TMI3280
600KHz, 18V,2A Synchronous Step-Down Converter
FEATURES
High Efficiency: Up to 95%
600KHz Frequency Operation
2.0A Output Current
Up to 96% duty
3.5V to 18V Input Voltage Range
0.6V Reference
Constant on time(COT) Mode Control for
Excellent Line and Load Transient Response
Integrated internal compensation
Stable with Low ESR Ceramic Output
Capacitors
Over Current Protection with Hiccup-Mode
Input overvoltage protection(OVP)
Thermal Shutdown
Inrush Current Limit and Soft Start
Available in SOT23-6 Package
-40°C to +85°C Temperature Range
GENERAL DESCRIPTION
The TMI3280 is a fully integrated, high–
efficiency 2.0A synchronous rectified
step-down converter.
The TMI3280 operates at high efficiency
over a wide output current load range. This
TMI3280 adopts the instant PWM
architecture to achieve fast transient
responses for high step down applications
and high efficiency at light loads.
The TMI3280 requires a minimum number
of readily available standard external
components and is available in a 6-pin
SOT23 ROHS compliant package.
APPLICATIONS
Distributed Power Systems
Digital Set Top Boxes
Flat Panel Television and Monitors
Notebook computer
Wireless and DSL Modems
TYPICAL APPILCATION
Figure 1. Basic Application Circuit
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TMI3280 V1.2
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TMI3280
ABSOLUTE MAXIMUM RATINGS (Note 1)
Parameter
Value
Unit
Input Supply Voltage ,EN
-0.3~23
V
LX Voltages
-0.3~23
V
FB Voltage
-0.3~6
V
BS Voltage
-0.3~25
V
Storage Temperature Range
-65~150
°C
Junction Temperture(Note2)
155
°C
Power Dissipation
600
mW
Lead Temperature(Soldering,10s)
260
°C
PIN CONFIGURATION
SOT23-6
Top Mark: S4CXXX (S4C: Device Code, XXX: Inside Code)
Part Number
Package
Top mark
Quantity/ Reel
TMI3280
SOT23-6
S4CXXX
3000
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TMI3280 V1.2
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TMI3280
PIN FUNCTIONS
Pin
Name
Function
1
BS
2
GND
3
FB
4
EN
5
IN
Power supply Pin
6
LX
Switching Pin
Bootstrap. A capacitor connected between LX and BST pins is required to form a floating
supply across the high-side switch driver.
Ground
Adjustable version feedback input. Connect FB to the center point of the external resistor
divider.
Drive this pin to a logic-high to enable the IC. Drive to a logic-low to disable the IC and
enter micro-power shutdown mode.
ESD RATING
Items
Description
Value
Unit
VESD
Human Body Model for all pins
±2000
V
JEDEC specification JS-001
RECOMMENDED OPERATING CONDITIONS
Items
Description
Voltage Range
IN
3.5
18
V
TA
Operating Temperature Range
-40
85
°C
TMI and SUNTO are the brands of TOLL microelectronic
TMI3280 V1.2
2016.11
Min
Max
Unit
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TMI3280
ELECTRICAL CHARACTERISTICS (Note 3)
( VIN=12V ,VOUT=5V , TA = 25°C, unless otherwise noted.)
Parameter
Conditions
Input Voltage Range
OVP
Min
Typ
3.5
Threshold
Max
Unit
18
V
19
UVLO Threshold
V
3.0
V
Supply Current in Operation
VEN=2.0V, VFB=1.1V
0.4
Supply Current in Shutdown
VEN =0 or EN = GND
1
Regulated Feedback Voltage
TA = 25°C,3.5V≤VIN ≤18V
0.588
0.6
0.6
mA
uA
0.612
V
High-Side Switch On-Resistance
120
mΩ
Low-Side Switch On-Resistance
80
mΩ
High-Side Switch Leakage Current
VEN=0V, VLX=0V
Upper Switch Current Limit
Minimum Duty Cycle
Oscillation Frequency
0
10
2.5
uA
A
600
KHz
96
%
Minimum On-Time
60
nS
Thermal Shutdown
160
℃
Maximum Duty Cycle
VFB=0.6V
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may
be impaired.
Note 2: TJ is calculated from the ambient temperature TA and power dissipation PD
according to the following formula: TJ = TA + (PD) x (250°C/W).
Note 3: 100% production test at +25°C. Specifications over the temperature range are
guaranteed by design and characterization.
Note 4: Dynamic supply current is higher due to the gate charge being delivered at the
switching frequency
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TMI3280 V1.2
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FUNCTIONAL DESCRIPTION
Internal Regulator
The TMI3280 is a current mode step down DC/DC converter that provides excellent transient
response with no extra external compensation components. This device contains an internal,
low resistance, high voltage power MOSFET, and operates at a high 600K operating
frequency to ensure a compact, high efficiency design with excellent AC and DC
performance.
