RT6204 [RICHTEK]
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| 型号： | RT6204 |
| 厂家： | 
RICHTEK TECHNOLOGY CORPORATION |
| 描述： | 暂无描述 |
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| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
RT6204
500mA, 60V, 350kHz Synchronous Step-Down Converter
General Description
Features
0.8V Feedback Reference Voltage with 1.5%
The RT6204 is a 60V, 500mA, 350kHz, high-efficiency,
synchronous step-down DC-DC converter with an
input-voltage range of 5.2V to 60V and a programmable
output-voltage range of 0.8V to 50V. It features
current-mode control to simplify external compensation
and to optimize transient response with a wide range of
inductors and output capacitors. High efficiency can be
achieved through integrated N-MOSFETs, and
pulse-skipping mode at light loads. With EN pin,
power-up sequence can be more flexible and shutdown
quiescent current can be reduced to < 3A.
Accuracy
Wide Input Voltage Range : 5.2V to 60V
Output Current : 500mA
Integrated N-MOSFETs
Current-Mode Control
Fixed Switching Frequency : 350kHz
Programmable Output Voltage : 0.8V to 50V
Low < 3AShutdown Quiescent Current
Up to 92% Efficiency
Pulse-Skipping Mode for Light-Load Efficiency
Programmable Soft-Start Time
Cycle-by-Cycle Current Limit Protection
Input Under-Voltage Lockout, Output Under-Voltage
and Thermal Shutdown Protection
The RT6204 features cycle-by-cycle current limit for
over-current protection against short-circuit outputs,
and user-programmable soft-start time to prevent inrush
current during startup. It also includes input under-voltage
lockout, output under-voltage, and thermal shutdown
protection to provide safe and smooth operation in all
operating conditions.
Applications
4-20mA Loop-Powered Sensors
OBD-II Port Power Supplies
The RT6204 is available in the SOP-8 (Exposed pad)
package.
Low-Power Standby or Bias Voltage Supplies
Industrial Process Control, Metering, and Security
Systems
High-Voltage LDO Replacement
Telecommunications Systems
Commercial Vehicle Power Supplies
General Purpose Wide Input Voltage Regulation
Simplified Application Circuit
C
BOOT
Efficiency vs. Output Current
100
90
80
V
VIN
BOOT
SW
IN
L1
C
IN
V
OUT
70
60
50
40
30
20
10
0
V
V
V
V
= 24V
= 36V
= 48V
= 60V
IN
IN
IN
IN
RT6204
R1
C
EN
SS
Enable
FB
GND
COMP
FF
C
C
C
OUT
R2
C
C
P
SS
R
C
V
= 12V
OUT
0
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
Output Current (A)
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6204-04 February 2018
www.richtek.com
1
RT6204
Ordering Information
Marking Information
RT6204GSP : Product Number
YMDNN : Date Code
RT6204
RT6204
Package Type
GSPYMDNN
SP : SOP-8 (Exposed Pad-Option 2)
Lead Plating System
G : Green (Halogen Free and Pb Free)
Pin Configuration
Note :
(TOP VIEW)
Richtek products are :
RoHS compliant and compatible with the current
8
7
6
5
SS
BOOT
VIN
2
3
4
EN
requirements of IPC/JEDEC J-STD-020.
GND
SW
COMP
FB
9
Suitable for use in SnPb or Pb-free soldering processes.
GND
SOP-8 (Exposed Pad)
Functional Pin Description
Pin No.
Pin Name
Pin Function
Bootstrap capacitor connection node for High-Side Gate Driver. Connect a
0.1F ceramic capacitor from BOOT to SW to power the internal gate driver.
1
BOOT
Supply voltage input, 5.2V to 60V. Bypass VIN to GND with a large
high-quality capacitor.
2
3
VIN
SW
Switch node for output inductor connection.
Power ground. The exposed pad must be connected to GND and well
soldered to the input and output capacitors and a large PCB copper area for
maximum power dissipation.
4, 9
(Exposed Pad)
GND
FB
Feedback voltage input. Connect FB to the midpoint of the external
feedback resistor divider to sense the output voltage. The device regulates
the FB voltage at 0.8V (typical) Feedback Reference Voltage.
5
6
Compensation node for the compensation of the regulation control loop.
Connect a series RC network from COMP to GND. In some cases, another
capacitor from COMP to GND may be required.
COMP
Enable control input. A logic High (VEN > 1.35V) enables the device, and a
logic Low (VEN < 0.925V) shuts down the device, reducing the supply
current to 3A or below. Connect EN pin to VIN pin with a 100k pull-up
resistor for automatic startup.
7
8
EN
SS
Soft-start capacitor connection node. Connect an external capacitor from SS
to GND to set the soft-start time. Do not leave SS pin unconnected. A
capacitor of capacitance from 10nF to 100nF is recommended, which can
set the soft-start time from 1.33ms to 13.3ms, accordingly.
