RICHTEK RT8004QS

RT8004
3A, 4MHz, Synchronous Step-Down Regulator
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 75mΩ 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.
Ordering Information
RT8004
Package Type
S : SOP-16
CP : TSSOP-16 (Exposed Pad)
QV : VQFN-16L 4x4 (V-Type)
Operating Temperature Range
P : Pb Free with Commercial Standard
G : Green (Halogen Free with Commercial Standard)
Features
High Efficiency : Up to 95%
μA
Low Quiescent Current : 100μ
Ω
Low RDS(ON) Internal Switches : 75mΩ
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 SOP-16, 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
Pin Configurations
(TOP VIEW)
PGOOD
VDD
PVDD
LX
General Description
Note :
Richtek Pb-free and Green products are :
ments of IPC/JEDEC J-STD-020.
`Suitable for use in SnPb or Pb-free soldering processes.
`100% matte tin (Sn) plating.
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.
DS8004-03
September 2007
16 15 14 13
COMP
FB
RT
SYNC
1
2
3
4
GND
17
12
11
10
9
LX
PGND
PGND
LX
5 6 7 8
EN/SS
GND
PVDD
LX
`RoHS compliant and compatible with the current require-
VQFN-16L 4x4
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RT8004
(TOP VIEW)
VDD
PGOOD
COMP
FB
RT
SYNC
EN/SS
GND
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
PVDD
LX
LX
PGND
PGND
LX
LX
PVDD
VDD
PGOOD
COMP
FB
RT
SYNC
EN/SS
GND
SOP-16
16
15
14
13
GND
12
17 11
10
9
2
3
4
5
6
7
8
PVDD
LX
LX
PGND
PGND
LX
LX
PVDD
TSSOP-16 (Exposed Pad)
Typical Application Circuit
V IN
2.65V to 5.5V
47uF
RPG
100k
VDD
PGOOD
CTH RTH
1000pF 10k
R OCS
309k
External Clock
R SS
4.7M
V OUT
2.5V/3A
LX
FB
RT
C OUT
47uF
PVDD
SYNC
EN/SS
C SS
470pF
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2
L1
1uH
COMP
R2
240k
C1
22pF
R1
510k
RT8004
PGOOD
GND
C IN1
10uF
PVDD
C IN2
10uF
PGND
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RT8004
Functional Pin Description
Pin No.
RT8004PS RT8004PCP RT8004PQP
Pin
Name
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
PGOOD ground when the output voltage is not within ±12.5% of regulation
point.
Error Amplifier Compensation Pin. The current comparator
COMP
threshold increases with this control voltage. Connect external
compensation elements to this pin to stabilize the control loop.
1
1
15
2
2
16
3
3
1
4
4
2
FB
Feedback Pin. Receives the feedback voltage from a resistive
divider connected across the output.
5
5
3
RT
Oscillator Resistor Input. Connecting a resistor to ground from this
pin sets the switching frequency.
6
6
4
SYNC
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.
EN/SS
Enable Control and Soft-Start Input. Forcing this pin below 0.5V
shuts down the RT8004. In shutdown all functions are disabled
drawing < 1uA of supply current. A capacitor to ground from this
pin sets the ramp time to full output current.
7
7
8
8, Exposed
Pad (17)
9, 16
9, 16
10,11,
14, 15
10,11,
14, 15
12, 13
12, 13
DS8004-03
5
VDD
6, Exposed
GND
Pad (17)
7, 14
PVDD
8, 9, 12, 13 LX
September 2007
10, 11
PGND
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 C IN and
COUT .
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RT8004
Function Block Diagram
RT
PVDD
ISEN
SYNC
ExtSyn
Slope Com
Oscillator
COMP
0.8V
EA
FB
EN/SS
OC
Limit
Output
Clamp
LX
Driver
Ext_SS
SD
0.9V
Int_SS
Control
Logic
0.7V
NISEN
0.4V
POR
PGOOD
0.8V
POR
BG
VDD
PGND
NMOS I Limit
VREF
OTP
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-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-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 −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
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.
