RT2875BQGCP [RICHTEK]
Synchronous Step-Down Converter;
| 型号： | RT2875BQGCP |
| 厂家： | 
RICHTEK TECHNOLOGY CORPORATION |
| 描述： | Synchronous Step-Down Converter |
| 文件： | 总16页 (文件大小：295K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
®
RT2875A/B
3A, 36V, Synchronous Step-Down Converter
General Description
Features
3A Output Current
Internal N-MOSFETs
Current Mode Control
The RT2875A/B is a high efficiency, current-mode
synchronousDC/DC step-down converter that can deliver
up to 3A output current over a wide input voltage range
from 4.5V to 36V. The device integrates 95mΩ high-side
and 70mΩ low-side MOSFETs to achieve high conversion
efficiency. The current-mode control architecture
supports fast transient response and simple external
compensation. A cycle-by-cycle current limit function
provides protection against shorted output and an external
soft-start eliminates input current surge during start-up.
The RT2875A/B provides complete protection functions
such as input under-voltage lockout, output under-voltage
protection, over-current protection and thermal shutdown.
Adjustable Switching Frequency : 300kHz to 2.1MHz
Adjustable Current Limit : 1.5A to 6A
Synchronous to External Clock : 300kHz to 2.1MHz
Adjustable Output Voltage from 0.6V to 24V
High Efficiency Up to 95%
Stable with Low ESR Ceramic Output Capacitors
Cycle-by-Cycle Current Limit
Input Under-Voltage Lockout
Output Under-Voltage Protection
Thermal Shutdown
AEC-Q100 Grade 2 Certification
The RT2875A/B is available in the thermal enhanced
TSSOP-14 (Exposed Pad) package.
RoHS Compliant and Halogen Free
Applications
Point of Load Regulator in Distributed Power Systems
Pin Configurations
(TOP VIEW)
Digital Set Top Boxes
14
SW
SW
PGND
RT/SYNC
AGND
RLIM
BOOT
VIN
VIN
PGOOD
EN
SS
Broadband Communications
Vehicle Electronics
2
3
4
5
6
7
13
12
11
10
9
PGND
15
8
COMP
FB
TSSOP-14 (Exposed Pad)
Simplified Application Circuit
VIN
BOOT
V
IN
C
BOOT
C
IN
RT2875A/B
L
V
SW
FB
OUT
Enable
EN
R1
R2
PGOOD
PGOOD
C
OUT
RLIM
C
COMP
R
COMP
RT/SYNC
COMP
SS
R
LIM
R
OSC
C
SS
AGND PGND
Copyright 2016 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS2875A/B-05 October 2016
www.richtek.com
1
RT2875A/B
Ordering Information
RT2875A/B
Marking Information
RT2875AQGCP
RT2875AQGCP : Product Number
Package Type
CP: TSSOP-14 (Exposed Pad)
RT2875AQ
GCPYMDNN
YMDNN : Date Code
Lead Plating System
G : Green (Halogen Free and Pb Free)
RT2875BQGCP
AQ : Latched UVP
BQ : Hiccup Mode UVP
RT2875BQGCP : Product Number
YMDNN : Date Code
RT2875BQ
GCPYMDNN
Note :
Richtek products are :
RoHS compliant and compatible with the current require-
ments of IPC/JEDEC J-STD-020.
Suitable for use in SnPb or Pb-free soldering processes.
Functional Pin Description
Pin No.
Pin Name
Pin Function
1, 2
SW
Switch Node. Connect to external L-C filter.
3,
Power Ground. The exposed pad must be soldered to a large PCB and
connected to PGND for maximum power dissipation.
PGND
15 (Exposed Pad)
Oscillator Resistor and External Frequency Synchronization Input. Must connect
a resistor from this pin to GND to set the switching frequency. If SYNC clock is
requested, connect an external clock to change the switching frequency.