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
2
DS6204-04 February 2018
RT6204
Functional Block Diagram
VIN
Thermal
Shutdown
Internal
Regulator
HV
Protection
UVLO
-
EN
Current
Sense
1.2V
+
Shutdown
Comparator
1μA
BOOT
UVLO
BOOT
SW
HS
LS
Logic &
Clamp
Control
Gate Driver
with Dead
Time Control
0.4V
+
-
UV
Comparator
HS Switch
Current
Comparator
Current
Sense
FB
SS
-
LS Switch
Current
EA
+
0.8V
+
Comparator
GND
6μA
Slope
Compensation
Oscillator
COMP
Operation
The RT6204 is a synchronous step-down converter,
integrated with both high-side (HS) and low-side (LS)
MOSFETs to reduce external component count and a
gate driver with dead-time control logic to prevent
shoot-through condition from happening. The RT6204
also features constant frequency and peak
current-mode control with slope compensation. During
PWM operation, output voltage is regulated down, and
is sensed from the FB pin to be compared with an
internal 0.8V reference voltage VREF. In normal
operation, the high-side N-MOSFET is turned on when
an S-R latch is set by the rising edge of an internal
oscillator output as the PWM clock, and is turned off
when the S-R latch is reset by the output of a (high-side)
current comparator, which compares the high-side
sensed current signal with the current signal related to
the COMP voltage. While the high-side N-MOSFET is
turned off, the low-side N-MOSFET will be turned on. If
the output voltage is not established, the high-side
power switch will be turned on again and another cycle
begins.
Pulse Skipping Operation
At very light-load condition, the RT6204 provides pulse
skipping technique to decrease switching loss for better
efficiency. When load current decreases, the FB
voltage VFB will increase slightly. With VFB 1% higher
than VREF, the COMP voltage will be clamped at a
minimum value and the converter will enter into pulse
skipping mode. When the converter operates in pulse
skipping mode, the internal oscillator will be stopped,
which makes the switching period being extended. In
pulse skipping mode, as the load current decreases,
VFB will be discharged more slowly, which in turn will
extend the switching period even more.
Error Amplifier
The RT6204 adopts a transconductance amplifier as
the error amplifier. The error amplifier of a typical
970A/V transconductance (gm) compares the
feedback voltage VFB with the lower one of the
soft-start voltage or the internal reference voltage VREF
0.8V. As VFB drops due to the load current increase,
the output voltage of the error amplifier will go up so
is a registered trademark of Richtek Technology Corporation.
,
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
DS6204-04 February 2018
www.richtek.com
3
RT6204
that the device will supply more inductor current to
match the load current. The frequency compensation
components, such as the series resistor and capacitor,
and an optional capacitor, are placed between the
COMP pin and ground.
reduce input inrush current. The soft-start time can be
programmed by selecting the value of the capacitor
CSS connected from the SS pin to GND. An internal
current source ISS (typically, 6A) charges the external
capacitor CSS to build a soft-start ramp voltage. The
feedback voltage VFB will be compared with the
soft-start ramp voltage during soft-start time. For the
RT6204, the external capacitor CSS is required, and for
soft-start control, the SS pin should never be left
unconnected, and it is not recommended to be
connected to an external voltage source. The soft-start
time depends on RC time constant; for example, a
0.1F capacitor for programming soft-start time will
result in 18.333ms (typ.) soft-start time.
Oscillator
The internal oscillator frequency is set to a typical
350kHz as a fixed frequency for PWM operation.
Slope Compensation
In order to prevent sub-harmonic oscillations that may
occur over all specified load and line conditions when
operating at duty cycle higher than 50%, the RT6204
features an internal slope compensation, which adds a
compensating slope signal to the sensed current signal
to support applications with duty cycle up to 93%.
Output Under-Voltage Protection (UVP) with Hiccup
Mode
There is an exclusive condition for the internal slope
compensation. When the loading current is reducing to
make the COMP voltage touch its minimum level, the
internal slope compensation will be disabled to keep a
minimum peak current no matter what duty cycle it is.
Thus a sub-harmonic oscillation may happen if the
operation duty cycle is larger than 50% and it results in
a higher output ripple voltage during such kind of light
load operation.
The RT6204 provides under-voltage protection with
hiccup mode. When the feedback voltage VFB drops
below under-voltage protection threshold VTH-UVP, half
of the feedback reference voltage VREF, the UVP
function will be triggered to turn off the high-side
MOSFET immediately. The converter will attempt
auto-recovery soft-start after under-voltage condition
has occurred for a period of time. Once the
under-voltage condition is removed, the converter will
resume switching and be back to normal operation.
Internal Regulator
Current Limit Protection
When the VIN is plugged in, the internal regulator will
generate a low voltage to drive internal control circuitry
and to supply the bootstrap power for the high-side
gate driver.
