ETC TD6817

 Techcode®
ÓÉ Foxit PDF Editor ±à¼°æȨËùÓÐ (c) by Foxit Software Company, 2004
½öÓÃÓÚÆÀ¹À¡£
DATASHEET
1.5MHz 2A Synchronous Step-Down Regulator Dropout
TD6817
汪工 TEL:13828719410 QQ:1929794238
General Description Features The TD6817 is a high efficiency monolithic synchronous
buck regulator using a constant frequency, current mode
architecture. The device is available in an adjustable
version and fixed output voltages of 1.5V and 1.8V.
Supply current during operation is only 20mA and drops
to ≤1mA in shutdown. The 2.5V to 5.5V input voltage
range makes the TD6817 ideally suited for single Li-Ion
battery-powered applications. 100% duty cycle provides
low dropout operation, extending battery life in portable
systems.Automatic Burst Mode operation increases
efficiency at light loads, further extending battery life.
Switching frequency is internally set at 1.5MHz, allowing
the use of small surface mount inductors and capacitors.
The internal synchronous switch increases efficiency and
eliminates the need for an external Schottky diode. Low
output voltages are easily supported with the 0.6V
feedback reference voltage. The TD6817 is available in
TSOT23-5 package.
z
z
z
z
High Efficiency: Up to 96%
High Efficiency at light loads
Very Low Quiescent Current: Only 20uA During
Operation
2A Output Current
2.5V to 5.5V Input Voltage Range
1.5MHz Constant Frequency Operation
No Schottky Diode Required
Low Dropout Operation: 100% Duty Cycle
0.6V Reference Allows Low Output Voltages
Shutdown Mode Draws ≤1uA Supply Current
Current Mode Operation for Excellent Line and Load
Transient Response
Overtemperature Protected
z
TSOT23-5 Package is Available
z
z
z
z
z
z
z
z
Applications Cellular Telephones
Personal Information Appliances
Wireless and DSL Modems
Digital Still Cameras
MP3 Players
Portable Instruments
z
z
z
z
z
z
Package Types TSOT23­5 Figure 1. Package Types of TD6817 December, 20, 2009 Techcode Semiconductor Limited www.tongchuangwei.com
1 Techcode®
DATASHEET
1.5MHz 2A Synchronous Step-Down Regulator Dropout
TD6817
Pin Assignments Pin
Name
Description
1
RUN
Run Control Input. Forcing this pin
above 1.5V enables the part.
Forcing this pin below 0.3V shuts
down the device. In shutdown, all
functions are disabled drawing
<1μA supply current. Do not leave
RUN floating.
2
GND
Ground Pin.
3
SW
Switch Node Connection to
Inductor. This pin connects to the
drains of the internal main and
synchronous power MOSFET
switches.
4
VIN
Main Supply Pin. Must be closely
decoupled to GND, Pin 2, with a
TSOT23‐5 5
VFB
5
VOUT
2.2μF or greater ceramic capacitor.
Feedback Pin. Receives the
feedback voltage from an external
resistive divider across the output.
Output Voltage Feedback Pin. An
internal resistive divider divides the
output voltage down for comparison
to the internal reference voltage.
Ordering Information TD6817 □
□ Circuit Type Output Versions
Blank:Adj
12:1.2V
Package
15:1.5V
T:TSOT23-5
18:1.8V
December, 20, 2009 Techcode Semiconductor Limited www.tongchuangwei.com
2 Techcode®
DATASHEET
1.5MHz 2A Synchronous Step-Down Regulator Dropout
TD6817
Functional Block Diagram Figure2:Functional Block Diagram of TD6817 Type Application Circuit Figure 3. Type Application Circuit of TD6817 December, 20, 2009 Techcode Semiconductor Limited www.tongchuangwei.com
3 Techcode®
DATASHEET
1.5MHz 2A Synchronous Step-Down Regulator Dropout
TD6817
Absolute Maximum Ratings Note1: Stresses greater than those listed under Maximum Ratings may cause permanent damage to the device. This is a stress rating only and functional operation
of the device at these or any other conditions above those indicated in the operation is not implied. Exposure to absolute maximum rating conditions for extended
periods may affect reliability.
