RT7296A - Farnell

RT7296A
3A, 17V Current Mode Synchronous Step-Down Converter
General Description
Features
The RT7296A 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 : 500kHz
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

TTH Power-Save Mode
against

Internal 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
complete
output.
protection
and
The
30m
RT7296A provides
functions
such
as
PWM frequency is adjustable by the EN/SYNC pin. The
RT7296A is available in the TSOT-23-8 (FC) package.
Ordering Information
RT7296A
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
02= : Product Code
DNN : Date Code
02=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
RT7296A
BOOT
C3
C1
L1
VOUT
SW
Enable
EN/SYNC
PVCC
C2
R3
TTH
R5
R1
FB
R2
C4
GND
R4
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RT7296A
Pin Configurations
PVCC
EN/SYNC
BOOT
8
7
6
5
2
3
4
VIN
SW
GND
TTH
FB
(TOP VIEW)
TSOT-23-8 (FC)
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
TTH
Transition Threshold. Connect a resistor divider to let the RT7296A into power
saving mode under light loads. Connect to PVCC to force RT7296A into forced
PWM mode.
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
Bootstrap Supply for High-Side Gate Driver. Connect a 0.1F ceramic
capacitor between the BOOT and SW pins.
6
EN/SYNC
Enable Control Input. High = Enable. Apply an external clock to adjust the
switching frequency
7
PVCC
5V Bias Supply Output. Connect a 0.1F capacitor to ground.
8
FB
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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DS7296A-01
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RT7296A
Function Block Diagram
TTH
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
Internal SS
+ EA
+
Oscillator
LS Switch
Current
Comparator
Current
Sense
GND
Slope
Compensation
Operation
Under-Voltage Lockout Threshold
Operating Frequency and Synchronization
The IC includes an input Under Voltage Lockout
The internal oscillator runs at 500kHz (typ.) when the
Protection (UVLO). If the input voltage exceeds the
EN/SYNC pin is at logic-high level (>1.6V). If the EN
UVLO rising threshold voltage (3.9V), the converter
pin is pulled to low-level over 8s, the IC will shut down.
resets and prepares the PWM for operation. If the input
The RT7296A can be synchronized with an external
voltage falls below the UVLO falling threshold voltage
clock ranging from 200kHz to 2MHz applied to the
(3.25V) during normal operation, the device stops
EN/SYNC pin. The external clock duty cycle must be
switching. The UVLO rising and falling threshold
from 20% to 80% with logic-high level = 2V and
voltage includes a hysteresis to prevent noise caused
logic-low level = 0.8V.
reset.
Internal Regulator
Chip Enable
The internal regulator generates 5V power and drive
The EN pin is the chip enable input. Pulling the EN pin
internal circuit. When VIN is below 5V, PVCC will drop
low (<1.1V) will shut down the device. During shutdown
with VIN. A capacitor (>0.1F) between PVCC and
mode, the RT7296A’s quiescent current drops to lower
GND is required.
than 1A. Driving the EN pin high (>1.6V) will turn on
the device.
Internal Soft-Start Function
The RT7296A provides internal soft-start function. The
soft-start function is used to prevent large inrush
current while converter is being powered-up. The
soft-start time (VFB from 0V to 0.8V) is 1.5ms.
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May 2015
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RT7296A
Over-Current Protection
RT7296A
provides
cycle-by-cycle
over
current
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)
RT7296A provides Hiccup Mode of Under-Voltage
Protection (UVP). When the FB voltage drops below
half of the feedback reference voltage, VFB, the UVP
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 150C, the chip
will shut down the switching operation. The chip is
automatically
re-enabled
when
the
junction
temperature cools down by approximately 20C.
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DS7296A-01
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RT7296A
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 = 25C
TSOT-23-8 (FC) ---------------------------------------------------------------------------------------------------------- 1.428W

Package Thermal Resistance
(Note 2)
TSOT-23-8 (FC), JA --------------------------------------------------------------------------------------------------- 70C/W
TSOT-23-8 (FC), JC --------------------------------------------------------------------------------------------------- 15C/W

Lead Temperature (Soldering, 10 sec.) --------------------------------------------------------------------------- 260C

Junction Temperature --------------------------------------------------------------------------------------------------- 40C to 150C

Storage Temperature Range ----------------------------------------------------------------------------------------- 65C to 150C

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 ---------------------------------------------------------------------------------------- 40C to 125C