The RT6204 provides cycle-by-cycle current limit
protection against over-load or short-circuited condition.
When the peak inductor current reaches the current
limit, the high-side MOSFET will be turned off
immediately with no violating minimum on-time tON_MIN
requirement to prevent the device from operating in an
over-current condition.
Chip Enable
The RT6204 provides an EN pin, as an external chip
enable control, to enable or disable the device. When
VIN is higher than the input under-voltage lockout
threshold (VUVLO) with the EN voltage (VEN) higher
than 1.35V, the converter will be turned on. When VEN
is lower than 0.925V, the converter will enter into
shutdown mode, during which the supply current can
be even reduced to 3A or below.
Thermal Shutdown
The RT6204 provides over-temperature protection
(OTP) function to prevent the chip from damaging due
to over-heating. The over-temperature protection
function will shut down the switching operation when
the junction temperature exceeds 165C. Once the
over-temperature condition is removed, the converter
will resume switching and be back to normal operation.
External Soft-Start
The RT6204 provides external soft-start feature to
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS6204-04 February 2018
RT6204
Absolute Maximum Ratings (Note 1)
VIN
(Note 5)------------------------------------------------------------------------------------------------- 0.3V to 80V
SW
DC----------------------------------------------------------------------------------------------------------------- 0.3V to (VIN + 0.3V)
<200ns----------------------------------------------------------------------------------------------------------- 5V to (VIN + 4V)
EN Pin------------------------------------------------------------------------------------------------------------ 0.3V to 80V
BOOT to SW, VBOOT VSW -------------------------------------------------------------------------------- 0.3V to 6V
Other Pins------------------------------------------------------------------------------------------------------- 0.3V to 6V
Power Dissipation, PD @ TA = 25C
SOP-8 (Exposed Pad) --------------------------------------------------------------------------------------- 3.44W
Package Thermal Resistance
(Note 2)
SOP-8 (Exposed Pad), JA --------------------------------------------------------------------------------- 29C/W
SOP-8 (Exposed Pad), JC --------------------------------------------------------------------------------- 2C/W
Lead Temperature (Soldering, 10 sec.)------------------------------------------------------------------ 260C
Junction Temperature---------------------------------------------------------------------------------------- 150C
Storage Temperature Range------------------------------------------------------------------------------- 65C to 150C
ESD Susceptibility
(Note 3)
HBM (Human Body Model) --------------------------------------------------------------------------------- 2kV
Recommended Operating Conditions
(Note 4)
Supply Input Voltage ----------------------------------------------------------------------------------------- 5.2V to 60V
Ambient Temperature Range------------------------------------------------------------------------------- 40C to 85C
Junction Temperature Range ------------------------------------------------------------------------------ 40C to 125C
Electrical Characteristics
(VIN = 12V, TA = 25C, unless otherwise specified)
Parameter
Supply Current
Symbol
Test Conditions
Min
Typ
Max Unit
VEN = 0V
--
--
--
0.5
20
3
Shutdown Supply Current
ISHDN
A
VEN = 0V, VIN = 60V
VEN = 3V, VFB = 0.9V
--
Quiescent Supply Current
Reference
IQ
0.6
--
mA
V
Feedback Reference Voltage
Enable and UVLO
VREF
5V VIN 60V
0.788
0.8
0.812
Input Under-Voltage Lockout
Threshold
VUVLO
VIN Rising
4
4.6
5.2
V
Input Under-Voltage Lockout
Hysteresis
VUVLO_HYS
200
350
500
mV
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6204-04 February 2018
www.richtek.com
5
RT6204
Parameter
Symbol
VTH_EN
Hysteresis VTH_EN_HYS Falling
Test Conditions
Min
1.15
25
Typ
1.25
--
Max Unit
Rising
1.35
225
V
EN Input Threshold
Voltage
mV
Error Amplifier
Error Amplifier Transconductance
Error Amplifier Source/Sink Current
gm_EA
IC = 10A
--
--
970
160
--
--
A/V
A
COMP to Current Sense
Transconductance
gm_CS
--
0.9
--
A/V
Internal MOSFET
RDS(ON)_H1
RDS(ON)_H2
RDS(ON)_L
--
--
--
660
890
330
850
1200
500
High-Side Switch On-Resistance
m
m
VIN = 60V
Low-Side Switch On-Resistance
Switching
Oscillation Frequency
fOSC1
--
--
--
--
350
100
93
--
--
--
--
kHz
kHz
%
Short-Circuit Oscillation Frequency fOSC2
VFB = 0V
Maximum Duty Cycle
Minimum On-Time
DMAX
VFB = 0.7V
tON_MIN
90
ns
Soft-Start
Soft-Start Current
ISS
VSS = 0V
--
6
--
A
Protection Function
High-Side Switch Leakage Current
High-Side Switch Current Limit
VEN = 0V, VSW = 0V
Minimum duty cycle
--
0
10
--
A
ILIM_HS
600
860
mA
After soft-start, with respect
to VFB
Under-Voltage Protection Threshold VTH_UVP
Thermal Shutdown TSD
--
--
50
--
--
%
165
C
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 under natural convection (still air) at TA = 25°C with the component mounted on a high
effective-thermal-conductivity four-layer test board on a JEDEC 51-7 thermal measurement standard. 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.