Parameter
Value
Unit
Input Supply Voltage
-0.3 ~6
V
RUN, VFB Voltages
-0.3 ~ VIN
V
SW Voltage
-0.3V ~(VIN+0.3)
V
P-Channel Switch Source Current (DC)
2250
mA
N-Channel Switch Sink Current (DC)
2250
mA
Peak SW Sink and Source Current
2.5
A
Operating Temperature Range
-40~+85
ºC
Junction Temperature
125
ºC
Lead Temperature (Soldering, 10 sec)
300
ºC
Storage Temperature Range
-65~150
ºC
Electrical Characteristics Unless otherwise specified, VIN= 3.6V TA=25 ºC. Symbol
Parameter
IVFB
Feedback Current
VFB
Regulated Feedback
Voltage
VFB
Reference Voltage Line
Regulation
VOUT
Regulated Output
Voltage
VOUT
Output Voltage Line
Regulation
Conditions
Min.
Typ.
Max.
Unit
30
nA
TA = 25°C
0.5880
0.6000
0.6120
0°C TA ≤ 85°C
0.5865
0.6000
0.6135
–40°C ≤ TA ≤ 85°C
0.5850
0.6000
0.6150
0.04
0.4
%/ V
V
VIN = 2.5V to 5.5V
TD6817-1.5, IOUT = 150mA
1.455
1.500
1.545
TD6817-1.8, IOUT = 150mA
1.746
1.800
1.854
0.04
0.4
VIN = 2.5V to 5.5V
V
%/ V
December, 20, 2009 Techcode Semiconductor Limited www.tongchuangwei.com
4 Techcode®
DATASHEET
1.5MHz 2A Synchronous Step-Down Regulator Dropout
TD6817
Electrical Characteristics(Cont.) Unless otherwise specified, VIN= 3.6V TA=25 ºC. Symbol
Parameter
Conditions
Min.
Typ.
Max.
Unit
IPK
Peak Inductor Current
VIN = 3V, VFB = 0.5V or
VOUT = 90%, Duty Cycle <
35%
2.4
2.5
2.6
A
VLOADREG
Output Voltage Load
Regulation
VIN
Input Voltage Range
0.5
2.5
%
5.5
V
Input DC Bias Current
Active Mode
VFB = 0.5V or VOUT =
90%, ILOAD = 0A
300
400
uA
Sleep Mode
VFB = 0.62V or VOUT =
103%, ILOAD = 0A
20
35
uA
Shutdown
VRUN = 0V, VIN = 4.2V
0.1
1
uA
Oscillator Frequency
VFB = 0.6V or VOUT =
100%
1.5
1.8
MHz
IS
fOSC
1.2
VFB = 0V or VOUT = 0V
400
KHz
RPFET
RDS(ON) of P-Channel
FET
ISW = 100mA
0.11
0.13
Ω
RNFET
RDS(ON) of N-Channel
FET
ISW = -100mA
0.12
0.15
Ω
ILSW
SW Leakage
VRUN = 0V, VSW = 0V or
5V, VIN = 5V
0.01
1
uA
VRUN
RUN Threshold
1
1.5
V
IRUN
RUN Leakage Current
0.01
1
uA
0.3
December, 20, 2009 Techcode Semiconductor Limited www.tongchuangwei.com
5 Techcode®
DATASHEET
1.5MHz 2A Synchronous Step-Down Regulator Dropout
TD6817
Typical Operating Characteristics Reference Voltage
Oscillator Frequency Oscillator Frequency vs Supply Voltage
RDS(ON) vs Temperature December, 20, 2009 Techcode Semiconductor Limited www.tongchuangwei.com
6 Techcode®
DATASHEET
1.5MHz 2A Synchronous Step-Down Regulator Dropout
TD6817
Typical Operating Characteristics(Cont.) RDS(ON) vs Input Voltage
Efficiency vs Output Current Efficiency vs Output Current
Efficiency vs Output Current December, 20, 2009 Techcode Semiconductor Limited www.tongchuangwei.com
7 Techcode®
DATASHEET
1.5MHz 2A Synchronous Step-Down Regulator Dropout
TD6817
Typical Operating Characteristics(Cont.) Efficiency vs Output Current Output Voltage vs Output Current Efficiency vs Input Voltage
Dynamic Supply Current vs Supply Voltage December, 20, 2009 Techcode Semiconductor Limited www.tongchuangwei.com
8 Techcode®
DATASHEET
1.5MHz 2A Synchronous Step-Down Regulator Dropout
TD6817
Typical Operating Characteristics P-FET Leakage vs Temperature
N-FET Leakage vs Temperature December, 20, 2009 Techcode Semiconductor Limited www.tongchuangwei.com
9 Techcode®
DATASHEET
1.5MHz 2A Synchronous Step-Down Regulator Dropout
TD6817
Function Description Main Control Loop The TD6817 uses a constant frequency, current mode
step-down architecture. Both the main (P-channel
MOSFET) and synchronous (N-channel MOSFET)
switches are internal. During normal operation, the
internal top power MOSFET is turned on each cycle
when the oscillator sets the RS latch, and turned off
when the current comparator, ICOMP, resets the RS
latch. The peak inductor current at which ICOMP resets
the RS latch, is controlled by
the output of error amplifier EA. When the load current
increases, it causes a slight decrease in the feedback
voltage, FB, relative to the 0.6V reference, which in turn,
causes the EA amplifier’s output voltage to increase until
the average inductor current matches the new load
current. While the top MOSFET is off, the bottom
MOSFET is turned on until either the inductor current
starts to reverse, as indicated by the current reversal
comparator IRCMP, or the beginning of the next clock
cycle.