4
RT/SYNC
5
6
AGND
RLIM
Analog Ground.
Current Limit Setting. Connect a resistor from this pin to GND to set the current
limit value.
Feedback Voltage Input. The pin is used to set the output voltage of the
converter to regulate to the desired via a resistive divider. Feedback reference =
0.6V.
7
FB
Compensation Node. COMP is used to compensate the regulation control loop.
Connect a series RC network from COMP to GND. In some cases, an additional
capacitor from COMP to GND is required.
8
9
COMP
SS
Soft-Start Time Setting. Connect a capacitor from SS to GND to set the
soft-start period.
10
11
EN
Enable Control Input. High = Enable.
Power Good Indicator Output.
PGOOD
Power Input. Support 4.5V to 36V input voltage. Must bypass with a suitable
large ceramic capacitor at this pin.
12, 13
14
VIN
Bootstrap Supply for High-Side Gate Driver. Connect a 0.1F ceramic capacitor
between the BOOT and SW pins.
BOOT
Copyright 2016 Richtek Technology Corporation. All rights reserved.
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2
DS2875A/B-05 October 2016
RT2875A/B
Function Block Diagram
PGOOD
VIN
6k
Internal
Regulator
-
+
EN
UVLO
Current
Sense
1.5V
Shutdown
Comparator
3.8V
BOOT
UVLO
+
0.55V
-
BOOT
SW
PGOOD
Comparator Protection
Logic &
Power
Stage &
Deadtime
Control
Control
0.3V
+
-
UV
Comparator
HS Switch
Current
Comparator
LS Switch
Current
Comparator
-
FB
Current
Sense
EA
0.6V
6µA
+
+
PGND
AGND
Slop
Compensation
Oscillator
COMP RT/SYNC
RLIM
SS
Operation
The RT2875A/B is current-mode synchronous step-down
converter. In normal operation, the high-sideN-MOSFET
is turned on when the S-R latch is set by the oscillator
and is turned off when the current comparator resets the
S-R latch. While the high-side N-MOSFET is turned off,
the low-side N-MOSFET is turned on to conduct the
inductor current until next cycle begins.
Internal Regulator
The regulator provides low voltage power to supply the
internal control circuits and the bootstrap power for high-
side gate driver.
Enable
The converter is turned on when the ENpin is higher than
1.6V. When the EN pin is lower than 0.4V, the converter
will enter shutdown mode and reduce the supply current
lower than 10μA.
Error Amplifier
The error amplifier adjusts its output voltage by comparing
the feedback signal (VFB) with the internal 0.6V reference.
When the load current increases, it causes a drop in the
feedback voltage relative to the reference, and then the
error amplifier's output voltage rises to allow higher inductor
current to match the load current.
Soft-Start (SS)
In order to prevent the converter output voltage from
overshooting during the startup period, the soft-start
function is necessary. The soft-start time is adjustable
by an external capacitor.
Switching Frequency
The switching frequency can be set by using extra resister
RT or external clock. Switching frequency range is from
300kHz to 2.1MHz.
Copyright 2016 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
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3
RT2875A/B
UV Comparator
If the feedback voltage is lower than 0.3V, the UV
Comparator will go high to turn off the high-side MOSFET.
The output under voltage protection is designed to operate
in Hiccup mode. When the UV condition is removed, the
converter will resume switching.
Current Setting
The current limit of high side MOSFET is adjustable by
an external resistor connected to the RLIM pin. The current
limit range is from 1.5A to 6A.
Thermal Shutdown
The over-temperature protection function will shut down
the switching operation when the junction temperature
exceeds 180°C. Once the junction temperature cools
down by approximately 15°C, the converter will
automatically resume switching.