Note 5. When VIN is beyond the recommended operating voltage (60V) and within the absolute maximum voltage (80V), the
conducting current through SW pin has to be less than 0.5A to avoid instant damage to the devices.
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
6
DS6204-04 February 2018
RT6204
Typical Application Circuit
C
BOOT
100nF
D
*
BOOT
3.3V
R
BOOT
0
V
BOOT
VIN
IN
L1
C
IN
SW
V
OUT
C
OUT
R *
S
RT6204
D1*
R1
EN
Enable
C *
S
FB
C
FF
*
SS
C
GND
COMP
C
C
R2
C
SS
P
R
C
0.1μF
* : Optional
Table 1. Suggested component selections for the application of 500mA load current for some common
output voltages
VOUT (V) R1 (k) R2 (k) L1 (H) COUT (F) RC (k) CC (nF) CP (pF)
1
2.49
4.99
12.4
21
10
10
10
10
10
10
10
22
22
20
20
20
20
20
20
20
4.02
4.99
6.98
10
6.8
6.8
6.8
6.8
6.8
6.8
6.8
NC
NC
NC
NC
68
1.2
1.8
2.5
3.3
5
33
33
30.9
52.3
102
47
16
100
150
24
47
9
34.9
47
12
(Note)
140
10
220
20
34.9
6.8
47
Note : For VIN 17V & VOUT = 12V application, the snubber components need to be added (RS = 3.9, CS = 1nF)
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6204-04 February 2018
www.richtek.com
7
RT6204
Typical Operating Characteristics
(TA = 25C, unless otherwise specified)
Efficiency vs. Output Current
Efficiency vs. Output Current
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
V
V
V
V
V
= 12V
= 24V
= 36V
= 48V
= 60V
IN
IN
IN
IN
IN
V
V
= 48V
= 60V
40
30
20
10
0
V
V
V
V
= 5V
IN
IN
IN
IN
IN
IN
= 12V
= 24V
= 36V
V
= 3.3V
V
= 5V
OUT
OUT
0.4
0
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
Output Current (A)
0
0.1
0.2
0.3
0.5
Output Current (A)
Efficiency vs. Output Current
Efficiency vs. Input Voltage
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
V
V
V
V
= 24V
= 36V
= 48V
= 60V
IN
IN
IN
IN
V
= 12V
V
= 5V, I
= 500mA
OUT
OUT
OUT
0
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
Output Current (A)
12 16 20 24 28 32 36 40 44 48 52 56 60
Input Voltage (V)
Output Voltage vs. Output Current
Output Voltage vs. Input Voltage
3.50
3.46
3.42
3.38
3.34
3.30
3.26
3.22
3.18
3.14
3.10
5.04
5.03
5.02
5.01
5.00
4.99
4.98
4.97
4.96
4.95
4.94
V
V
= 48V
= 60V
IN
IN
V
V
V
V
= 5V
IN
IN
IN
IN
= 12V
= 24V
= 36V
V
= 3.3V
V
= 5V, I
= 500mA
OUT
OUT
OUT
50 100 150 200 250 300 350 400 450 500
Output Current (mA)
12 16 20 24 28 32 36 40 44 48 52 56 60
Input Voltage (V)
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
8
DS6204-04 February 2018
RT6204
Typical Operating Characteristics (continued)
Feedback Voltage vs. Input Voltage
Feedback Voltage vs. Temperature
0.8018
0.8014
0.8010
0.8006
0.8002
0.7998
0.7994
0.7990
0.820
0.815
0.810
0.805
0.800
0.795
0.790
0.785
0.780
V
V
V
= 5V
IN
IN
IN
= 50V
= 60V
5
10 15 20 25 30 35 40 45 50 55 60
Input Voltage (V)
-50
-25
0
25
50
75
100
125
Temperature (°C)
Oscillation Frequency vs. Input Voltage
Oscillation Frequency vs. Temperature
370
367
364
361
358
355
352
349
346
343
340
380
375
370
365
360
355
350
345
340
335
330
325
320
V
= 5V, I
= 250mA
OUT
OUT
V
V
V
V
V
= 12V
= 24V
= 36V
= 48V
= 60V
IN
IN
IN
IN
IN
I
= 250mA
OUT
5
10 15 20 25 30 35 40 45 50 55 60
Input Voltage (V)
-50
-25
0
25
50
75
100
125
Temperature (°C)
Upper SW Current Limit vs. Input Voltage
Upper SW Current Limit vs. Temperature
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
V
V
= 10V
= 50V
IN
IN
-50
-25
0
25
50
75
100
125
10 15 20 25 30 35 40 45 50 55 60
Intput Voltage (V)
Temperature (°C)
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6204-04 February 2018