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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DS8004-03
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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-Channel 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-Channel and N-Channel 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°C
SOP-16 -------------------------------------------------------------------------------------------------------------- 1.25W
TSSOP-16 ----------------------------------------------------------------------------------------------------------- 2.66W
VQFN-16L 4x4 ----------------------------------------------------------------------------------------------------- 2.315W
Package Thermal Resistance (Note 4)
SOP-16, θJA -------------------------------------------------------------------------------------------------------- 100°C/W
TSSOP-16, θJA ----------------------------------------------------------------------------------------------------- 47°C/W
VQFN-16L 4x4, θJA ------------------------------------------------------------------------------------------------ 54°C/W
VQFN-16L 4x4, θJC ----------------------------------------------------------------------------------------------- 7°C/W
Lead Temperature (Soldering, 10 sec.) ----------------------------------------------------------------------- 260°C
Junction Temperature --------------------------------------------------------------------------------------------- 150°C
Storage Temperature Range ------------------------------------------------------------------------------------ −65°C to +150°C
ESD Susceptibility (Note 2)
HBM (Human Body Mode) -------------------------------------------------------------------------------------- 2kV
MM (Machine Mode) ---------------------------------------------------------------------------------------------- 200V
Recommended Operating Conditions
(Note 3)
Supply Input Voltage ---------------------------------------------------------------------------------------------- 2.65V to 5.5V
Ambient Temperature Range ------------------------------------------------------------------------------------ −40°C to 85°C
Junction Temperature Range ------------------------------------------------------------------------------------ −40°C to 125°C
Electrical Characteristics
(VDD = 3.3V, TA = 25°C, unless otherwise specified)
Parameter
Symbol
Min
Typ
Max
Units
2.65
--
5.5
V
0.784
0.8
0.816
V
--
--
0.4
μA
180
400
520
μA
--
--
1
μA
--
0.04
0.2
%/V
--
0.05
+/-0.2
%
Power Good Range
--
+/-12.5
+/-15
%
Power Good Pull-Down Resistance
--
--
120
Ω
Input Voltage Range
VDD
Feedback Voltage
VFB
Feedback Leakage Current
IFB
Input DC Bias Current
Test Conditions
(Note 5)
Active, VFB = 0.78V,
Not switching
Shutdown, VEN < 0.1V
Reference Voltage Line Regulation
VIN = 2.7V to 5.5V
Output Voltage Load Regulation
Measured in Servo Loop,
VCOMP = 1.2V to 1.6V
(Note 5)
(Note 5)
(Note 5)
Power Good
To be continued
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DS8004-03
September 2007
RT8004
Parameter
Switching Frequency
Symbol
fOSC
Sync Frequency Range
Test Conditions
Min
Typ
Max
Units
ROSC = 309k
0.8
1
1.2
MHz
Switching Frequency Range
0.3
--
4
MHz
(Note 6)
0.3
--
4
MHz
Switch On Resistance, High
RPFET
ISW = 1A
45
75
110
mΩ
Switch On Resistance, Low
RNFET
ISW = 1A
45
69
100
mΩ
Peak Current Limit
ILIM
4
5.2
7
A
VDD Rising
2.25
2.52
2.7
V
Hysteresis
--
0.15
--
V
VEN = 0V, VIN = 5.5V
--
--
1
μA
--
--
1
μA
0.65
--
0.95
V
Undervoltage Lockout Threshold
SW Leakage Current
EN/SS Leakage Current
Enable Threshold
VEN
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. Devices are ESD sensitive. Handling precaution recommended.
Note 3. The device is not guaranteed to function outside its operating conditions.
Note 4. θJA is measured in the natural convection at TA = 25°C on 4-layers high effective thermal conductivity test board of
JEDEC 51-7 thermal measurement standard. The case point of θJC is on the expose pad for the QFN package.
Note 5. The specifications over the -40°C to 85°C 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 reange is guaranteed by design but not production tested.