Copyright 2016 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
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4
DS2875A/B-05 October 2016
RT2875A/B
Absolute Maximum Ratings (Note 1)
Supply Voltage, VIN ------------------------------------------------------------------------------------------------ −0.3V to 40V
Switch Voltage, SW ------------------------------------------------------------------------------------------------ −0.3V to (VIN + 0.3V)
BOOT to SW --------------------------------------------------------------------------------------------------------- −0.3V to 6V
PowerGood Voltage, PGOOD------------------------------------------------------------------------------------ −0.3V to 40V
Other Pins------------------------------------------------------------------------------------------------------------- −0.3V to 6V
Power Dissipation, PD @ TA = 25°C
TSSOP-14 (Exposed Pad) ---------------------------------------------------------------------------------------- 4.464W
Package Thermal Resistance (Note 2)
TSSOP-14 (Exposed Pad), θJA ---------------------------------------------------------------------------------- 28°C/W
TSSOP-14 (Exposed Pad), θJC ---------------------------------------------------------------------------------- 4.3°C/W
Lead Temperature (Soldering, 10 sec.)------------------------------------------------------------------------- 260°C
Junction Temperature ----------------------------------------------------------------------------------------------- 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 36V
Junction Temperature Range-------------------------------------------------------------------------------------- −40°C to 150°C
Ambient Temperature Range-------------------------------------------------------------------------------------- −40°C to 105°C
Electrical Characteristics
(VIN = 12V, TA = −40°C to 105°C, unless otherwise specified)
Parameter
Symbol
Test Conditions
= 0V
Min
Typ
Max Unit
Shutdown Supply Current
V
V
R
--
‐‐
10
A
EN
EN
Switching quiescent current with
no load at DCDC output
= 2V, V = 0.64V,
FB
--
--
1.3
mA
= 91k, R
= 169k
LIM
OSC
Feedback Voltage
V
4.5V V 36V
0.588
0.6
950
95
0.612
V
FB
IN
Error Amplifier Trans-conductance G
IC = ±10A
--
--
--
--
--
--
A/V
EA
High-Side
Low-Side
R
R
DS(ON)1
DS(ON)2
Switch On-
Resistance
m
70
High-Side Switch Leakage
Current
V
= 0V, V
= 0V
--
1
--
A
EN
SW
Current Limit Setting Rage
(Note 5)
1.5
1.79
3.52
4.84
--
--
2.1
4
6
A
A
A
A
A
High-Side Switch Current Limit 1
High-Side Switch Current Limit 2
High-Side Switch Current Limit 3
Low-Side Switch Current Limit
H
H
H
R
LIM
R
LIM
R
LIM
= 100k
2.41
4.48
6.16
--
OC1
OC2
OC3
= 47k
= 33k
5.5
2
From Drain to Source
COMP to Current Sense
Transconductance
G
--
5.2
--
--
A/V
CS
Include Sync mode and RT mode
set point
Switching Frequency Range
300
2100 kHz
Copyright 2016 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
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5
RT2875A/B
Parameter
Switching Frequency1
Switching Frequency2
Switching Frequency3
Symbol
Test Conditions
Min
275
Typ
305
0.98
2.1
Max Unit
335 kHz
f
f
f
R = 169k
OSC1
OSC2
OSC3
t
R = 51k
0.83
1.89
1.13 MHz
2.31 MHz
t
R = 23k
t
Short Circuit Oscillation
Frequency
V
V
= 0V, R
= 12V
= 100k,
FB
IN
OSC
--
31.25
--
kHz
ns
Minimum SYNC Pulse width
--
--
20
--
--
2
High-Level
SYNC Input Voltage
Minimum On-Time
EN Input Voltage
V
ns
V
Low- Level
0.8
--
--
--
t
100
1.5
0.2
4.1
300
90
--
ON
Logic-High
Hysteresis
V
1.4
--
1.6
--
IH
EN hysteresis voltage
VIN Rising
Hysteresis
Rising
V
--
--
V
UVLO
Input Under-Voltage Lockout
Threshold
V
--
--
mV
UVLO
--
--
Power Good Threshold
%
Falling
--
85
--
Power Good Output High
Leakage Current
V
= V
, V = 5.5V
REF PGOOD
--
30
--
nA
FB
Power Good Output Low
Soft-Start Charge Current
SW Discharge Resistance
Thermal Shutdown
I
= 0.4mA
--
--
--
6
0.3
--
V
A
PGOOD
I
SS
--
80
180
15
--
T
160
--
200
--
C
C
SD
Thermal Shutdown Hysteresis
T
SD
Note 1. Stresses beyond those listed “Absolute Maximum Ratings” may cause permanent damage to the device. These are
stress ratings only, and 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 may
affect device reliability.
