RT7296F - Richtek

RT7296F
3A, 17V Current Mode Synchronous Step-Down Converter
General Description
Features
The RT7296F 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

Internal Soft-Start Function
against

Power Save mode at light load
input

Power Good Indicator
under-voltage lockout, output under-voltage protection,

Input Under-Voltage Lockout
over-current protection, and thermal shutdown. The

Output Under-Voltage Protection
PWM frequency is adjustable by the EN/SYNC pin. The

Thermal Shutdown
high-side
mode control
shorted
complete
output.
protection
and
The
30m
RT7296F
functions
such
provides
as
RT7296F is available in the TSOT-23-8 (FC) package.
Ordering Information
RT7296F
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
Note :
Richtek products are :


08= : Product Code
DNN : Date Code
08=DNN
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
RT7296F
BOOT
C3
C1
L1
VOUT
SW
Enable
EN/SYNC
R5
PVCC
C2
R3
PG
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DS7296F-00
July 2015
R1
FB
R2
C4
GND
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RT7296F
Pin Configurations
PVCC
EN/SYNC
BOOT
8
7
6
5
2
3
4
VIN
SW
GND
PG
FB
(TOP VIEW)
TSOT-23-8 (FC)
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
PG
Power Good Output. This pin is an open drain which can be connected to
PVCC by a resistor. If output voltage achieve 90% of the normal voltage, the
PG pin will go high after 400s delay.
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. If using pull high resistor connected to VIN, the
recommended value range is 60k to 300k.
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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RT7296F
Function Block Diagram
PG
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 RT7296F 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 RT7296F’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 RT7296F 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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July 2015
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RT7296F
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
--
7
--
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
--
A
From Drain to Source
--
2
--
A
440
500
570
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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m
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RT7296F
Parameter
EN Input Voltage
Symbol
Test Conditions
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
--
VIL
Unit
V
A
EN Input Current
IEN
EN Turn-off Delay
ENtd-off
--
8
--
s
Power-Good Rising Threshol
PGvth-Hi
--
0.9
--
VFB
Power-Good Falling Threshol
PGvth-Lo
--
0.85
--
VFB
Power-Good Delay
PGTd
--
0.4
--
ms
Power-Good Sink Current
Capability
VPG
--
--
0.4
V
Power-Good Leakage Current
IPG-LEAK
--
--
1
A
3.7
3.9
4.1
V
--
650
--
mV
--
5
--
V
Input Under-Voltage
Lockout Threshold
VIN Rising
VUVLO
Sink 4mA
VIN Rising
Hysteresis VUVLO
PVCC Regulator
VCC
PVCC 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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RT7296F
Typical Application Circuit
VIN
4.5V to 17V
C1
22μF
C3
0.1μF
RT7296F
5
2
BOOT
VIN
6
Enable
EN/SYNC
SW
R6
L1
10
4.7μH
3
VOUT
Cff
7
C2
0.1μF
PVCC
R3
100k 1
PG
FB
8
R5
16k
GND
4
15pF
R1
40.2k
R2
13k
C4
44μF
Table 1. Suggested Component Values
VOUT (V)
R1 (k)
R2 (k)
R5 (k)
Cff (pF)
C2 (F)
C4 (F)
L1 (H)
1.0
20.5
84.5
82
15
0.1
44
2.2
3.3
40.2
13
16
15
0.1
44
4.7
5.0
40.2
7.68
16
15
0.1
44
4.7
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RT7296F
Typical Operating Characteristics
Output Voltage vs. Input Voltage
Efficiency vs. Output Current
3.46
100
3.42
80
VIN = 4.5V
70
VIN = 12V
60
VIN = 17V
Output Voltage(V)
Efficiency (%)
90
50
40
30
20
3.38
3.34
3.30
3.26
3.22
3.18
10
VOUT = 3.3V
0
IOUT = 3A
3.14
0
0.5
1
1.5
2
2.5
3
4
5
6
7
8
Output Current (A)
Output Voltage vs. Output Current
0.84
3.46
0.83
3.42
0.82
3.38
Output Voltage (V)
Reference Voltage (V)
10 11 12 13 14 15 16 17
Input Voltage (V)
Reference Voltage vs. Temperature
0.81
0.80
0.79
0.78
3.34
3.30
3.26
3.22
3.18
0.77
VIN = 12V, VOUT = 3.3V
IOUT = 1A
3.14
0.76
-50
-25
0
25
50
75
100
0
125
0.5
1
Temperature (°C)
2
2.5
3
EN Threshold vs. Temperature
1.50
4.20
1.45
EN Threshold (V)
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
Output Current (A)
UVLO Voltage vs. Temperature
UVLO Voltage (V)
9
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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RT7296F
Load Transient Response
Output Ripple Voltage
VOUT
VOUT
(50mV/Div)
(20mV/Div)
VLX
IOUT
(1A/Div)
(5V/Div)
VIN = 12V, VOUT = 3.3V,
IOUT = 1.5A to 3A to 1.5A, L = 4.7H
VIN = 12V, VOUT = 3.3V, IOUT = 3A, L = 4.7H
Time (200s/Div)
Time (1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
Time (2ms/Div)
Time (2ms/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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VIN = 12V, VOUT = 3.3V, IOUT = 3A
VIN = 12V, VOUT = 3.3V, IOUT = 3A
Time (5ms/Div)
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RT7296F
Application Information
The RT7296F 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.
RT7296F
SW
Output Voltage Selection
The resistive voltage divider allows the FB pin to sense
Figure 2. External Bootstrap Diode
a fraction of the output voltage as shown in Figure 1.
FB
R5
RT7296F
100nF
Inductor Selection
R1
VOUT
The inductor value and operating frequency determine
R2
the ripple current according to a specific input and
GND
output voltage. The ripple current ΔIL increases with
higher VIN and decreases with higher inductance.
Figure 1. Output Voltage Setting
by an external resistive voltage divider according to the

