RT7296D
3A, 17V Current Mode Synchronous Step-Down Converter
General Description
Features
The RT7296D is a high-efficiency, 3A current mode
4.5V to 17V Input Voltage Range
synchronous step-down DC/DC converter with a wide
3A Output Current
input voltage range from 4.5V to 17V. The device
Internal N-Channel MOSFETs
integrates 80m
low-side
Current Mode Control
MOSFETs to achieve high efficiency conversion. The
Fixed Switching Frequency : 800kHz
current
architecture supports fast
Synchronous to External Clock : 200kHz to 2MHz
transient response and internal compensation. A
Cycle-by-Cycle Current Limit
cycle-by-cycle current limit function provides protection
Power Save Mode at Light Load
against
External Soft-Start Function
input
Input Under-Voltage Lockout
under-voltage lockout, output under-voltage protection,
Output Under-Voltage Protection
over-current protection, and thermal shutdown. The
Thermal Shutdown
high-side
mode control
shorted output.
complete
protection
and
30m
The RT7296D provides
functions
such
as
PWM frequency is adjustable by the EN/SYNC pin. The
RT7296D is available in the TSOT-23-8 (FC) package.
Ordering Information
RT7296D
Package Type
J8F : TSOT-23-8 (FC)
Lead Plating System
G : Green (Halogen Free and Pb Free)
Applications
Industrial and Commercial Low Power Systems
Computer Peripherals
LCD Monitors and TVs
Set-top Boxes
Marking Information
0A= : Product Code
DNN : Date Code
0A=DNN
Note :
Richtek products are :
RoHS compliant and compatible with the current
requirements of IPC/JEDEC J-STD-020.
Suitable for use in SnPb or Pb-free soldering processes.
Simplified Application Circuit
VIN
VIN
RT7296D
BOOT
C3
C1
L1
VOUT
SW
Enable
EN/SYNC
R5
C2
SS
C5
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July 2015
R1
FB
PVCC
R2
C4
GND
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RT7296D
Pin Configurations
PVCC
EN/SYNC
BOOT
8
7
6
5
2
3
4
VIN
SW
GND
SS
FB
(TOP VIEW)
TSOT-23-8 (FC)
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
SS
Soft-Start Control Input. SS control the soft-start period. Connect a capacitor
from SS to GND to set the soft-start period.
2
VIN
Power Input. Support 4.5V to17V Input Voltage. Must bypass with a suitable
large ceramic capacitor at this pin.
3
SW
Switch Node. Connect to external L-C filter.
4
GND
System Ground.
5
BOOT
6
EN/SYNC
7
PVCC
8
FB
Bootstrap Supply for High-Side Gate Driver. Connect a 0.1F ceramic
capacitor between the BOOT and SW pins.
Enable Control Input. High = Enable. Apply an external clock to adjust the
switching frequency. If using pull high resistor connected to VIN, the
recommended value range is 60k to 300k.
5V Bias Supply Output. Connect a 0.1F capacitor to ground.
Feedback Voltage Input. The pin is used to set the output voltage of the
converter to regulate to the desired voltage via a resistive divider. Feedback
reference = 0.8V.
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RT7296D
Function Block Diagram
VIN
PVCC
Internal
Regulator
Current
Sense
UVLO
BOOT
UVLO
Shutdown
- Comparator
EN/SYNC
1.4V
0.4V
+
BOOT
Logic &
Protection
Control
Power
Stage &
Deadtime
Control
+
SW
UV
Comparator
HS Switch
Current
Comparator
1pF
50pF 400k
FB
0.807V
+ EA
+
Oscillator
LS Switch
Current
Comparator
Current
Sense
GND
Slope
Compensation
11µA
SS
Operation
Under Voltage Lockout Threshold
Operating Frequency and Synchronization
The IC includes an input Under Voltage Lockout
Protection (UVLO). If the input voltage exceeds the
UVLO rising threshold voltage (3.9V), the converter
resets and prepares
The internal oscillator runs at 500kHz (typ.) when the
EN/SYNC pin is at logic-high level (>1.6V). If the EN
the PWM for operation. If the input voltage falls below
the UVLO falling threshold voltage (3.25V) during
normal operation, the device stops switching. The
UVLO rising and falling threshold voltage includes a
hysteresis to prevent noise caused reset.
clock ranging from 200kHz to 2MHz applied to the
EN/SYNC pin. The external clock duty cycle must be
Chip Enable
The internal regulator generates 5V power and drive
internal circuit. When VIN is below 5V, PVCC will drop
with VIN. A capacitor(>0.1F) between PVCC and
GND is required.