www.richtek.com
9
RT6204
Typical Operating Characteristics (continued)
Shutdown Current vs. Input Voltage
Shutdown Current vs. Temperature
20
18
16
14
12
10
8
3.0
2.5
2.0
1.5
1.0
0.5
0.0
6
4
2
V
= 12V, V = 0V
EN
IN
0
5
5
5
10 15 20 25 30 35 40 45 50 55 60
Input Voltage (V)
-50
-25
0
25
50
75
100
125
Temperature (°C)
Supply Current vs. Input Voltage
Supply Current vs. Temperature
2.0
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
V
= 3.3V, V = high
V
= 12V, V
= 3.3V, V = high
OUT EN
OUT
EN
IN
10 15 20 25 30 35 40 45 50 55 60
Input Voltage (V)
-50
-25
0
25
50
75
100
125
Temperature (°C)
Switching
EN Input Current vs. Input Voltage
1.2
1.1
1.0
0.9
0.8
0.7
0.6
V
= 60V, V
= 3.3V, I
= 10mA
OUT
IN
OUT
V
OUT
(20mV/Div)
V
SW
(30V/Div)
I_Inductor
(100mA/Div)
EN = VIN
Time (40s)
10 15 20 25 30 35 40 45 50 55 60
Input Voltage (V)
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
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DS6204-04 February 2018
RT6204
Typical Operating Characteristics (continued)
Switching
Switching
V
V
OUT
OUT
(10mV/Div)
(10mV/Div)
SW
(30V/Div)
SW
(30V/Div)
I_Inductor
(100mA/Div)
I_Inductor
(200mA/Div)
V
= 60V, V
= 3.3V, I
= 250mA
OUT
V
= 60V, V
= 3.3V, I
= 500mA
OUT
IN
OUT
IN
OUT
Time (2s/Div)
Time (2s/Div)
Power On from EN
Power Off from EN
V
= 60V, V
= 3.3V, I
= 500mA
OUT
V
= 60V, V
= 3.3V, I
= 500mA
OUT
IN
OUT
IN
OUT
V
OUT
(2V/Div)
V
OUT
(2V/Div)
EN
(2V/Div)
I
OUT
EN
(200mA/Div)
(2V/Div)
I
OUT
(200mA/Div)
Time (10ms/Div)
Time (1ms/Div)
Power On from VIN
Power Off from VIN
V
= EN = 60V, V
= 3.3V, I
= 500mA
OUT
V
= EN = 60V, V
= 3.3V, I
= 500mA
OUT
IN
OUT
IN
OUT
V
IN
(20V/Div)
V
OUT
(1V/Div)
V
IN
(20V/Div)
I_Inductor
(200mA/Div)
V
OUT
(1V/Div)
I_Inductor
(200mA/Div)
Time (10ms/Div)
Time (1ms/Div)
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11
RT6204
Application Information
Output Voltage Setting
UVLO
The output voltage can be adjusted by setting the
feedback resistors R1 and R2, as Figure 1. Choose a
10k resistor for R2 and calculate R1 by using the
equation below :
V
V
IN
EN
R1
R2
VOUT = VFB 1 +
3V
V
REF
where VFB is the feedback voltage (typically equal to
VREF
)
V
SS
V
OUT
R1
V
OUT
FB
RT6204
GND
R2
V
SW
Normal Operation with
pulse skipping mode
Soft-Start
Delay time
Soft-Start
Figure 1. Output Voltage Setting by a Resistive
Voltage Divider
Figure 2. Power-Up Sequence Controlled by the EN Pin
Chip Enable Operation
UVLO
V
The RT6204 provides enable/disable control through
the EN pin. The chip remains in shutdown mode by
pulling the EN pin below Logic-Low threshold (0.95V).
During the shutdown mode, the RT6204 disables most
of the logic circuitry to lower the quiescent current.
When VEN rises above Logic-High threshold (VTH_EN
1.35V), the RT6204 will begin initialization for a new
soft-start cycle.
IN
V
EN
3V
V
REF
If the EN pin is floating, VEN will be pulled Low by a 1A
current drawn from the EN pin. Connecting a 1k to
100k pull-up resistor is recommended. An external
MOSFET can be added to implement a logic-controlled
enable control. Figure 2 shows the power up sequence,
which is controlled by the EN pin.
V
SS
V
OUT
V
SW
The RT6204 also provides enable control through VIN
pin. If the VEN is above Logic-High threshold first, the
chip will remain in shutdown mode until the VIN rises
Normal Operation with
pulse skipping mode
Soft-Start
Delay time
Soft-Start
Figure 3. Power-Up Sequence Controlled by the VIN Pin
above VUVLO
.