Ambient Temperature Range ---------------------------------------------------------------------------------------- 40C to 85C
Electrical Characteristics
(VIN = 12V, TA = 25C, 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, TTH = 0.5V
--
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
--
A
From Drain to Source
--
2
--
A
440
500
580
kHz
200
--
2000
kHz
VFB < 400mV
--
125
--
kHz
VFB = 0.7V
90
95
--
%
--
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
VFB = 0.75V
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RT7296A
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
--
650
--
mV
--
5
--
V
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
IVCC = 5mA
--
3
--
%
Soft-Start Time
tSS
FB from 0V to 0.8V
--
1.5
--
ms
Thermal Shutdown Temperature
TSD
--
150
--
o
Thermal Shutdown Hysteresis
TSD
--
o
--
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 = 25C 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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RT7296A
Typical Application Circuit
C3
0.1μF
RT7296A
5
2
BOOT
VIN
VIN
4.5V to 17V
Enable
C2
0.1μF
C1
22μF
6
EN/SYNC
7
PVCC
R3
91k
1 TTH
GND
R4
4
10k
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SW
FB
R6
L1
10
4.7μH
3
8
VOUT
R5
33k
R1
40.2k
R2
13k
C4
44μF
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RT7296A
Typical Operating Characteristics
Efficiency vs. Output Current
Output Voltage vs. Input Voltage
100
3.60
3.55
80
VIN = 7V
3.50
3.45
70
VIN = 12V
Output Voltage (V)
Efficiency (%)
90
VIN = 17V
60
50
40
30
20
3.40
3.35
3.30
3.25
3.20
3.15
3.10
10
3.05
VOUT = 3.3V
0
0
0.5
1
1.5
2
2.5
VOUT = 3.3V, IOUT = 3A
3.00
4
3
5
6
7
8
Reference Voltage vs. Temperature
10 11 12 13 14 15 16 17
Output Voltage vs. Load Current
0.84
3.46
0.83
3.42
0.82
3.38
Output Voltage (V)
Reference Voltage (V)
9
Input Voltage (V)
Output Current (A)
0.81
0.80
0.79
0.78
0.77
3.34
3.30
3.26
3.22
3.18
VIN = 12V, VOUT = 3.3V
TTH = 3V
0.76
3.14
-50
-25
0
25
50
75
100
125
0
0.5
1
2
2.5
3
EN Threshold vs. Temperature
1.50
4.20
1.45
EN Threshold (V)
UVLO Voltage (V)
UVLO Voltage vs. Temperature
4.40
4.00
Rising
3.80
3.60
1.40
Rising
1.35
1.30
1.25
3.40
Falling
3.20
1.5
Load Current (A)
Temperature (°C)
Falling
1.20
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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RT7296A
Load Transient Response
Output Ripple Voltage
VOUT
VOUT
(100mV/Div)
(20mV/Div)
IOUT
(1A/Div)
VIN = 12V, VOUT = 3.3V,
IOUT = 1.5A to 3A to 1.5A, L = 4.7H
VLX
(5V/Div)
Time (200s/Div)
Time (2s/Div)
Power On from EN
Power Off from EN
VOUT
VOUT
(2V/Div)
(2V/Div)
VEN
VEN
(2V/Div)
(2V/Div)
VLX
VLX
(10V/Div)
(10V/Div)
ILX
ILX
(3A/Div)
(3A/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 3A
VIN = 12V, VOUT = 3.3V, IOUT = 3A
Time (5ms/Div)
Time (500s/Div)
Power On from VIN
Power Off from VIN
VOUT
VOUT
(2V/Div)
(2V/Div)
VIN
VIN
(10V/Div)
(10V/Div)
VLX
(10V/Div)
VLX
(10V/Div)
ILX
ILX
(3A/Div)
(3A/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 3A
Time (5ms/Div)
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DS7296A-01
VIN = 12V, VOUT = 3.3V, IOUT = 3A, L = 4.7H
May 2015
VIN = 12V, VOUT = 3.3V, IOUT = 3A
Time (5ms/Div)
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RT7296A
Application Information
The RT7296A 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
BOOT
current can be up to 3A.
RT7296A
SW
Output Voltage Selection
The resistive voltage divider allows the FB pin to sense
a fraction of the output voltage as shown in Figure 1.
FB
R5
RT7296A
100nF
Figure 2. External Bootstrap Diode
The TTH Voltage setting
R1
VOUT
The TTH voltage is used to be change the transition
R2
threshold between power saving mode and CCM.
GND
Higher TTH voltage gets higher efficiency at light load
Figure 1. Output Voltage Setting
condition but larger output ripple; a lower TTH voltage
For adjustable voltage mode, the output voltage is set
can improve output ripple but degrades efficiency
by an external resistive voltage divider according to the
during light load condition. A resistor divider from PVCC
following equation :
(5V) of RT7296A can help to build TTH voltage, as
 R1 
VOUT  VFB  1 

 R2 
shown in Figure 3. It is recommended that TTH voltage
should be less than 0.6V.
Where VFB is the feedback reference voltage (0.8V
PVCC
typ.). Table 1 lists the recommended resistors value for
R3
TTH
RT7296A
common output voltages.
Table1. Recommended Resistors Value
VOUT (V)
R1 (k)
R2 (k)
R5 (k)
1.0
20.5
82
82
3.3
40.2
13
33
5.0
40.2
7.68
33
R4
GND
Figure 3. TTH Voltage Setting
Inductor Selection
The inductor value and operating frequency determine
External Bootstrap Diode
the ripple current according to a specific input and
Connect a 100nF low ESR ceramic capacitor between
output voltage. The ripple current ΔIL increases with
the BOOT pin and SW pin. This capacitor provides the
higher VIN and decreases with higher inductance.
gate driver voltage for the high side MOSFET. It is
recommended to add an external bootstrap diode