The EN pin is the chip enable input. Pulling the EN pin
low (1.6V) will turn on
pin is pulled to low-level over 8s, the IC will shut down.
The RT7296D can be synchronized with an external
from 20% to 80% with logic-high level = 2V and
logic-low level = 0.8V.
Internal Regulator
Soft-Start Function
the device.
The RT7296D provides external soft-start function. The
soft-start function is used to prevent large inrush
current while converter is being powered-up. The
soft-start timing can be programmed by the external
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RT7296D
capacitor between SS pin and GND. The Chip provides
a 11A charge current for the external capacitor.
Over Current Protection
RT7296D provides cycle-by-cycle over current limit
protection. When the inductor current peak value
reaches current limit, IC will turn off High Side MOS to
avoid over current.
Under Voltage Protection (Hiccup Mode)
RT7296D provides Hiccup Mode of Under Voltage
Protection (UVP). When the FB voltage drops below
half of the feedback reference voltage, VFB, the UVP
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function will be triggered and the IC will shut down for a
period of time and then recover automatically. The
Hiccup Mode of UVP can reduce input current in
short-circuit conditions.
Thermal Shutdown
Thermal shutdown is implemented to prevent the chip
from operating at excessively high temperatures. When
the junction temperature is higher than 150oC, the chip
will shutdown the switching operation. The chip is
automatically re-enabled when the junction temperature
cools down by approximately 20oC.
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RT7296D
Absolute Maximum Ratings
(Note 1)
Supply Input Voltage, VIN --------------------------------------------------------------------------------------------- 0.3V to 20V
Switch Voltage, SW ------------------------------------------------------------------------------------------------------ 0.3V to VIN + 0.3V
BOOT to SW, VBOOT – SW --------------------------------------------------------------------------------------------- 0.3V to 6V
Other Pins------------------------------------------------------------------------------------------------------------------- 0.3V to 6V
Power Dissipation, PD @ TA = 25C
TSOT-23-8 (FC) ---------------------------------------------------------------------------------------------------------- 1.428W
Package Thermal Resistance
(Note 2)
TSOT-23-8 (FC), JA --------------------------------------------------------------------------------------------------- 70C/W
TSOT-23-8 (FC), JC --------------------------------------------------------------------------------------------------- 15C/W
Lead Temperature (Soldering, 10 sec.) --------------------------------------------------------------------------- 260C
Junction Temperature --------------------------------------------------------------------------------------------------- 40C to 150C
Storage Temperature Range ----------------------------------------------------------------------------------------- 65C to 150C
ESD Susceptibility
(Note 3)
HBM (Human Body Model) ------------------------------------------------------------------------------------------- 2kV
Recommended Operating Conditions
(Note 4)
Supply Input Voltage, VIN ---------------------------------------------------------------------------------------4.5V to 17V
Junction Temperature Range ---------------------------------------------------------------------------------------- 40C to 125C
Ambient Temperature Range ---------------------------------------------------------------------------------------- 40C to 85C
Electrical Characteristics
(VIN = 12V, TA = 25C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
Shutdown Supply Current
VEN = 0V
--
--
1
A
Quiescent Current with no Load
at DCDC Output
VEN = 2V, VFB = 1V
--
0.8
1
mA
0.799
0.807
0.815
V
--
10
50
nA
Feedback Voltage
VFB
Feedback Current
IFB
Switch
On-Resistance
VFB = 820mV
High-Side