DS8004-03
September 2007
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RT8004
Typical Operating Characteristics
Efficiency vs. Output Current
Load Regulation
100%
100
0.30%
0.30
90
90%
80
80%
0.20%
0.20
VIN = 3.3V
70%
70
V OUT Devation (%)
Efficiency (%)
VIN = 3.3V, VOUT = 2.5V
VIN = 5V
60%
60
50
50%
40
40%
30
30%
20%
20
0.10%
0.10
0.00%0
-0.10%
-0.10
10
10%
VOUT = 2.5V, 1M Continuous Mode Operation
0
0%
0
500
1000
1500
2000
2500
-0.20%
-0.20
3000
50
550
1050
1550
2050
Output Current (mA)
Output Current (mA)
Soft-Star Power On
Power Off
VIN = 3.3V, VOUT = 2.5V
RLOAD = 0.83Ω
2550
VIN = 3.3V, VOUT = 2.5V, IOUT = 0A
VOUT
(1V/Div)
IL
(2A/Div)
VSS
(1V/Div)
VOUT
(2V/Div)
VLX
(5V/Div)
IL
(1A/Div)
VIN
(2V/Div)
Time (2.5ms/Div)
Time (25ms/Div)
Ripple Voltage
Load Transien Response
VIN = 3.3V, VOUT = 2.5V
ILOAD = No Load to 3A
VIN = 3.3V, VOUT = 2.5V, IOUT = 3A
VOUT
(10mV/Div)
3050
VOUT
(50mV/Div)
VLX
(2V/Div)
IL
(2A/Div)
IL
(1A/Div)
Time (500ns/Div)
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Time (25μs/Div)
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RT8004
Frequency vs. RRT
VREF Deviation vs. Temperature
1.50%
4500
VIN = 3.3V
1.00%
3500
Frequency (kHz)
V REF Deviation (%)
VIN = 3.3V
4000
0.50%
0.00%
-0.50%
3000
2500
2000
1500
1000
-1.00%
500
-1.50%
0
-50
-25
0
25
50
75
100
125
0
200
400
Temperature (°C)
1000
1200
1400
Frequency vs. Temperature
5
R = 309k
VIN = 3.3V
4
Frequency Deviation (%)
1.032
Frequency (MHz)
800
RRT (k
ٛ)
(kΩ)
Frequency vs. Input Voltage
1.04
600
1.024
1.016
1.008
1
3
2
1
0
-1
-2
-3
-4
-5
0.992
3
3.5
4
4.5
5
5.5
-50
-25
0
25
50
75
100
125
Temperature (°C)
Input Voltage (V)
Peak Current Limited vs. Input Voltage
Quiescent Current vs. Input Voltage
5.5
500
VOUT = 2.5V
Peak Current Limited (A)
Quiescent Current (uA)
450
400
350
300
250
200
150
100
5.3
5.1
4.9
4.7
50
4.5
0
3
3.5
4
4.5
Input Voltage (V)
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5
5.5
3
3.5
4
4.5
5
5.5
Input Voltage (V)
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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 ΔIL increases with higher VIN and decreases
with higher inductance.
⎤
⎡V
⎤⎡ V
ΔIL = ⎢ OUT ⎥ ⎢1 − OUT ⎥
f
×
L
V
⎣
⎦⎣
IN ⎦
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 ΔIL = 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
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chosen according to the following equation :
VOUT ⎤
⎡ VOUT ⎤ ⎡
L=⎢
⎢1 −
⎥
⎥
⎣ f × ΔIL(MAX) ⎦ ⎢⎣ VIN(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 = 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, ΔVOUT, is determined by :
ΔVOUT
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.
1 ⎤
⎡
≤ ΔIL ⎢ESR +
8fCOUT ⎥⎦
⎣
The output ripple is highest at maximum input voltage
since ΔIL 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
DS8004-03
September 2007
VOUT
R2
VFB
RT8004
R1
GND
Figure 1. Setting the Output Voltage
Frequency Synchronization
The RT8004’ s internal oscillator can be synchronized to
an external clock signal. During synchronization, the top
MOSFET turn-on is locked to the falling edge of the
external frequency source. The synchronization frequency
range is 300kHz to 4MHz. Synchronization only occurs if
the external frequency is greater than the frequency set
by the external resistor. Because slope compensation is
generated by the oscillator’ s RC circuit, the external
frequency should be set 25% higher than the frequency
set by the external resistor to ensure that adequate slope
compensation is present.
Soft-Start
The EN/SS pin provides a means to shut down the RT8004
as well as a timer for soft-start. Pulling the EN/SS pin
below 0.5V places the RT8004 in a low quiescent current
shutdown state (IQ < 1μA).
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RT8004
The RT8004 contains an internal soft-start clamp that
gradually raises the clamp on COMP after the EN/SS pin
is pulled above 0.8V. The full current range becomes
available on COMP after 1024 switching cycles. If a longer
soft-start period is desired, the clamp on COMP can be
set externally with a resistor and capacitor on the EN/SS
pin as shown in Typical Application Circuit. The soft-start
duration can be calculated by using the following formula:
TSS = RSS x CSS x ln(
VIN
) (s)
VIN − 1.8V
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: VDD quiescent current and I2R
losses. The VDD 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 VDD 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
high to low to high again, a packet of charge ΔQ moves
from VDD to ground. The resulting ΔQ/Δt is the current out
of VDD 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
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losses are proportional to VDD 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 LX pin is a function of both top and bottom
MOSFET RDS(ON) and the duty cycle (DC) as follows :
RSW = RDS(ON)TOP x DC + RDS(ON)BOT x (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 RT8004 does not dissipate much
heat due to its high efficiency. But, in applications where
the RT8004 is running at high ambient temperature with
low supply voltage and high duty cycles, such as in
dropout, the heat dissipated may exceed the maximum
junction temperature of the part. If the junction temperature
reaches approximately 150°C, both power switches will
be turned off and the SW node will become high
impedance. To avoid the RT8004 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 x θJA
Where PD is the power dissipated by the regulator and
θJA 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.