Burst Mode Operation The TD6817 is capable of Burst Mode operation in which
the internal power MOSFETs operate intermittently
based on load demand.
In Burst Mode operation, the peak current of the inductor
is set to approximately 200mA regardless of the output
load. Each burst event can last from a few cycles at light
loads to almost continuously cycling with short sleep
intervals at moderate loads. In between these burst
events, the power MOSFETs and any unneeded circuitry
are turned off, reducing the quiescent current to 20uA. In
this sleep state, the load current is being supplied solely
from the output capacitor. As the output voltage droops,
the EA amplifier’s output rises above the sleep threshold
signaling the BURST comparator to trip and turn the top
MOSFET on. This process repeats at a rate that is
dependent on the load demand.
Short­Circuit Protection When the output is shorted to ground, the frequency of
the oscillator is reduced to about 400kHz, 1/4 the
nominal frequency. This frequency foldback ensures that
the inductor current has more time to decay, thereby
preventing runaway. The oscillator’s frequency will
progressively increase to 1.5MHz when VFB or VOUT
rises above 0V.
Dropout Operation As the input supply voltage decreases to a value
approaching 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 until it reaches 100% duty cycle.
The output voltage will then be determined by the input
voltage minus the voltage drop across the P-channel
MOSFET and the inductor.
An important detail to remember is that at low input
supply voltages, the RDS(ON) of the P-channel switch
increases (see Typical Performance Characteristics).
Therefore, the user should calculate the power
dissipation when the TD6817 is used at 100% duty cycle
with low input voltage (See Thermal Considerations in
the Applications Information section).
December, 20, 2009 Techcode Semiconductor Limited www.tongchuangwei.com
10 Techcode®
DATASHEET
1.5MHz 2A Synchronous Step-Down Regulator Dropout
TD6817
Function Description(Cont.) Low Supply Operation The TD6817 will operate with input supply voltages as
low as 2.5V, but the maximum allowable output current is
reduced at this low voltage. Figure 2 shows the reduction
in the maximum output current as a function of input
voltage for various output voltages.
Slope Compensation and Inductor Peak Current Slope compensation provides stability in constant
frequency architectures by preventing subharmonic
oscillations at high duty cycles. It is accomplished
internally by adding a compensating ramp to the inductor
current signal at duty cycles in excess of 40%. Normally,
this results in a reduction of maximum inductor peak
current for duty cycles >40%. However, the TD6817 uses
a patent-pending scheme that counteracts this
compensating ramp, which allows the maximum inductor
peak current to remain unaffected throughout all duty
cycles.
The basic TD6817 application circuit is shown in Figure
3. External component selection is driven by the load
requirement and begins with the selection of L followed
by CIN and COUT.
Inductor Selection For most applications, the value of the inductor will fall in
the range of 1uH to 4.7uH. Its value is chosen based on
the desired ripple current. Large value inductors lower
ripple current and small value inductors result in higher
ripple currents. Higher VIN or VOUT also increases the
ripple current as shown in equation 1. A reasonable
starting point for setting ripple current is DIL = 800mA
(40% of 2000mA).
The DC current rating of the inductor should be at least
equal to the maximum load current plus half the ripple
current to prevent core saturation. Thus, a 2200mA rated
inductor should be enough for most applications
(2000mA + 200mA). For better efficiency, choose a low
DC-resistance
inductor.
The inductor value also has an effect on Burst Mode
operation. The transition to low current operation begins
when the inductor current peaks fall to approximately
200mA. Lower inductor values (higher DIL) will cause
this to occur at lower load currents, which can cause a
dip in efficiency in the upper range of low current
operation. In Burst Mode operation, lower inductance
values will cause the burst frequency to increase.
Maximum Output Current vs Input Voltag December, 20, 2009 Techcode Semiconductor Limited www.tongchuangwei.com
11 Techcode®
DATASHEET
1.5MHz 2A Synchronous Step-Down Regulator Dropout
TD6817
Function Description(Cont.) Inductor Core Selection 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 electrical characteristics. The choice of which
style inductor to use often depends more on the price vs
size requirements and any radiated field/EMI
requirements than on what the TD6817 requires to
operate.