Note 2. θJA is measured at TA = 25 °C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. θJC is
measured at the exposed pad of the package.
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
Note 5. Guarantee by design.
Copyright 2016 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
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DS2875A/B-05 October 2016
RT2875A/B
Typical Application Circuit
12, 13
14
BOOT
VIN
V
IN
C
IN
C
BOOT
RT2875A/B
L
10µF x 2
1, 2
V
SW
FB
OUT
10
Enable
EN
R1
R2
11
6
PGOOD
PGOOD
C
7
OUT
22µF x 2
RLIM
C
COMP
R
COMP
8
9
4
RT/SYNC
COMP
SS
R
LIM
R
OSC
C
SS
AGND PGND
5
3, 15 (Exposed Pad)
For 500kHz Only
VOUT
12
R1 (k)
102
R2 (k)
5.36
8.25
15
ROSC (k)
100
RCOMP (k)
CCOMP (nF)
L (H)
10
32
20
3.9
3.3
3.3
3.3
3.3
3.9
8
102
100
8.2
5
110
100
15
6.8
3.3
2.5
1.2
115
25.5
8.06
10
100
10
4.7
25.5
10
100
7.5
4.3
3.6
100
2.2
Copyright 2016 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
DS2875A/B-05 October 2016
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RT2875A/B
Typical Operating Characteristics
Efficiency vs. Load Current
Output Voltage vs. Load Current
100
3.37
3.36
3.35
3.34
3.33
3.32
3.31
3.30
90
80
VIN = 5V
VIN = 12V
IN = 23V
VIN = 30V
VIN = 36V
70
60
50
40
30
20
10
0
V
VIN = 12V
VIN = 5V
VIN = 24V
VIN = 30V
VIN = 36V
VOUT = 3.3V
2.5 3
VOUT = 3.3V
2.5
0
0.5
1
1.5
2
3
0
0.5
1
1.5
2
Load Current (A)
Load Current (A)
Referecnec Voltage vs. Input Voltage
Reference Voltage vs. Temperature
0.610
0.65
0.64
0.63
0.62
0.61
0.60
0.59
0.58
0.57
0.56
0.55
0.608
0.605
0.603
0.600
0.598
0.595
0.593
0.590
VIN = 12V, VOUT = 1.2V, IOUT = 0A
VIN = 4.5V to 36V, VOUT = 3.3V, IOUT = 0A
2
9.6
17.2
24.8
32.4
40
-50
-25
0
25
50
75
100
125
Temperature (°C)
Input Voltage (V)
Switching Frequency vs. RT
Switching Frequency vs. Input Voltage
2000
1800
1600
1400
1200
1000
800
600
590
580
570
560
550
540
530
520
510
500
600
400
200
VIN = 12V, VOUT = 3.3V, IOUT = 0A
VIN = 12V, VOUT = 3.3V, IOUT = 0A, RT = 100kΩ
0
20
40
60
80 100 120 140 160 180 200
4
8 12 16 20 24 28 32 36
RT(kΩ)
Input Voltage (V)
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DS2875A/B-05 October 2016
RT2875A/B
Current Limit vs. RLIM
Switching Frequency vs. Temperature
7
6
5
4
3
2
1
0
600
580
560
540
520
500
480
460
440
420
400
VIN = 4.5V
VIN = 12V
VIN = 24V
VIN = 36V
VOUT = 3.3V, IOUT = 0A
50 75 100 125
20
30
40
50
60
70
80
90
100
-50
-25
0
25
RLIM (kΩ)
Ambient Temperature (°C)
Current Limit vs. Temperature
Enable Voltage vs. Temperature
8
7
6
5
4
3
2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Enable_Rising
Enable_Falling
VIN = 12V, VOUT = 3.3V
50 75 100 125
VIN = 12V, VOUT = 3.3V, RLIM = 39kΩ
25 50 75 100 125
-50
-25
0
-50
-25
0
25
Temperature (°C)
