
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2.6 |
-- |
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5.5 |
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V |
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Feedback Reference Voltage |
VREF |
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0.784 |
0.8 |
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0.816 |
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V |
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Feedback Leakage Current |
IFB |
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-- |
0.1 |
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0.4 |
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μA |
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DC Bias Current |
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Active , VFB = 0.78V, Not Switching |
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460 |
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-- |
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μA |
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Shutdown |
-- |
-- |
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1 |
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μA |
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Output Voltage Line Regulation |
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VIN = 2.7V to 5.5V |
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0.04 |
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-- |
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%/V |
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Output Voltage Load Regulation |
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Measured in Servo Loop, |
−0.2 |
±0.02 |
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0.2 |
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% |
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V |
= 0.2V to 0.7V (Note 5) |
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COMP |
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Error Amplifier |
gm |
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-- |
800 |
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μs |
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Transconductance |
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Current Sense Transresistance |
RT |
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0.4 |
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Ω |
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Switching Leakage Current |
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SHDN/RT = VIN = 5.5V |
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1 |
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μA |
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Switching Frequency |
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ROSC = 332k |
0.8 |
1 |
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1.2 |
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MHz |
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Switching Frequency |
0.3 |
-- |
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2 |
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MHz |
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Switch On Resistance, High |
RPMOS |
ISW = 0.5A |
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110 |
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160 |
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mΩ |
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Switch On Resistance, Low |
RNMOS |
ISW = 0.5A |
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110 |
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170 |
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mΩ |
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To be continued |
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DS8015A-04 March 2011 |
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www.richtek.com |
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5 |
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