ETC TD2776

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DATASHEET
2.2A 32V Synchronous Rectified Step-Down Converter
TD2776(A)
汪工 TEL:13828719410 QQ:1929794238
General Description Features • 2.2A Output Current
• Wide 4.75V to 32V Operating Input Range
The TD2776(A) is a monolithic synchronous buck
regulator. The device integrates two 90mΩ
MOSFETs, and provides 2.2A of continuous load
current over a wide input voltage of 4.75V to 32V.
Current mode
control provides fast
transient
response and cycle-by-cycle current limit.
An adjustable soft-start prevents inrush current at
turn-on, and in shutdown mode the supply current
drops to 1µA.
This device, available in an SOP8-PP package,
provides a very compact solution with minimal external
components. • Integrated 90mΩ Power MOSFET Switches
• Output Adjustable from 0.923V to 30V
• Up to 93% Efficiency
• Programmable Soft-Start
• Stable with Low ESR Ceramic Output Capacitors
• Fixed 340KHz(TD2776)/600KHz(TD2776A)
Frequency
• Cycle-by-Cycle Over Current Protection
• Input Under Voltage Lockout
Applications • Distributed Power Systems
• Networking Systems
• FPGA, DSP, ASIC Power Supplies
• Green Electronics/ Appliances
•
Notebook Computers
Package Types Figure 1. Package Types of TD2776 October, 20, 2010. Techcode Semiconductor Limited www.tongchuangwei.com
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DATASHEET
2.2A 32V Synchronous Rectified Step-Down Converter
TD2776(A)
Pin Configurations Figure 2 Pin Configuration of TD2776(Top View) Pin Description Pin Number
Pin Name
Description
High-Side Gate Drive Boost Input. BS supplies the drive for the high-side N-Channel
1
BS
MOSFET switch. Connect a 0.01µF or greater capacitor from SW to BS to power the
high side switch.
Power Input. IN supplies the power to the IC, as well as the step-down converter
2
IN
switches. Drive IN with a 4.75V to 32V power source. Bypass IN to GND with a
suitably large capacitor to eliminate noise on the input to the IC. See Input Capacitor.
Power Switching Output. SW is the switching node that supplies power to the output.
3
SW
Connect the output LC filter from SW to the output load. Note that a capacitor is
required from SW to BS to power the high-side switch.
4
GND
5
FB
Ground.
Feedback Input. FB senses the output voltage to regulate that voltage. Drive FB
with a resistive voltage divider from the output voltage. The feedback threshold
is 0.923V. See Setting the Output Voltage.
Compensation Node. COMP is used to compensate the regulation control loop.
6
COMP
Connect a series RC network from COMP to GND to compensate the regulation
control loop. In some cases, an additional capacitor from COMP to GND is
required. See Compensation Components.
Enable Input. EN is a digital input that turns the regulator on or off. Drive EN high to
7
EN
turn on the regulator, drive it low to turn it off. Pull up with 100kΩ resistor for
automatic startup.
Soft-Start Control Input. SS controls the soft start period. Connect a capacitor from SS
8
SS
to GND to set the soft-start period. A 0.1µF capacitor sets the soft-start period to 15ms.
To disable the soft-start feature, leave SS unconnected.
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2.2A 32V Synchronous Rectified Step-Down Converter
TD2776(A)
Ordering Information TD2776 □
□ Circuit Type Packing:
Blank:Tube
R:Type and Reel
Package
M:SOP8-PP
Function Block
Figure 3 Function Block Diagram of TD2776 October, 20, 2010. Techcode Semiconductor Limited www.tongchuangwei.com
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2.2A 32V Synchronous Rectified Step-Down Converter
TD2776(A)
Absolute Maximum Ratings Parameter
Symbol
Value
Unit
Supply Voltage
VIN
-0.3 to 32
V
Switch Node Voltage
VSW
30
V
Boost Voltage
VBS
VSW – 0.3V to VSW+6V
V
Output Voltage
VOUT
0.923V to 30
V
–0.3V to +6V
V
TJ
150
ºC
Storage Temperature
TSTG
-65 to 150
ºC
Lead Temperature (Soldering, 10 sec)
TLEAD
260
ºC
2000
V
All Other Pins
Operating Junction Temperature
ESD (HBM)
MSL
Level3
Thermal Resistance-Junction to Ambient
Thermal Resistance-Junction to Case
RθJA
RθJC
90
45
ºC / W
ºC / W
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2.2A 32V Synchronous Rectified Step-Down Converter
TD2776(A)
Electrical Characteristics VIN = 12V, Ta = 25℃ unless otherwise specified. Parameters
Symbol
Shutdown Supply Current
Min.
VEN = 0V
VEN = 2.0V; VFB =
Supply Current
Feedback Voltage
Test Condition
1.0V
VFB
4.75V ≤ VIN ≤ 23V
0.900
Feedback Overvoltage Threshold
Typ.
Max.