Temperature (°C)
UVLO vs. Temperature
Turn On
Load Transient Response
4.7
4.6
4.5
4.4
4.3
4.2
4.1
4.0
3.9
3.8
3.7
3.6
3.5
3.4
3.3
3.2
3.1
3.0
VOUT
(200mV/Div)
Turn Off
IOUT
(2A/Div)
VIN = 12V, VOUT = 3.3V
50 75 100 125
VIN = 12V, VOUT = 3.3V, IOUT = 0A to 3A
-50
-25
0
25
Time (250μs/Div)
Temperature (°C)
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RT2875A/B
Load Transient Response
Switching
VOUT
VOUT
(5mV/Div)
(200mV/Div)
VSW
(10V/Div)
IOUT
(1A/Div)
IOUT
(2A/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 1.5A
VIN = 12V, VOUT = 1.2V, IOUT = 0A to 2.5A
Time (1μs/Div)
Time (250μs/Div)
Switching
Power On from EN
VOUT
(5mV/Div)
VEN
(2V/Div)
VOUT
VSW
(2V/Div)
(10V/Div)
IOUT
(2A/Div)
IOUT
(2A/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 3A
Time (5ms/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 3A
Time (1μs/Div)
Power Off from EN
Power On from VIN
VEN
VIN
(2V/Div)
(5V/Div)
VOUT
VOUT
(2V/Div)
(2V/Div)
IOUT
(2A/Div)
IOUT
(2A/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 3A
VIN = 12V, VOUT = 3.3V, IOUT = 3A
Time (10ms/Div)
Time (50μs/Div)
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DS2875A/B-05 October 2016
RT2875A/B
Power Off from VIN
VIN
(5V/Div)
VOUT
(2V/Div)
IOUT
(2A/Div)
VIN = 12V, VOUT = 3.3V, sIOUT
=
Time (5ms/Div)
Copyright 2016 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
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RT2875A/B
Application Information
Output Voltage Setting
Chip Enable Operation
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 1.
The EN pin is the chip enable input. Pulling the EN pin
low (<0.4V) will shutdown the device. During shutdown
mode, the RT2875A/B quiescent current drops to lower
than 10μA.Driving the ENpin high (>1.6V) will turn on the
device again. For external timing control, the EN pin can
also be externally pulled high by adding a REN resistor
and CEN capacitor from the VIN pin (see Figure 3).
V
OUT
R1
FB
RT2875A/B
GND
R2
EN
R
EN
V
IN
EN
RT2875A/B
Figure 1. Output Voltage Setting
C
EN
GND
The output voltage is set by an external resistive voltage
divider according to the following equation :
Figure 3. Enable Timing Control
R1
R2
VOUT = VREF 1
An external MOSFET can be added to implement digital
control on the EN pin when no system voltage above 2.5V
is available, as shown in Figure 4. In this case, a 100kΩ
pull-up resistor, REN, is connected between VIN and the
EN pin. MOSFET Q1 will be under logic control to pull
down the EN pin.
Where VREF is the reference voltage (0.6V typ.).
External Bootstrap Diode
Connect a 0.1μF low ESR ceramic capacitor between the
BOOT and SW pins. This capacitor provides the gate driver
voltage for the high side MOSFET.
