TD6821 [ETC]
1.5MHz Dual 1.5A S ynch r onous Step Down Re g ulato; 1.5MHz，双路1.5AS ynch ř onous降压再克ulato
| 型号： | TD6821 |
| 厂家： | 
ETC |
| 描述： | 1.5MHz Dual 1.5A S ynch r onous Step Down Re g ulato |
| 文件： | 总18页 (文件大小：1042K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
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DATASHEET
Techcode®
1.5MHz Dual 1.5A Synchronous Step Down Regulator
TD6821
汪工 TEL:13828719410 QQ:1929794238
General Description
Features
z
z
z
low Rds(on) for internal switches (top/bottom):
3-5.5V input voltage range
1.5MHz switching frequency minimizes
the external components
The TD6821 are high-efficiency 1.5MHz synchronous
step-down DC-DC regulator ICs capable of delivering
up to 1.5A output currents, respectively. The
TD6821 operates over a wide input voltage range
from 3V to 5.5V and integrate main switch and
synchronous switch with very low RDS(ON) to minimize
the conduction loss.
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z
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Internal softstart limits the inrush current
100% dropout operation
Compact and thermally enhanced package:
SOP8-PP
Low output voltage ripple and small external inductor
and
capacitor
sizes
are
achieved
with
1.5MHz switching frequency.
Applications
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LCD TV WiFi
Card GPS
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Access Point Router
Set Top Box
Package Types
Figure 1. Package Types of TD6821
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Techcode Semiconductor Limited
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DATASHEET
Techcode®
1.5MHz Dual 1.5A Synchronous Step Down Regulator
TD6821
Pin Configurations
Figure 2 Pin Configuration of TD6821 (Top View)
Pin Description
Pin Number
Pin Name Description
Enable Pin. EN is a digital input that turns the regulator on or off .Drive EN pin
high to turn on the regulator, drive it low to turn it off.
7,5
EN1,2
GND
Exposed
paddle
Ground Pin..
Power Switch Output Pin. SW is the switch node that supplies power to the
output.
2,4
1,3
8,6
LX1,2
IN1,2
FB1,2
Input pin. Decouple this pin to GND paddle with at least 10uF ceramic cap.
Feedback Pin. Through an external resistor divider network, FB senses the
output voltage and regulates it. The feedback threshold voltage is 0.6V.
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DATASHEET
Techcode®
1.5MHz Dual 1.5A Synchronous Step Down Regulator
TD6821
Ordering Information
TD6821 □ □
Circuit Type
M:SOP8-PP
Packing:
Blank：Tube
R:Type and Reel
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
Symbol
VIN
Value
-0.3 to 6
Vin+0.6
Vin+0.6
Internally limited
150
Unit
V
Input Voltage
Feedback Pin Voltage
Enable Pin Voltage
Power Dissipation
VFB
V
VEN
V
PD
mW
ºC
ºC
ºC
V
Operating Junction Temperature
Storage Temperature
TJ
TSTG
TLEAD
-65 to 150
260
Lead Temperature (Soldering, 10 sec)
ESD (HBM)
2000
MSL
Level3
50
Thermal Resistance-Junction to Ambient
Thermal Resistance-Junction to Case
ºC / W
ºC / W
RθJA
RθJC
10
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DATASHEET
Techcode®
1.5MHz Dual 1.5A Synchronous Step Down Regulator
TD6821
Recommended Operating Conditions
Parameter
Symbol
VIN
Min.
3
Max.
5.5
Unit
V
Input Voltage
Operating Junction Temperature
Operating Ambient Temperature
TJ
-40
-40
125
85
ºC
TA
ºC
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DATASHEET
Techcode®
1.5MHz Dual 1.5A Synchronous Step Down Regulator
TD6821
Electrical Characteristics
VCC = 5V Vout=2.5V,L=2.2uH,Cout=10uF,Imax=1A, Ta = 25℃ unless otherwise specified.
Parameters
Input voltage
Symbol
VIN
Test Condition
Min.
Typ.
Max.
5.5
Unit
V
3
Shutdown Supply Current
Feedback Voltage
ISTBY
VEN=0V
10
uA
VFB
IFB
0.588
-50
0.6
0.612
V
Feedback Bias Current
Oscillator Frequency
VFB=Vin
nA
FOSC
1.5
200
150
MHz
NFET RON
RDS(ON)N
RDS(ON)N
ILIM
mΩ
mΩ
A
PFET RON
PFET Current Limit
EN rising threshold
EN falling threshold
Input UVLO threshold
UVLO hyesteresis
Min ON Time
1.8
1.5
VENH
V
VENL
0.4
2.4
V
VUVLO
VHYS
V
0.1
50
V
ns
%
Max Duty Cyele
100
ºC
Thermal Shutdown Temperature
TSD
160
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DATASHEET
Techcode®
1.5MHz Dual 1.5A Synchronous Step Down Regulator
TD6821
Type Application Circuit
Figure 3. Type Application Circuit of TD6821
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DATASHEET
Techcode®
1.5MHz Dual 1.5A Synchronous Step Down Regulator
TD6821
Typical Operating Characteristics
Reference Voltage
Oscillator Frequency
Oscillator Frequency vs Supply Voltage
RDS(ON) vs Temperature
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DATASHEET
Techcode®
1.5MHz Dual 1.5A Synchronous Step Down Regulator
TD6821
Typical Operating Characteristics(Cont.)