Error Amplifier
The error amplifier compares the FB pin voltage with the internal FB reference (VFB) and
outputs a current proportional to the difference between the two. This output current is
then used to charge or discharge the internal compensation network to form the COMP
voltage, which is used to control the power MOSFET current. The optimized internal
compensation network minimizes the external component counts and simplifies the control
loop design.
Internal Soft-Start
The soft-start is implemented to prevent the converter output voltage from overshooting
during startup. When the chip starts, the internal circuitry generates a soft-start voltage (SS)
ramping up from 0V to 0.6V. When it is lower than the internal reference (REF), SS overrides
REF so the error amplifier uses SS as the reference. When SS is higher than REF, REF regains
control. The SS time is internally fixed to 1ms.
Over-Current-Protection and Hiccup
The TMI3280 has cycle-by-cycle over current limit when the inductor current peak value
exceeds the set current limit threshold. Meanwhile, output voltage starts to drop until FB is
below the Under-Voltage (UV) threshold, typically 30% below the reference. Once a UV is
triggered, the TMI3280 enters hiccup mode to periodically restart the part. This protection
mode is especially useful when the output is dead-short to ground. The average short circuit
current is greatly reduced to alleviate the thermal issue and to protect the regulator. The
TMI3280 exits the hiccup mode once the over current condition is removed.
Startup and Shutdown
If both VIN and EN are higher than their appropriate thresholds, the chip starts. The
reference block starts first, generating stable reference voltage and currents, and then the
internal regulator is enabled. The regulator provides stable supply for the remaining
circuitries. Three events can shut down the chip: EN low, VIN low and thermal shutdown. In
the shutdown procedure, the signaling path is first blocked to avoid any fault triggering. The
COMP voltage and the internal supply rail are then pulled down. The floating driver is not
subject to this shutdown command.
TMI and SUNTO are the brands of TOLL microelectronic
TMI3280 V1.2
2016.11
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TMI3280
FUNCTIONAL BLOCK DIAGRAM
Figure 2. TMI3280 Block Diagram
APPLICATION INFORMATION
Setting the Output Voltage
The external resistor divider is used to set the output voltage (see Typical Application on
page 1). The feedback resistor R1 also sets the feedback loop bandwidth with the internal
compensation capacitor. Choose R1 to be around 51kΩ for optimal transient response. R2 is
then given by:
R2
R1
Vout / VFB 1
Vout
R1(KΩ)
5.1V
3.3V
1.8V
1.55V
1.25V
1.20V
1.05V
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51
51
51
51
51
51
33
R2(KΩ)
6.8
11.3
25.5
33
47
51
47
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TMI3280 V1.2
2016.11
TMI3280
Selecting the Inductor
A 4.7μH to 22μH inductor with a DC current rating of at least 25% percent higher than the
maximum load current is recommended for most applications. For highest efficiency, the
inductor DC resistance should be less than 15mΩ. For most designs, the inductance value can
be derived from the following equation.
L
Vout (Vin Vout )
Vin I L f OSC
Where ΔIL is the inductor ripple current. Choose inductor ripple current to be approximately
30% if the maximum load current, 2A. The maximum inductor peak current is:
I L ( MAX ) I LOAD
IL
2
Under light load conditions below 100mA, larger inductance is recommended for improved
efficiency.
Setting the Output Capacitor
The output capacitor (C2) 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:
VOUT
VOUT VOUT
1
1
RESR
fS L VIN
8 fS C2
Where L is the inductor 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:
VOUT
VOUT
VOUT
1
2
8 fS L C2 VIN
In the case of tantalum or electrolytic capacitors, the ESR dominates the impedance at the
switching frequency. For simplification, the output ripple can be approximated to:
VOUT
VOUT VOUT
1
RESR
fS L VIN
The characteristics of the output capacitor also affect the stability of the regulation system.
The TMI3280 can be optimized for a wide range of capacitance and ESR values.
PCB Layout Guide
PCB layout is very important to achieve stable operation. It is highly recommended to
duplicate EVB layout for optimum performance. If change is necessary, please follow these
guidelines and take Figure 4 for reference.
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TMI3280 V1.2
2016.11
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TMI3280
1) Keep the path of switching current short and minimize the loop area formed by Input
capacitor, high-side MOSFET and low-side MOSFET.
2) Bypass ceramic capacitors are suggested to be put close to the Vin Pin.
3) Ensure all feedback connections are short and direct. Place the feedback resistors and
compensation components as close to the chip as possible.
4) VOUT, LX away from sensitive analog areas such as FB.
5) Connect IN, LX, and especially GND respectively to a large copper area to cool the chip to
improve thermal performance and long-term reliability.
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TMIandSUNTOarethebrandsofTOLLmicroelectronic.
TMI3280 V1.2
2016.11
TMI3280
PACKAGE INFORMATION
SOT23-6
TMI and SUNTO are the brands of TOLL microelectronic
TMI3280 V1.2
2016.11
www.toll-semi.com www.suntosemi.com
9
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