RDS(ON)_H
--
80
--
Low-Side
RDS(ON)_L
--
30
--
--
--
1
A
Under 40% duty-cycle
4.2
5
5.8
A
From Drain to Source
--
2
--
A
VFB = 0.75V
--
800
--
kHz
200
--
2000
kHz
VFB < 400mV
--
125
--
kHz
VFB = 0.7V
87
92
--
%
--
60
--
ns
Switch Leakage
Current Limit
VEN = 0V, VSW = 0V
ILIM
Low-Side Switch Current Limit
Oscillation Frequency
fOSC
SYNC Frequency Range
f SYNC
Fold-Back Frequency
Maximum Duty-Cycle
DMAX
Minimum On-Time
tON
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RT7296D
Parameter
EN Input Voltage
Symbol
Min
Typ
Max
Logic-High VIH
1.2
1.4
1.6
Logic-Low
1.1
1.25
1.4
VEN = 2V
--
2
--
VEN = 0V
--
0
--
--
8
--
s
3.7
3.9
4.1
V
530
610
690
mV
--
5
--
V
--
1.5
3
%
A
VIL
EN Input Current
IEN
EN Turn-off Delay
ENtd-off
Input Under-Voltage
Lockout Threshold
VIN Rising
Test Conditions
VUVLO
VIN Rising
Hysteresis VUVLO
Unit
V
A
VCC Regulator
VCC
VCC Load Regulation
VLOAD
Soft-Start Charge Current
ISS
8
11
14
Thermal Shutdown Temperature
TSD
--
150
--
o
Thermal Shutdown Hysteresis
TSD
--
o
IVCC = 5mA
--
20
C
C
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. JA is measured at TA = 25C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. JC is
measured at the exposed pad of the package.
Note 3. Devices are ESD sensitive. Handling precaution recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
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RT7296D
Typical Application Circuit
C3
0.1μF
RT7296D
5
2
BOOT
VIN
VIN
4.5V to 17V
C1
22μF
6
Enable
7
C2
0.1μF
EN/SYNC
SW
R6
10 L1
3.3μH
3
PVCC
1 SS
C5
22nF
VOUT
3.3V
Cff
FB
8
R5
5.6k
R1
40.2k
R2
13k
GND
4
C4
44μF
Table 1. Suggested Component Values
VOUT (V)
R1 (k)
R2 (k)
R5 (k)
Cff (pF)
C4 (F)
L1 (H)
1.0
20.5
84.5
34
33
44
1
3.3
40.2
13
5.6
33
44
3.3
5.0
40.2
7.68
2
33
44
3.3
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RT7296D
Typical Operating Characteristics
Efficiency vs. Output Current
Output Voltage vs. Input Voltage
120
3.46
3.42
VIN = 7V
80
VIN = 12V
60
3.38
Output Voltage (V)
Efficiency (%)
100
VIN = 17V
40
3.34
3.30
3.26
3.22
20
3.18
VOUT = 3.3V
VOUT = 3.3V
0
0
0.5
1
1.5
2
3.14
2.5
4
3
5
6
7
8
10 11 12 13 14 15 16 17
Input Voltage (V)
Output Current (A)
Reference Voltage vs. Temperature
Output Voltage vs. Output Current
0.84
3.46
0.83
3.42
0.82
3.38
Output Voltage (V)
Reference Voltage (V)
9
0.81
0.80
0.79
0.78
3.34
3.30
3.26
3.22
3.18
0.77
IOUT = 1A
VIN = 12V, VOUT = 3.3V
3.14
0.76
-50
-25
0
25
50
75
100
0
125
0.5
1
UVLO Voltage vs. Temperature
2
2.5
3
EN Threshold vs. Temperature
4.40
1.50
4.20
1.45
Rising
4.00
EN Threshold (V)
UVLO Voltage (V)
1.5
Output Current (A)
Temperature (°C)
3.80
3.60
Falling
3.40
Rising
1.40
1.35
1.30
Falling
1.25
1.20
3.20
VOUT = 3.3V, IOUT = 0A
VOUT = 3.3V, IOUT = 0A
1.15
3.00
-50
-25
0
25
50
75
100
Temperature (°C)
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125
-50
-25
0
25
50
75
100
125
Temperature (°C)
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Load Transient Response
Output Ripple Voltage
VOUT
(20mV/Div)
VOUT
(50mV/Div)
VIN = 12V, VOUT = 3.3V,
L = 3.3H, IOUT = 3A
VIN = 12V, VOUT = 3.3V, L = 3.3H,
IOUT = 1.5A to 3A to 1.5A
IOUT
(1A/Div)
VLX
(5V/Div)
Time (200s/Div)
Time (2s/Div)
Power On from EN
Power Off from EN
VOUT
(2V/Div)
VOUT
(2V/Div)
VEN
(2V/Div)
VEN
(2V/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 3A
VLX
(10V/Div)
VLX
(10V/Div)
ILX
(3A/Div)
ILX
(3A/Div)
Time (2ms/Div)
Time (2ms/Div)
Power On from VIN
Power Off from VIN
VOUT
(2V/Div)
VOUT
(2V/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 3A
VIN = 12V, VOUT = 3.3V, IOUT = 3A
VIN
(10V/Div)
VIN
(10V/Div)
VLX
(10V/Div)
VLX
(10V/Div)
ILX
(3A/Div)
ILX
(3A/Div)
Time (5ms/Div)
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VIN = 12V, VOUT = 3.3V, IOUT = 3A
July 2015
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RT7296D
Application Information
The RT7296D is a high voltage buck converter that can
5V
support the input voltage range from 4.5V to 17V and
the input voltage range from 4.5V to 17V and the output
current can be up to 3A.