Figure 3 shows the power up sequence, which is
controlled by the VIN pin.
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12
DS6204-04 February 2018
RT6204
MOSFET will be turned off immediately, while the
minimum on-time tON_MIN requirement still needs to be
met, to prevent the device from operating in an
over-current condition.
Soft-Start
When start-up, a large inrush may be observed for high
output voltages and output capacitances. To solve this,
the RT6204 provides an external soft-start function to
reduce input inrush current to meet various
applications.
In the current-limited condition, the maximum sourcing
current is fixed because the peak inductor current is
limited. When the load is further increasing and is over
the sourcing capability of the high-side switch, the
output voltage will start to drop and eventually be lower
than the under-voltage protection threshold so that the
IC will enter shutdown mode and may restart with
hiccup mechanism.
For the RT6204, the soft-start time tSS can be
programmed by selecting the value of the capacitor
CSS connected between the SS pin and GND. During
the soft-start period, an internal pull-up current source
ISS (typically, 6A) charges the external capacitor CSS
to generate a soft-start voltage ramp, and then output
voltage will follow this voltage ramp to monotonically
start up.
UV Threshold
VFB
The soft-start time can be calculated as :
+ 0.3V
REF
C
SS
V
t
=
SS
I
SS
ILOAD
where ISS = 6A (typical), VREF is the feedback
reference voltage, and CSS is the external capacitor
placed from the SS pin to GND, where a 10nF to 100nF
capacitor is recommended to set the soft-start time from
1.833ms to 18.333ms.
Current Limit
I_inductor
tON_MIN
VSW
Figure 5. Peak-Current Limit
Under-Voltage Protection
1.1V
ISS
SS
CSS
VSS
The feedback voltage is constantly monitored for
under-voltage protection. When the feedback voltage is
VOUT
lower than under-voltage protection threshold VTH-UVP
,
tSS
the under-voltage protection is triggered, the high-side
MOSFET will be turned off and the low-side MOSFET
is turned on to discharge the output voltage. The
under-voltage protection is not a latched mechanism; if
the under-voltage condition remains for a period of time,
the RT6204 will enter hiccup mode. During the
soft-start time, the under-voltage protection is masked
and a 5s deglitch time is built in the UVP circuit to
prevent false transitions.
Figure 4. External Soft-Start Time Setting
Current Limit
The RT6204 provides peak current limit function to
prevent chip damaging from short-circuited output, the
VIN voltage and SW voltage are sensed when the
internal high-side MOSFET is turned on. During this
period, the VIN-SW voltage is increasing when the
inductor current is increasing. The peak inductor
current will be monitored every switching cycle. If the
current sense signal exceeds the internal current limit,
clamped by the maximum COMP voltage, the high-side
Hiccup Mode
If the under-voltage protection condition continues for a
period of time, the RT6204 will enter hiccup mode, in
which the soft-start process will be initialized without
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13
RT6204
VIN being re-powered on. During this period of time,
the SW starts to switch since the under-voltage
protection is masked during soft-start time. When
soft-start finishes, if the under-voltage condition is
removed, the converter will resume normal operation; if
the under-voltage condition, however, still remains, that
is, the FB voltage is still lower than under-voltage
threshold VTH_UVP, the under-voltage protection will be
triggered again. The cycle will repeat until this fault
condition is removed.
External Bootstrap Diode
A 0.1F capacitor CBOOT, where a low ESR ceramic
capacitor is typically used, is connected between the
BOOT and SW pins to provide the gate driver supply
voltage for the high-side N-MOSFET.
It is recommended to add an external bootstrap diode
from an external 3.3V supply voltage to the BOOT pin
to improve efficiency when the input voltage VIN is
lower than 5.5V or duty cycle is higher than 65%. A
low-cost bootstrap diode can be used, such as IN4148
or BAT54.
Output Voltage Limitation
Note that the external BOOT voltage must be lower
than 5.5V.
The output voltage must be set higher than (VIN x 6.3%)
due to the limitation of the minimum on-time tON_MIN
and switching frequency. When current limiting
protection is triggered and the load current is still
increasing slowly, the output voltage will start to drop
and the on-time of the high-side MOSFET will decrease
3.3V
BOOT
C
100nF
BOOT
RT6204
as well. When the output voltage drops below VTH_UVP
,
SW
the under-voltage protection is triggered to turn off the
internal driver to protect the converter. If the output
voltage, however, does not drop below VTH_UVP yet
when the on-time of the high-side MOSFET has
decreased to tON_MIN (~90ns), the internal gate driver
will keep switching to maintain the output voltage,
which may damage the chip under this over-current
condition.
Figure 6. External Bootstrap Diode
Inductor Selection
Output inductor plays a very important role in
step-down converters because it stores energy from
input power rail and releases to output load. For better
efficiency, DC resistance (DCR) of the inductor must be
minimized to reduce copper loss. In addition, since the
inductor takes up most of the PCB space, its size also
matters. Low-profile inductors can also save board
space if height limitation exists. However, low-DCR and
low-profile inductors are usually not cost effective.