Unit
1
3.0
µA
1.3
1.5
mA
0.923
0.946
V
1.1
V
400
V/V
800
µA/V
Error Amplifier Voltage Gain *
AEA
Error Amplifier Transconductance
GEA
High-Side Switch On Resistance *
RDS(ON)1
90
mΩ
Low-Side Switch On Resistance *
RDS(ON)2
90
mΩ
High-Side Switch Leakage
∆IC = ±10µA
VEN = 0V, VSW = 0V
Current
Upper Switch Current Limit
Minimum Duty Cycle
Lower Switch Current Limit
From Drain to Source
COMP to Current Sense
Transconductance
Oscillation Frequency
Short Circuit Oscillation
Frequency
Maximum Duty Cycle
10
2.4
GCS
3.4
A
1.1
A
4.8
A/V
TD2776
340
TD2776A
600
Fosc2
VFB = 0V
100
KHz
DMAX
VFB = 1.0V
90
%
220
ns
Fosc1
Minimum On Time *
EN Shutdown Threshold Voltage
µA
VEN Rising
1.1
1.5
KHz
2.0
V
EN Shutdown Threshold Voltage
Hysteresis
EN Lockout Threshold Voltage
EN Lockout Hysterisis
210
2.2
2.5
210
mV
2.7
V
mV
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2.2A 32V Synchronous Rectified Step-Down Converter
TD2776(A)
Electrical Characteristics(Cont.) VIN = 12V, Ta = 25℃ unless otherwise specified. Parameters
Symbol
Input Under Voltage Lockout
Threshold
Test Condition
VIN Rising
Input Under Voltage Lockout
Threshold Hysteresis
Soft-Start Current
VSS = 0V
Soft-Start Period
CSS = 0.1µF
Thermal Shutdown *
Min.
3.80
Typ.
4.10
Max.
4.40
Unit
V
210
mV
6
µA
15
160
ms
°C
Typical Performance Characteristics Figure 4. Steady State Test Figure 5. Steady State Test October, 20, 2010. Techcode Semiconductor Limited www.tongchuangwei.com
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2.2A 32V Synchronous Rectified Step-Down Converter
TD2776(A)
Figure 6. Startup through Enable Figure 7. Startup through Enable Figure 8. Shutdown through Enable Figure 9. Shutdown through Enable October, 20, 2010. Techcode Semiconductor Limited www.tongchuangwei.com
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2.2A 32V Synchronous Rectified Step-Down Converter
TD2776(A)
Figure 11. Short Circuit Test Figure 10. Load Transient Test Figure 12. Short Circuit Recovery October, 20, 2010. Techcode Semiconductor Limited www.tongchuangwei.com
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2.2A 32V Synchronous Rectified Step-Down Converter
TD2776(A)
Typical Application Circuit Fig13. TD2776 with 5V Output, 470µF/16V Electrolytic Output Capacitor October, 20, 2010. Techcode Semiconductor Limited www.tongchuangwei.com
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2.2A 32V Synchronous Rectified Step-Down Converter
TD2776(A)
Function Description Component Selection
Setting the Output Voltage
The output voltage is set using a resistive voltage
divider from the output voltage to FB pin.The voltage
divider divides the output voltage down to the
feedback voltage by the ratio:
Where VFB is the feedback voltage and VOUT is the
output voltage.Thus the output voltage is:
Where VOUT is the output voltage, VIN is the input
voltage, fS is the switching frequency, and ∆IL is the
peak-to-peak inductor ripple current.
Choose an inductor that will not saturate under the
maximum inductor peak current. The peak inductor
current can be calculated by:
Where ILOAD is the load current.
The choice of which style inductor to use mainly
depends on the price vs. size requirements and any
EMI requirements.
Optional Schottky Diode
R2 can be as high as 100kΩ, but a typical value is
During the transition between high-side switch and
10kΩ. Using the typical value for R2, R1 is determined
low-side switch, the body diode of the lowside power
by:
MOSFET conducts the inductor current. The forward
voltage of this body diode is high. An optional Schottky
diode may be paralleled between the SW pin and
For example, for a 3.3V output voltage, R2 is 10kΩ,
GND pin to improve overall efficiency. Table 1 lists
and R1 is 26.1kΩ.
example Schottky diodes and their Manufacturers.
Inductor
The inductor is required to supply constant current to
Part Number Voltage/Current Vendor
the output load while being driven by the switched
B140
40V, 1A
Diodes, Inc.
input voltage. A larger value inductor will result in less
SK14
40V, 1A Diodes, Inc. ripple current that will result in lower output ripple
MBRS140
40V, 1A International Rectifier
voltage. However,the larger value inductor will have a
Input Capacitor
larger physical size, higher series resistance, and/or
The input current to the step-down converter is
lower saturation current. A good rule for determining
discontinuous, therefore a capacitor is required to
the inductance to use is to allow the peak-to-peak
ripple current in the inductor to be approximately 30% supply the AC current to the step-down converter
while maintaining the DC input voltage. Use low ESR
of the maximum switch current limit. Also, make sure
capacitors for the best performance. Ceramic
that the peak inductor current is below the maximum
capacitors are preferred, but tantalum or low-ESR
switch current limit. The inductance value can be
electrolytic capacitors may also suffice. Choose X5R
calculated by:
or X7R dielectrics when using ceramic capacitors.