BOOT
RT7296D
Output Voltage Selection
SW
The resistive voltage divider allows the FB pin to sense
a fraction of the output voltage as shown in Figure 1.
FB
R5
RT7296D
100nF
R1
Figure 2. External Bootstrap Diode
External Soft-Start Capacitor
VOUT
R2
RT7296D provides external soft-start function. The
soft-start function is used to prevent large inrush
GND
current while converter is being powered-up. The
soft-start timing can be programmed by the external
Figure 1. Output Voltage Setting
For adjustable voltage mode, the output voltage is set
by an external resistive voltage divider according to the
following equation :
R1
VOUT VFB 1
R2
capacitor (CSS) between SS pin and GND. The Chip
provides a 11A charge current (ISS) for the external
capacitor. The soft-start time (tSS, VREF is from 0V to
0.8V) can be calculated by the following formula :
tSS (ms) =
Where VFB is the feedback reference voltage (0.807V
typ.). Table 1 lists the recommended resistors value for
common output voltages.
Table 2. Recommended Resistors Value
VOUT (V)
R1 (k)
R2 (k)
R5 (k)
1.0
20.5
84.5
34
3.3
40.2
13
5.6
5.0
40.2
7.68
2
CSS (nF) 1.3
ISS ( A)
Inductor Selection
The inductor value and operating frequency determine
the ripple current according to a specific input and
output voltage. The ripple current IL increases with
higher VIN and decreases with higher inductance.
V
V
IL OUT 1 OUT
VIN
f L
Having a lower ripple current reduces not only the ESR
External Bootstrap Diode
Connect a 100nF low ESR ceramic capacitor between
the BOOT pin and SW pin. This capacitor provides the
gate driver voltage for the high side MOSFET. It is
recommended to add an external bootstrap diode
between an external 5V and BOOT pin, as shown as
Figure 2, for efficiency improvement when input voltage
is lower than 5.5V or duty ratio is higher than 65% .The
bootstrap diode can be a low cost one such as IN4148
or BAT54. The external 5V can be a 5V fixed input from
system or a 5V output (PVCC) of the RT7296D.
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losses in the output capacitors but also the output
voltage ripple. High frequency with small ripple current
can achieve highest efficiency operation. However, it
requires a large inductor to achieve this goal.
For the ripple current selection, the value of IL =
0.3(IMAX) will be a reasonable starting point. The
largest ripple current occurs at the highest VIN. To
guarantee that the ripple current stays below the
specified maximum, the inductor value should be
chosen according to the following equation :
VOUT
VOUT
L
1
f IL(MAX)
VIN(MAX)
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RT7296D
The inductor's current rating (caused a 40C
temperature rising from 25C ambient) should be
greater than the maximum load current and its
current rating and long term reliability considerations.
Ceramic capacitors have excellent low ESR
characteristics but can have a high voltage coefficient
saturation current should be greater than the short
circuit peak current limit.
and audible piezoelectric effects. The high Q of
ceramic capacitors with trace inductance can also lead
to significant ringing.