V
  V
IL   OUT    1  OUT 
f

L
V

 
IN 
between an external 5V and BOOT pin, as shown as
Having a lower ripple current reduces not only the ESR
Figure 2, for efficiency improvement when input voltage
losses in the output capacitors but also the output
is lower than 5.5V or duty ratio is higher than 65% .The
voltage ripple. High frequency with small ripple current
bootstrap diode can be a low cost one such as IN4148
can achieve highest efficiency operation. However, it
or BAT54. The external 5V can be a 5V fixed input from
requires a large inductor to achieve this goal.
system or a 5V output (PVCC) of the RT7296A.
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RT7296A
For the ripple current selection, the value of IL = 0.3
voltage since IL increases with input voltage. Multiple
(IMAX) will be a reasonable starting point. The largest
capacitors placed in parallel may be needed to meet
ripple current occurs at the highest VIN. To guarantee
the ESR and RMS current handling requirement. Dry
that the ripple current stays below the specified
tantalum, special polymer, aluminum electrolytic and
maximum, the inductor value should be chosen
ceramic capacitors are all available in surface mount
according to the following equation :
packages. Special polymer capacitors offer very low
 VOUT
 
VOUT 
L
 1

 f  IL(MAX)  
VIN(MAX) 

 
ESR value. However, it provides lower capacitance
The
inductor's
current
rating
density than other types. Although Tantalum capacitors
(caused
a
40°C
have the highest capacitance density, it is important to
temperature rising from 25°C ambient) should be
only use types that pass the surge test for use in
greater than the maximum load current and its
switching
saturation current should be greater than the short
capacitors have significantly higher ESR. However, it
circuit peak current limit.
can be used in cost-sensitive applications for ripple
power
supplies.
Aluminum
electrolytic
current rating and long term reliability considerations.
CIN and COUT Selection
Ceramic
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
capacitors
have
excellent
low
ESR
characteristics but can have a high voltage coefficient
and audible piezoelectric effects. The high Q of
ceramic capacitors with trace inductance can also lead
to significant ringing.
Thermal Considerations
For continuous operation, do not exceed absolute
This formula has a maximum at VIN = 2VOUT, where
maximum junction temperature. The maximum power
IRMS = IOUT / 2. This simple worst-case condition is
dissipation depends on the thermal resistance of the IC
commonly used for design because even significant
package, PCB layout, rate of surrounding airflow, and
deviations do not offer much relief.
difference between junction and ambient temperature.
Choose a capacitor rated at a higher temperature than
The maximum power dissipation can be calculated by
required. Several capacitors may also be paralleled to
the following formula :
meet size or height requirements in the design. The
PD(MAX) = (TJ(MAX) TA) / θJA
selection of COUT is determined by the required
where TJ(MAX) is the maximum junction temperature,
Effective Series Resistance (ESR) to minimize voltage
TA is the ambient temperature, and θJA is the junction
ripple. Moreover, the amount of bulk capacitance is
to ambient thermal resistance.
also a key for COUT selection to ensure that the control
For recommended operating condition specifications,
loop is stable. Loop stability can be checked by viewing
the maximum junction temperature is 125°C. The
the load transient response as described in a later
junction to ambient thermal resistance, θJA, is layout
section. The output ripple, VOUT, is determined by :
dependent. For TSOT-23-8 (FC) package, the thermal


1
VOUT  IL   ESR 

8fC
OUT 

resistance, θJA, is 70°C/W on a standard four-layer
The output ripple will be highest at the maximum input
TA = 25°C can be calculated by the following formula :
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RT7296A
PD(MAX) = (125°C 25°C) / (70°C/W) = 1.428W for
Layout Considerations
TSOT-23-8 (FC) package
For best performance of the RT7296A, the following
The maximum power dissipation depends on the
layout guidelines must be strictly followed.
operating ambient temperature for fixed TJ(MAX) and

thermal resistance, θJA. The derating curve in Figure 4
possible.
allows the designer to see the effect of rising ambient

temperature on the maximum power dissipation.
SW should be connected to inductor by wide and
short trace. Keep sensitive components away from
this trace.
1.5
Maximum Power Dissipation (W)1
Input capacitor must be placed as close to the IC as
Four-Layer PCB

1.2
Keep every trace connected to pin as wide as
possible for improving thermal dissipation.
0.9
0.6
0.3
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 4. 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
TTH
8
BOOT
5
SW
CIN
R2
The feedback components
must be connected as close
to the device as possible.
R4
R3
PVCC
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 5. PCB Layout Guide
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RT7296A
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.
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
DS7296A-01
May 2015
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
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