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2.2A 32V Synchronous Rectified Step-Down Converter
TD2776(A)
Since the input capacitor (C1) absorbs the input
switching current it requires an adequate ripple current
In the case of tantalum or electrolytic capacitors,the
rating. The RMS current in the input capacitor can be
ESR dominates the impedance at the switching
estimated by:
frequency. For simplification, the output ripple can be
approximated to:
The worst-case condition occurs at VIN = 2VOUT,where
IC1 = ILOAD/2. For simplification, choose the input
capacitor whose RMS current rating greater than half
of the maximum load current.
The input capacitor can be electrolytic, tantalum or
ceramic. When using electrolytic or tantalum
capacitors, a small, high quality ceramic capacitor, i.e.
0.1μF, should be placed as close to the IC as possible.
When using ceramic capacitors, make sure that they
have enough capacitance to provide sufficient charge
to prevent excessive voltage ripple at input. The input
voltage ripple for low ESR capacitors can be
estimated by:
Where C1 is the input capacitance value.
Output Capacitor
The output capacitor is required to maintain the DC
output voltage. Ceramic, tantalum, or low ESR
electrolytic capacitors are recommended. Low ESR
capacitors are preferred to keep the output voltage
ripple low. The output voltage ripple can be estimated
by:
Where C2 is the output capacitance value and RESR is
the equivalent series resistance (ESR) value of the
output capacitor.
In the case of ceramic capacitors, the impedance at
the switching frequency is dominated by the
capacitance. The output voltage ripple is mainly
caused by the capacitance. For simplification, the
output voltage ripple can be estimated by:
The characteristics of the output capacitor also affect
the stability of the regulation system. The TD2776 can
be optimized for a wide range of capacitance and ESR
values.
Compensation Components
TD2776 employs current mode control for easy
compensation and fast transient response. The
system stability and transient response are controlled
through the COMP pin. COMP pin is the output of the
internal transconductance error amplifier. A series
capacitor-resistor combination sets a pole-zero
combination to control the characteristics of the control
system.
The DC gain of the voltage feedback loop is given by:
Where AVEA is the error amplifier voltage gain;GCS is
the current sense transconductance and RLOAD is the
load resistor value.
The system has two poles of importance. One is due
to the compensation capacitor (C3) and the output
resistor of the error amplifier, and the other is due to
the output capacitor and the load resistor. These poles
are located at:
Where GEA is the error amplifier transconductance.
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2.2A 32V Synchronous Rectified Step-Down Converter
The system has one zero of importance, due to the
compensation capacitor (C3) and the compensation
resistor (R3). This zero is located at:
TD2776(A)
Determine the C3 value by the following equation:
The system may have another zero of importance, if
the output capacitor has a large capacitance and/or a
high ESR value. The zero,due to the ESR and
capacitance of the output capacitor, is located at:
Where R3 is the compensation resistor.
3. Determine if the second compensation capacitor
(C6) is required. It is required if the ESR zero of the
output capacitor is located at less than half of the
switching frequency, or the following relationship is
valid:
If this is the case, then add the second compensation
In this case (as shown in Figure 14), a third pole set by capacitor (C6) to set the pole fP3 at the location of the
the compensation capacitor (C6) and the
ESR zero. Determine the C6 value by the equation:
compensation resistor (R3) is used to compensate the
effect of the ESR zero on the loop gain. This pole is
located at:
External Bootstrap Diode
An external bootstrap diode may enhance the
efficiency of the regulator, the applicable
The goal of compensation design is to shape the
conditions of external BST diode are:
converter transfer function to get a desired loop gain.
z VOUT=5V or 3.3V; and
z Duty cycle is high:
The system crossover frequency where the feedback
loop has the unity gain is important. Lower crossover
frequencies result in slower line and load transient
In these cases, an external BST diode is
responses,while higher crossover frequencies could
cause system instability. A good rule of thumb is to set recommended from the output of the voltage regulator
to BST pin, as shown in Fig.14
the crossover frequency below one-tenth of the
switching frequency.
To optimize the compensation components, the
following procedure can be used.
1. Choose the compensation resistor (R3) to set the
desired crossover frequency.
Determine the R3 value by the following equation:
Figure14.Add Optional External Bootstrap Diode to Enhance Efficiency Where fC is the desired crossover frequency which is
The recommended external BST diode is IN4148, and
typically below one tenth of the switching frequency.
the BST cap is 0.1~1μF.
2. Choose the compensation capacitor (C3) to achieve
the desired phase margin. For applications with typical
inductor values, setting the compensation zero, fZ1,
below one-forth of the crossover frequency provides
sufficient phase margin. October, 20, 2010. Techcode Semiconductor Limited www.tongchuangwei.com
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2.2A 32V Synchronous Rectified Step-Down Converter
TD2776(A)
Package Information SOP8­pp Package Outline Dimensions October, 20, 2010. Techcode Semiconductor Limited www.tongchuangwei.com
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DATASHEET
2.2A 32V Synchronous Rectified Step-Down Converter
TD2776(A)
Design Notes
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