In order to make sure the output voltage can drop
below VTH_UVP once current limiting protection is
triggered, the output voltage setting must be satisfied
with the equation below :
The output voltage at the time, when the switch has
been turned on for the minimum on-time, is
On the other hand, while larger inductance may lower
ripple current, and then power loss, rise time of the
inductor current, however, increases with inductance,
which degrades the transient responses. Therefore, the
inductor design is a trade-off among performance, size
and cost.
VOUT_MIN = V tON MIN fOSC1 = 0.0315V
_
IN
IN
where tON_MIN = ~90ns, fOSC1 = 350kHz
The UVP is triggered when the VFB is lower than
VTH_UVP, which is 50%. That is to say VOUT_MIN should
be lower than 50% of the actual VOUT to guarantee the
UVP can be triggered under this condition.
The first thing to consider is inductor ripple current. The
inductor ripple current is recommended in the range of
20% to 40% of full-load current, and thus the
inductance can be calculated using the following
equation.
VOUT_MIN < 0.5VOUT
The duty cycle limitation can be obtained.
DMIN > 0.063
For example, if the VIN = 50V, the VOUT should be set
higher than 3.15V.
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14
DS6204-04 February 2018
RT6204
Output Capacitor Selection
V
V
V
OUT
IN
OUT
L
=
MIN
f
kI
V
IN
SW
OUT
Output capacitance affects stability of the control
feedback loop, ripple voltage, and transient response.
In steady state condition, inductor ripple current flows
into the output capacitor, which results in voltage ripple.
Output voltage ripple VRIPPLE can be calculated by the
following equation :
where k is the ratio of peak-to-peak ripple current to
rated output current. From above, 0.2 to 0.4 of the ratio
k is recommended.
The next thing to consider is inductor saturation current.
Choose an inductor with saturation current rating
greater than maximum inductor peak current. The peak
inductor current can be calculated using the following
equation :
1
VRIPPLE = IL ESR +
8COUT fSW
where IL is the peak-to-peak inductor current.
V VOUT
LMIN fSW
V
OUT
V
IN
IN
The output inductor and capacitor form a second-order
low-pass filter for the buck converter.
IL=
It takes a few switching cycles to respond to load
transient due to the delay from the control loop. During
the load transient, the output capacitor will supply
current before the inductor can supply current high
enough to output load. Therefore, a voltage drop,
caused by the current change onto output capacitor,
and the current flowing through ESR of the capacitor,
will occur. To meet the transient response requirement,
the output capacitance should be large enough and its
ESR should be as small as possible. The output
voltage drop (V) can be calculated by the equation
below :
where IL is the inductor peak to peak current, and
I
2
L
I
= I
+
L_PEAK
OUT
Input Capacitor Selection
A high-quality ceramic capacitor of 4.7F or greater,
such as X5R or X7R, are recommended for the input
decoupling capacitor. X5R and X7R ceramic capacitors
are commonly used in power regulator applications
because the dielectric material has less capacitance
variation and more temperature stability.
Voltage rating and current rating are the key
parameters to select an input capacitor. An input
capacitor with voltage rating 1.5 times greater than the
maximum input voltage is a conservative and safe
design choice. As for current rating, the input capacitor
is used to supply the input RMS current, which can be
approximately calculated using the following equation :
I
OUT
V = I
ESR +
t
S
OUT
C
OUT
IOUT tS
V I ESR
COUT
>
OUT
where IOUT is the size of the output current transient,
and tS is the control-loop delay time. For the worst-case
scenario, from no load to full load, tS is about 1 to 3
switching cycles.
V
V
V
OUT
V
IN
OUT
I
= I
1
IN_RMS
OUT
IN
It is practical to have several capacitors with low
equivalent series resistance (ESR), being paralleled to
form a capacitor bank, to meet size or height
requirements, and to be placed close to the drain of the
high-side MOSFET, which is very helpful in reducing
input voltage ripple at heavy load. Besides, the input
voltage ripple is determined by the input capacitance,
which can be approximately calculated by the following
equation :
Given that a transient response requirement is 4% for
5V output voltage VOUT, output current transient IOUT
is from 0A to 0.5A, ESR of the ceramic capacitor is
2m, tS is 3 switching cycles for the longest delay, and
switching frequency is 350kHz, a minimum output
capacitance 21.53F can then be calculated from
above.
Another factor for output voltage drop is equivalent
series inductance (ESL). A big change in load current,
i.e. large di/dt, along with the ESL of the capacitor,
IOUT(MAX)
VOUT
V
IN
VOUT
V
IN
V
=
1
IN
CIN fSW
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DS6204-04 February 2018
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15
RT6204
causes a drop on the output voltage. A better transient
performance can be obtained by using a capacitor with
low ESL. Generally, using several capacitors connected
in parallel can have better transient performance than
using a single capacitor with the same total ESR.
operating ambient temperature for fixed TJ(MAX) and
thermal resistance, JA. The derating curve in Figure 7
allows the designer to see the effect of rising ambient
temperature on the maximum power dissipation.