DS8004-03
September 2007
RT8004
As an example, consider the RT8004 in dropout at an
input voltage of 3.3V, a load current of 3A and an ambient
temperature of 70°C. From the typical performance graph
of switch resistance, the RDS(ON) of the P-Channel switch
LX node is with high frequency voltage swing and should
be kept small area. Keep all sensitive small-signal nodes
away from LX node to prevent stray capacitive noise
pick-up.
at 70°C is approximately 97mΩ. Therefore, power
dissipated by the part is :
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).
PD = (ILOAD)2 (RDS(ON)) = (3A)2 (97mΩ) = 0.873W
For the TSSOP package, the θJA is 47°C/W. Thus the
junction temperature of the regulator is :
TJ = 70°C+(0.873W) (47°C/W) = 111°C
Which is below the maximum junction temperature of
125°C. Note that at higher supply voltages, the junction
temperature is lower due to reduced switch resistance
(RDS(ON)).
Connect the FB pin directly to the feedback resistors.
The resistor divider must be connected between VOUT
and GND.
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 ΔILOAD(ESR), where ESR is the effective series
resistance of COUT. ΔILOAD 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 COMP pin external components and output capacitor
shown in Typical Application Circuit will provide adequate
compensation for most applications.
Layout Considerations
Follow the PCB layout guidelines for optimal performance
of RT8004.
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 the GND pin at one point that is then connected to
the PGND pin close to the IC. The exposed pad should
be connected to GND.
Connect the terminal of the input capacitor(s), CIN, as
close as possible to the PVDD pin. This capacitor
provides the AC current into the internal power
MOSFETs.
DS8004-03
September 2007
www.richtek.com
13
RT8004
Outline Dimension
H
A
M
B
J
F
C
I
D
Symbol
Dimensions In Millimeters
Dimensions In Inches
Min
Max
Min
Max
A
9.804
10.008
0.386
0.394
B
3.810
3.988
0.150
0.157
C
1.346
1.753
0.053
0.069
D
0.330
0.508
0.013
0.020
F
1.194
1.346
0.047
0.053
H
0.178
0.254
0.007
0.010
I
0.102
0.254
0.004
0.010
J
5.791
6.198
0.228
0.244
M
0.406
1.270
0.016
0.050
16–Lead SOP Plastic Package
www.richtek.com
14
DS8004-03
September 2007
RT8004
D
L
U
EXPOSED THERMAL PAD
(Bottom of Package)
E
V
E1
e
A2
A
A1
b
Symbol
Dimensions In Millimeters
Dimensions In Inches
Min
Max
Min
Max
A
0.850
1.200
0.033
0.047
A1
0.000
0.150
0.000
0.006
A2
0.800
1.050
0.031
0.041
b
0.190
0.300
0.007
0.012
D
4.900
5.100
0.193
0.201
e
0.65
0.026
E
6.200
6.600
0.244
0.260
E1
4.300
4.500
0.169
0.177
L
0.450
0.750
0.018
0.030
U
2.000
3.000
0.079
0.118
V
2.000
3.000
0.079
0.118
16-Lead TSSOP (Exposed Pad) Plastic Package
DS8004-03
September 2007
www.richtek.com
15
RT8004
D
SEE DETAIL A
D2
L
1
E
E2
e
b
A
A1
1
1
2
2
DETAIL A
Pin #1 ID and Tie Bar Mark Options
A3
Note : The configuration of the Pin #1 identifier is optional,
but must be located within the zone indicated.
Symbol
Dimensions In Millimeters
Dimensions In Inches
Min
Max
Min
Max
A
0.800
1.000
0.031
0.039
A1
0.000
0.050
0.000
0.002
A3
0.175
0.250
0.007
0.010
b
0.250
0.380
0.010
0.015
D
3.950
4.050
0.156
0.159
D2
2.000
2.450
0.079
0.096
E
3.950
4.050
0.156
0.159
E2
2.000
2.450
0.079
0.096
e
L
0.650
0.500
0.026
0.600
0.020
0.024
V-Type 16L VQFN 4x4 Package
Richtek Technology Corporation
Richtek Technology Corporation
Headquarter
Taipei Office (Marketing)
5F, No. 20, Taiyuen Street, Chupei City
8F, No. 137, Lane 235, Paochiao Road, Hsintien City
Hsinchu, Taiwan, R.O.C.
Taipei County, Taiwan, R.O.C.
Tel: (8863)5526789 Fax: (8863)5526611
Tel: (8862)89191466 Fax: (8862)89191465
Email: [email protected]
www.richtek.com
16
DS8004-03
September 2007