R
EN
100k
V
It is recommended to add an external bootstrap diode
between an external 5V and BOOT pin 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 of the
RT2875A/B. Note that the external boot voltage must be
lower than 5.5V
EN
RT2875A/B
GND
IN
Q1
EN
Figure 4. Digital Enable Control Circuit
Under Voltage Protection
5V
Hiccup Mode
The RT2875B provides Hiccup Mode Under Voltage
Protection (UVP). When the VFB voltage drops below
0.3V, the UVP function will be triggered to shut down
switching operation. If the UVP condition remains for a
period, the RT2875B will retry automatically. When the
UVP condition is removed, the converter will resume
operation. The UVP is disabled during soft-start period.
BOOT
100nF
RT2875A/B
SW
Figure 2. External Bootstrap Diode
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DS2875A/B-05 October 2016
RT2875A/B
Latch Mode
For the ripple current selection, the value of ΔIL= 0.24(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 :
For the RT2875A it provides Latch-Off Mode Under
Voltage Protection (UVP). When the VFB voltage drops
below 0.3V, UVP will be triggered and the RT2875A will
shut down in Latch-Off Mode. In shutdown condition, the
RT2875A can be reset by EN pin or power input VIN.
V
f I
V
OUT
V
IN(MAX)
OUT
L =
1
Hiccup Mode
L(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. Please
see Table 2 for the inductor selection reference.
VOUT
(2V/Div)
Table 2. Suggested Inductors for Typical
Application Circuit
ILX
(2A/Div)
Component
Supplier
Dimensions
(mm)
Series
IOUT = Short
TDK
TDK
VLF10045
SLF12565
10 x 9.7 x 4.5
Time (50ms/Div)
12.5 x 12.5 x 6.5
TAIYO
YUDEN
Figure 5. Hiccup Mode Under Voltage Protection
NR8040
8 x 8 x 4
Over Temperature Protection
The RT2875A/B features an Over Temperature Protection
(OTP) circuitry to prevent from overheating due to
excessive power dissipation. The OTP will shut down
switching operation when junction temperature exceeds
180°C. Once the junction temperature cools down by
approximately 15°C, the converter will resume operation.
To maintain continuous operation, the maximum junction
temperature should be lower than 150°C.
CIN and COUT Selection
The input capacitance, CIN, is needed to filter the
trapezoidal current at the Source of the high side MOSFET.
To prevent large ripple current, a low ESR input capacitor
sized for the maximum RMS current should be used. The
approximate RMS current equation is given :
V
V
V
IN
V
OUT
OUT
I
= I
1
RMS
OUT(MAX)
IN
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.
Inductor Selection
The inductor value and operating frequency determine
the ripple current according to a specific input and output
voltage. The ripple current ΔIL increases with higher VIN
and decreases with higher inductance.
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.
V
f L
VOUT
V
IN
OUT
1
IL =
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output voltage
ripple. High frequency with small ripple current can achieve
the highest efficiency operation. However, it requires a
large inductor to achieve this goal.
For the input capacitor, two 10μF low ESR ceramic
capacitors are suggested. For the suggested capacitor,
please refer to Table 3 for more details.
The selection of COUT is determined by the required ESR
to minimize voltage ripple.
Copyright 2016 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
DS2875A/B-05 October 2016
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13
RT2875A/B
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.