4.0
Four-Layer PCB
External Diode Selection
3.6
3.2
2.8
2.4
2.0
1.6
1.2
0.8
0.4
0.0
In order to reduce conduction loss, an external diode
between SW pin and GND is recommended. Since a
low forward voltage of a diode may cause low
conduction loss during OFF-time, SCHOTTKY diodes
with current rating greater than maximum inductor peak
current are good design choice for the application.
During the on-time, the diode can prevent the reverse
voltage back to the input voltage. Therefore, the
voltage rating should be higher than maximum input
voltage.
0
25
50
75
100
125
Ambient Temperature (°C)
Thermal Considerations
Figure 7. Derating Curve of Maximum Power
Dissipation
The junction temperature should never exceed the
absolute maximum junction temperature TJ(MAX), listed
under Absolute Maximum Ratings, to avoid permanent
damage to the device. The maximum allowable power
dissipation depends on the thermal resistance of the IC
package, the PCB layout, the rate of surrounding
airflow, and the difference between the junction and
ambient temperatures. The maximum power
dissipation can be calculated using the following
formula :
Layout Considerations
PCB layout is very important for high-frequency switching
converter applications. The PCB traces can radiate
excessive noise and contribute to converter instability with
improper layout. It is good design to mount power
components and route the power traces on the same
layer. If the power trace, for example, VIN trace, must be
routed to another layer, there must be enough vias on the
power trace for passing current through with less power
loss. The width of power trace is decided by the
maximum current which may go through. With wide
traces and enough vias, resistance of the entire power
trace can be reduced to minimum to improve converter
performance. Below are some other layout guidelines,
which should be considered :
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 continuous operation, the maximum operating
junction temperature indicated under Recommended
Operating Conditions is 125C. The junction-to-ambient
thermal resistance, JA, is highly package dependent.
For a SOP-8 (Exposed Pad) package, the thermal
resistance, JA, is 29C/W on a standard JEDEC 51-7
high effective-thermal-conductivity four-layer test board.
The maximum power dissipation at TA = 25C can be
calculated as below :
Place input decoupling capacitors close to the VIN pin.
Input capacitor can provide instant current to the
converter when high-side MOSFET is turned on. It is
better to connect the input capacitors to the VIN pin
directly with a trace on the same layer.
Place an inductor close to the SW pin and the trace
between them should be wide and short. It can gain
better efficiency with minimum resistance of the SW
trace since the output current will flow through the SW
trace. It is also a good design to keep the area of SW
PD(MAX) = (125C 25C) / (29C/W) = 3.44W for a
SOP-8 (Exposed Pad) package.
The maximum power dissipation depends on the
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
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www.richtek.com
16
DS6204-04 February 2018
RT6204
trace as large as possible, without affecting other paths.
The area can help dissipate the heat in the internal
power stages. However, since a large voltage and
current variation usually occur on the SW trace, any
sensitive trace should be kept away from this node.
The connection point of the feedback trace on the
VOUT side should be kept away from the current path
for the VOUT trace and be close to the output capacitor,
which is closest to the inductor. The feedback trace
should be also kept away from any dirty trace, for
example, a trace with high dv/dt, di/dt, or current rating,
etc., and the total length should be kept as short as
possible to reduce the risk of noise coupling, and the
signal delay.
If possible, tie the grounds of the input capacitor and the
output capacitor together as the same reference ground.
C
IN
GND
Route C
to another layer
BOOT
C
BOOT
VIN
C
SS
BOOT
VIN
SS
EN
Compensator
GND
SW
COMP
FB
SW
GND
Resistive Voltage
Divider
DIODE
L
GND
VOUT
C
OUT
Figure 8. PCB Layout Guide
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6204-04 February 2018
www.richtek.com
17
RT6204
Outline Dimension
Dimensions In Millimeters Dimensions In Inches
Symbol
Min
Max
Min
Max
A
B
C
D
F
H
I
4.801
3.810
1.346
0.330
1.194
0.170
0.000
5.791
0.406
2.000
2.000
2.100
3.000
5.004
4.000
1.753
0.510
1.346
0.254
0.152
6.200
1.270
2.300
2.300
2.500
3.500
0.189
0.150
0.053
0.013
0.047
0.007
0.000
0.228
0.016
0.079
0.079
0.083
0.118
0.197
0.157
0.069
0.020
0.053
0.010
0.006
0.244
0.050
0.091
0.091
0.098
0.138
J
M
X
Y
X
Y
Option 1
Option 2
8-Lead SOP (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.
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
18
DS6204-04 February 2018
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