V
  V
IL   OUT    1  OUT 
VIN 
 f L  
following equation :
Having a lower ripple current reduces not only the ESR
For adjustable voltage mode, the output voltage is set
 R1 
VOUT  VFB  1 

 R2 
losses in the output capacitors but also the output
voltage ripple. High frequency with small ripple current
Where VFB is the feedback reference voltage (0.8V
can achieve highest efficiency operation. However, it
typ.). Table 2 lists the recommended resistors value for
requires a large inductor to achieve this goal.
common output voltages.
For the ripple current selection, the value of IL = 0.3
Table 2. Recommended Resistors Value
(IMAX) will be a reasonable starting point. The largest
VOUT (V)
R1 (k)
R2 (k)
R5 (k)
ripple current occurs at the highest VIN. To guarantee
1.0
20.5
84.5
82
that the ripple current stays below the specified
3.3
40.2
13
16
maximum, the inductor value should be chosen
5.0
40.2
7.68
16
according to the following equation :
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
 VOUT
 
VOUT 
L
 1

 f  IL(MAX)  
VIN(MAX) 

 
The
inductor's
current
rating
(caused
a
40°C
temperature rising from 25°C ambient) should be
greater than the maximum load current and its
saturation current should be greater than the short
circuit peak current limit.
is lower than 5.5V or duty ratio is higher than 65% .The
CIN and COUT Selection
bootstrap diode can be a low cost one such as IN4148
The input capacitance, CIN, is needed to filter the
or BAT54. The external 5V can be a 5V fixed input from
trapezoidal current at the source of the top MOSFET.
system or a 5V output (PVCC) of the RT7296F.
To prevent large ripple current, a low ESR input
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RT7296F
capacitor sized for the maximum RMS current should
ceramic capacitors with trace inductance can also lead
be used. The RMS current is given by :
to significant ringing.
IRMS  IOUT(MAX)
VOUT
VIN
VIN
1
VOUT
Thermal Considerations
For continuous operation, do not exceed absolute
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 

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 :
PD(MAX) = (TJ(MAX) TA) / θJA
where TJ(MAX) is the maximum junction temperature,
TA is the ambient temperature, and θJA is the junction
to ambient thermal resistance.
For recommended operating condition specifications,
the maximum junction temperature is 125°C. The
junction to ambient thermal resistance, θJA, is layout
dependent. For TSOT-23-8 (FC) package, the thermal
resistance, θJA, is 70°C/W on a standard four-layer
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
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
thermal test board. The maximum power dissipation at
TA = 25°C can be calculated by the following formula :
PD(MAX) = (125°C 25°C) / (70°C/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, θJA. The derating curve in Figure 3
allows the designer to see the effect of rising ambient
temperature on the maximum power dissipation.
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
current rating and long term reliability considerations.
Ceramic
capacitors
have
excellent
low
ESR
characteristics but can have a high voltage coefficient
and audible piezoelectric effects. The high Q of
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RT7296F
Layout Considerations
Maximum Power Dissipation (W)1
1.5
Four-Layer PCB
For best performance of the RT7296F, the following
1.2
layout guidelines must be strictly followed.

Input capacitor must be placed as close to the IC as
0.9
possible.

0.6
SW should be connected to inductor by wide and
short trace. Keep sensitive components away from
0.3
this trace.

0.0
0
25
50
75
100
125
Keep every trace connected to pin as wide as
possible for improving thermal dissipation.
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
PG
8
BOOT
5
SW
CIN
R2
The feedback components
must be connected as close
to the device as possible.
PVCC
Css
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 3. PCB Layout Guide
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RT7296F
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.
www.richtek.com
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is a registered trademark of Richtek Technology Corporation.
DS7296F-00
July 2015