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 the
capacitor manufacturer’s ripple current ratings are often
based on 2000 hours of life. This makes it advisable to
further derate the capacitor, or choose a capacitor rated
at a higher temperature than required. Always consult
the manufacturer if there is any question.
The selection of COUT is driven by the required effective
series resistance (ESR). Typically, once the ESR
requirement for COUT has been met, the RMS current
rating generally far exceeds the IRIPPLE(P-P)
requirement. The output ripple DVOUT is determined by:
CIN and COUT Selection In continuous mode, the source current of the top
MOSFET is a square wave of duty cycle VOUT/VIN. To
prevent large voltage transients, a low ESR input
capacitor sized for the maximum RMS current must be
used. The maximum RMS capacitor current is given by:
where f = operating frequency, COUT = output
capacitanceand DIL = ripple current in the inductor. For a
fixed output voltage, the output ripple is highest at
maximum input voltage since DIL increases with input
voltage.
Aluminum electrolytic and dry tantalum capacitors are
both available in surface mount configurations. In the
case of tantalum, it is critical that the capacitors are
surge tested for use in switching power supplies. An
excellent choice is the AVX TPS series of surface mount
tantalum. These are specially constructed and tested for
low ESR so they give the lowest ESR for a given volume.
Other capacitor types include Sanyo POSCAP, Kemet
T510 and T495 series, and Sprague 593D and 595D
series. Consult the manufacturer for other specific
recommendations.
December, 20, 2009 Techcode Semiconductor Limited www.tongchuangwei.com
12 Techcode®
DATASHEET
1.5MHz 2A Synchronous Step-Down Regulator Dropout
TD6817
Function Description(Cont.) 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. Because
the TD6817’s control loop does not depend on the output
capacitor’s ESR for stable operation, ceramic capacitors
can be used freely to achieve very low output ripple and
small circuit size.
However, care must be taken when ceramic capacitors
are used at the input and the 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
dielectrics have the best temperature and voltage
characteristics of all the ceramics for a given value and
size.
Output Voltage Programming In the adjustable version, the output voltage is set by a
resistive divider according to the following formula:
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure4. Figure 4:Setting the output Voltage Vout R1 R2 1.2V 150K 150K 1.5V 160K 240K 1.8V 150K 300K 2.5V 150K 470K 3.3V 150K 680K Table 1. Vout VS. R1, R2, Cf Select Table 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 in TD6817 circuits: 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 as illustrated in
Figure 5.
December, 20, 2009 Techcode Semiconductor Limited www.tongchuangwei.com
13 Techcode®
DATASHEET
1.5MHz 2A Synchronous Step-Down Regulator Dropout
TD6817
Function Description(Cont.) 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
Charateristics 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% total additional
loss.
Thermal Considerations Figure 4:Power Lost VS Load Current 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 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
thustheir 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:
In most applications the TD6817 does not dissipate
much heat due to its high efficiency. But, in applications
where the TD6817 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 TD6817 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)(qJA)
where PD is the power dissipated by the regulator and
qJA 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.
December, 20, 2009 Techcode Semiconductor Limited www.tongchuangwei.com
14 Techcode®
DATASHEET
1.5MHz 2A Synchronous Step-Down Regulator Dropout
TD6817
Function Description(Cont.) 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, which generates a
feedback error signal. The regulator loop then acts to
return VOUT to its steadystate value. During this
recovery time VOUT can be monitored for overshoot or
ringing that would indicate a stability problem. For a
detailed explanation of switching control loop theory.
A second, more severe transient is caused by switching
in loads with large (>1μF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in
parallel with COUT, causing a rapid drop in VOUT. No
regulator can deliver enough current to prevent this
problem if the load switch resistance is low and it is
driven quickly. The only solution is to limit the rise time of
the switch drive so that the load rise time is limited to
approximately (25 • CLOAD).Thus, a 10μF capacitor
charging to 3.3V would require a 250μs rise time, limiting
the charging current to about 130mA.
December, 20, 2009 Techcode Semiconductor Limited www.tongchuangwei.com
15 Techcode®
DATASHEET
1.5MHz 2A Synchronous Step-Down Regulator Dropout
TD6817
Package Information TSOT23­5 Package Outline Dimensions December, 20, 2009 Techcode Semiconductor Limited www.tongchuangwei.com
16 Techcode®
DATASHEET
1.5MHz 2A Synchronous Step-Down Regulator Dropout
TD6817
Design Notes
December, 20, 2009 Techcode Semiconductor Limited www.tongchuangwei.com
17