CIN and COUT Selection
The input capacitance, CIN, is needed to filter the
trapezoidal current at the source of the top MOSFET.
To prevent large ripple current, a low ESR input
capacitor sized for the maximum RMS current should
be used. The RMS current is given by :
IRMS IOUT(MAX)
VOUT
VIN
VIN
1
VOUT
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature.
The maximum power dissipation can be calculated by
the following formula :
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.
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 required
Effective Series Resistance (ESR) to minimize voltage
ripple. Moreover, the amount of bulk capacitance is
also a key for COUT selection 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, VOUT, is determined by :
VOUT
1
IL ESR
8fCOUT
The output ripple will be highest at the maximum input
voltage since IL increases with input voltage. Multiple
capacitors placed in parallel may be needed to meet
PD(MAX) = (TJ(MAX) TA) / JA
where TJ(MAX) is the maximum junction temperature,
TA is the ambient temperature, and JA is the junction to
ambient thermal resistance.
For recommended operating condition specifications,
the maximum junction temperature is 125C. The
junction to ambient thermal resistance, JA, is layout
dependent. For TSOT-23-8 (FC) package, the thermal
resistance, JA, is 70C/W on a standard JEDEC 51-7
four-layer thermal test board. The maximum power
dissipation at TA = 25C can be calculated by the
following formula :
PD(MAX) = (125C 25C) / (70C/W) = 1.428W for
TSOT-23-8 (FC) package
The maximum power dissipation depends on the
operating ambient temperature for fixed TJ(MAX) and
thermal resistance, JA. The derating curve in Figure 3
allows the designer to see the effect of rising ambient
temperature on the maximum power dissipation.
the ESR and RMS current handling requirement. Dry
tantalum, special polymer, aluminum electrolytic and
ceramic capacitors are all available in surface mount
packages. Special polymer capacitors offer very low
ESR value. However, it provides lower capacitance
density than other types. Although Tantalum capacitors
have the highest capacitance density, it is important to
only use types that pass the surge test for use in
switching power supplies. Aluminum electrolytic
capacitors have significantly higher ESR. However, it
can be used in cost-sensitive applications for ripple
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RT7296D
Layout Considerations
Maximum Power Dissipation (W)1
1.6
Four-Layer PCB
For best performance of the RT7296D, the following
1.4
layout guidelines must be strictly followed.
1.2
1.0
Input capacitor must be placed as close to the IC as
possible.
0.8
0.6
SW should be connected to inductor by wide and
short trace. Keep sensitive components away from
0.4
this trace.
0.2
0.0
0
25
50
75
100
Keep every trace connected to pin as wide as
possible for improving thermal dissipation.
125
Ambient Temperature (°C)
Figure 3. Derating Curve of Maximum Power
Dissipation
SW should be connected to inductor by Wide and
short trace. Keep sensitive components away from
this trace. Suggestion layout trace wider for thermal.
R1 FB
VOUT
4
3
6
SW
2
PVCC
7
EN/SYNC
GND
VIN
VOUT
CIN COUT
COUT
SS
8
BOOT
5
SW
CIN
R2
Css
The feedback components
must be connected as close
to the device as possible.
GND
Via can help to reduce
power trace and improve
thermal dissipation.
Input capacitor must be placed as close
to the IC as possible. Suggestion layout
trace wider for thermal.
Figure 4. PCB Layout Guide
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RT7296D
Outline Dimension
Dimensions In Millimeters
Symbol
Dimensions In Inches
Min.
Max.
Min.
Max.
A
0.700
1.000
0.028
0.039
A1
0.000
0.100
0.000
0.004
B
1.397
1.803
0.055
0.071
b
0.220
0.380
0.009
0.015
C
2.591
3.000
0.102
0.118
D
2.692
3.099
0.106
0.122
e
0.585
0.715
0.023
0.028
H
0.080
0.254
0.003
0.010
L
0.300
0.610
0.012
0.024
TSOT-23-8 (FC) Surface Mount Package
Richtek Technology Corporation
14F, No. 8, Tai Yuen 1st Street, Chupei City
Hsinchu, Taiwan, R.O.C.
Tel: (8863)5526789
Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should
obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot assume
responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be accurate and
reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may
result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries.
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