Current Setting
The current limit of high side MOSFET is adjustable by
an external resistor connected to the RLIM pin. The current
limit range is from 1.5A to 6A. When the inductor current
reaches the current limit threshold, the COMP voltage
will be clamped to limit the inductor current. Inductor
current ripple current also should be considered into
current limit setting. Current limit minimum value should
be set as below :
The output ripple, ΔVOUT , is determined by :
1
VOUT I ESR
L
8fCOUT
The output ripple will be the highest at the maximum input
voltage since ΔIL increases with input voltage. Multiple
capacitors placed in parallel may be needed to meet the
ESR and RMS current handling requirement. Higher values,
lower cost ceramic capacitors are now becoming available
in smaller case sizes. Their high ripple current, high voltage
rating and low ESR make them ideal for switching regulator
applications. However, care must be taken when these
capacitors are used at input and output. When a ceramic
capacitor is used at the input and the power is supplied
by a wall adapter through long wires, a load step at the
output can induce ringing at the input, VIN. At best, this
ringing can couple to the output and be mistaken as loop
instability. At worst, a sudden inrush of current through
the long wires can potentially cause a voltage spike at
VIN large enough to damage the part.
Current limit minimum = (IO(max) + 1 / 2 inductor current
ripple) x 1.2
Through extra resister RLIMconnect to RLIM pin to setting
the current limit value below offer approximate formula
equation :
ISET = current limit value (A)
y = (ISET − 0.4206) / 167.79
RLIM (kΩ) = (1 / y)
Soft-Start
The RT2875A/B provides 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 capacitor CSS between
SS andGND.An internal current source ISS (6μA) charges
an external capacitor to build a soft-start ramp voltage.
The VFB voltage will track the internal ramp voltage during
softstart interval. The typical soft start time is calculated
as follows :
Switching Frequency Setting
The switching frequency can be set by using extra resister
RT or external clock. Switching frequency range is from
300kHz to 2.1MHz. Through extra resister RT connect to
RT/SYNC pin to setting the switching frequency FS, below
offer approximate formula equation :
Soft-Start time tSS = CSS x 0.6 / 6μA
Setting Frequency = FS (kHz)
x = [FS − 31.379] / 47691
ROSC (kΩ) = (1 / x)
The RT2875A/B can be synchronized with an external clock
ranging from 300kHz to 2.1MHz applied to the RT/SYNC
pin. The external clock duty cycle must be from 10% to
90%. The RT/SYNC pin is at logic-high level (>2V). If the
EN pin is pulled to low-level for 10μs above, the IC will
shut down.
Copyright 2016 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
14
DS2875A/B-05 October 2016
RT2875A/B
Thermal Considerations
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Four-Layer PCB
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 :
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.
0
25
50
75
100
125
150
Ambient Temperature (°C)
For recommended operating condition specifications, the
maximum junction temperature is 150°C. The junction to
ambient thermal resistance, θJA, is layout dependent. For
TSSOP-14 (Exposed Pad) package, the thermal
resistance, θJA, is 28°C/W on a standard JEDEC 51-7
four-layer thermal test board. The maximum power
dissipation at TA = 25°C can be calculated by the following
formula :
Figure 6. Derating Curve of Maximum PowerDissipation
PD(MAX) = (150°C − 25°C) / (28°C/W) = 4.464W for
TSSOP-14 (Exposed Pad) package
The maximum power dissipation depends on the operating
ambient temperature for fixed TJ(MAX) and thermal
resistance, θJA. The derating curve in Figure 6 allows the
designer to see the effect of rising ambient temperature
on the maximum power dissipation.
Copyright 2016 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS2875A/B-05 October 2016
www.richtek.com
15
RT2875A/B
Outline Dimension
Dimensions In Millimeters
Dimensions In Inches
Symbol
Min
Max
Min
Max
0.047
0.006
0.041
0.012
0.201
A
A1
A2
b
1.000
0.000
0.800
0.190
4.900
1.200
0.150
1.050
0.300
5.100
0.039
0.000
0.031
0.007
0.193
D
e
0.650
0.026
E
6.300
4.300
0.450
1.900
1.600
6.500
4.500
0.750
2.900
2.600
0.248
0.169
0.018
0.075
0.063
0.256
0.177
0.030
0.114
0.102
E1
L
U
V
14-Lead TSSOP (Exposed Pad) Plastic 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.
www.richtek.com
16
DS2875A/B-05 October 2016
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