RDS(ON) vs Input Voltage
Efficiency vs Output Current
Efficiency vs Output Current
Efficiency vs Output Current
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DATASHEET
Techcode®
1.5MHz Dual 1.5A Synchronous Step Down Regulator
TD6821
Typical Operating Characteristics(Cont.)
Efficiency vs Output Current
Output Voltage vs Output Current
Efficiency vs Input Voltage
Dynamic Supply Current vs Supply Voltage
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DATASHEET
Techcode®
1.5MHz Dual 1.5A Synchronous Step Down Regulator
TD6821
Typical Operating Characteristics(Cont.)
P-FET Leakage vs Temperature
N-FET Leakage vs Temperature
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DATASHEET
Techcode®
1.5MHz Dual 1.5A Synchronous Step Down Regulator
TD6821
Function Description
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.
Main Control Loop
The TD6821 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
ShortCircuit Protection
internal top power MOSFET is turned on each cycle When the output is shorted to ground, the frequency of
when the oscillator sets the RS latch, and turned off the oscillator is reduced to about 400kHz, 1/4 the
when the current comparator, ICOMP, resets the RS nominal frequency. This frequency foldback ensures that
latch. The peak inductor current at which ICOMP resets the inductor current has more time to decay, thereby
the RS latch, is controlled by
preventing runaway. The oscillator’s frequency will
the output of error amplifier EA. When the load current progressively increase to 1.5MHz when VFB or VOUT
increases, it causes a slight decrease in the feedback rises above 0V.
voltage, FB, relative to the 0.6V reference, which in turn,
causes the EA amplifier’s output voltage to increase until
Dropout Operation
the average inductor current matches the new load
current. While the top MOSFET is off, the bottom
As the input supply voltage decreases to a value
MOSFET is turned on until either the inductor current
approaching the output voltage, the duty cycle increases
starts to reverse, as indicated by the current reversal
toward the maximum on-time. Further reduction of the
comparator IRCMP, or the beginning of the next clock
supply voltage forces the main switch to remain on for
cycle.
more than one cycle until it reaches 100% duty cycle.
The output voltage will then be determined by the input
Burst Mode Operation
voltage minus the voltage drop across the P-channel
MOSFET and the inductor.
The TD6821 is capable of Burst Mode operation in
which the internal power MOSFETs operate
intermittently based on load demand.
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).
In Burst Mode operation, the peak current of the inductor Therefore, the user should calculate the power
is set to approximately 200mA regardless of the output dissipation when the TD6821 is used at 100% duty cycle
load. Each burst event can last from a few cycles at light with low input voltage (See Thermal Considerations in
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
20mA. In this sleep state, the load current is being
supplied solely from the output capacitor. As the output
voltage droops,
the Applications Information
section).
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DATASHEET
Techcode®
1.5MHz Dual 1.5A Synchronous Step Down Regulator
TD6821
Function Description(Cont.)
The basic TD6821 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.
Low Supply Operation
The TD6821 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.
Inductor Selection
For most applications, the value of the inductor will fall in
the range of 1mH to 4.7mH. 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 = 480mA
(40% of 1200mA).
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 TD6821
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 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 1320mA
rated inductor should be enough for most applications
(1200mA + 120mA). 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
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DATASHEET
Techcode®
1.5MHz Dual 1.5A Synchronous Step Down Regulator
TD6821
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.
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 TD6821 requires to
operate. Table 1 shows some typical surface mount
inductors that work well in TD6821 applications.
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:
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.
Table 1. Representative Surface Mount Inductors
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:
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DATASHEET
Techcode®
1.5MHz Dual 1.5A Synchronous Step Down Regulator
TD6821
Function Description(Cont.)
Using Ceramic Input and Output
Capacitors
Figure 4:Setting the output Voltage
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 TD6821’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.
Vout
1.2V
1.5V
1.8V
2.5V
3.3V
R1
R2
150K
160K
150K
150K
150K
150K
240K
300K
470K
680K
Table 2. Vout VS. R1, R2, Cf Select Table
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.
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:
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.
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 TD6821 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.
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 Figure4.
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DATASHEET
Techcode®
1.5MHz Dual 1.5A Synchronous Step Down Regulator
TD6821
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
In most applications the TD6821 does not dissipate
much heat due to its high efficiency. But, in applications
where the TD6821 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 TD6821 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:
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.
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.
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:
The junction temperature, TJ, is given by:TJ = TA + TR
where TA is the ambient temperature.
As an example, consider the TD6821 in dropout at an
input voltage of 2.7V, a load current of 800mA and an
ambient temperature of 70°C. From the typical
performance graph of switch resistance, the RDS(ON)
of the P-channel switch at 70°C is approximately 0.52W.
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DATASHEET
Techcode®
1.5MHz Dual 1.5A Synchronous Step Down Regulator
TD6821
Function Description(Cont.)
Therefore,
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
power dissipated by the part is:
PD = ILOAD 2 • RDS(ON) = 187.2mW
For the SOT-23 package, the qJA is 250°C/ W. Thus,
the junction temperature of the regulator is:
TJ = 70°C + (0.1872)(250) = 116.8°C
which is below the maximum junction temperature of
125°C.
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.
Note that at higher supply voltages, the junction
temperature is lower due to reduced switch resistance
(RDS(ON)).
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current.
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Techcode Semiconductor Limited
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DATASHEET
Techcode®
1.5MHz Dual 1.5A Synchronous Step Down Regulator
TD6821
Package Information
SOP8 Package Outline Dimensions
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Techcode Semiconductor Limited
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DATASHEET
Techcode®
1.5MHz Dual 1.5A Synchronous Step Down Regulator
TD6821
Design Notes
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