FR9855 [FITIPOWER]
18V, 5.5A Synchronous Step-Downn DC/DC Converter;
| 型号： | FR9855 |
| 厂家： | 
Fitipower |
| 描述： | 18V, 5.5A Synchronous Step-Downn DC/DC Converter |
| 文件： | 总16页 (文件大小：1031K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
FR9855
18V, 5.5A Synchronous Step-Down
DC/DC Converter
Description
Features
The FR9855 is a synchronous step-down DC/DC
converter with fast constant on time (FCOT) mode
control. The device provides 4.5V to 18V input
voltage range and 5.5A continuous load current
capability. Operation frequency depends on Input
and output voltage condition. At light load condition,
the FR9855 can operate at power saving mode to
support high efficiency and reduce power loss.
Low RDS(ON) Integrated Power MOSFET
(70mΩ/38mΩ)
Wide Input Voltage Range: 4.5V to 18V
Output Voltage Range: 0.765V to 8V
5.5A Output Current
FCOT Mode Enables Fast Transient Response
Pseudo 630kHz Frequency
Power Good Function (for SOP8-EP Only)
Input Under Voltage Lockout
The FR9855 fault protection includes cycle-by-cycle
current limit, short circuit protection, UVLO and
thermal shutdown. The soft-start function prevents
inrush current at turn-on. The FR9855 use fast
constant on time control that provides fast transient
response, the noise immunity and all kinds of very
low ESR output capacitor for ensuring performance
stabilization.
Adjustable Soft Start Function
Cycle-by-Cycle Current Limit
Hiccup Short Circuit Protection
Over Temperature Protection with Auto Recovery
SOP-8 Exposed Pad and TDFN-10(3mmx3mm)
Packages
Applications
STB (Set-Top-Box)
LCD Display, TV
Distributed Power System
Networking, XDSL Modem
The FR9855 is offered in SOP-8 (Exposed Pad) and
TDFN-10 (3mm x 3mm) packages, which provides
good thermal conductance.
Pin Assignments
SP Package (SOP-8 Exposed Pad)
Ordering Information
FR9855□
Package Type
SP: SOP-8 (Exposed Pad)
DA: TDFN-10(3mm x 3mm)
1
2
3
4
8
7
SHDN
FB
VIN
BST
LX
9
GND
PG
6
5
GND
SS
DA Package (TDFN-10)(3mm x 3mm)
VIN
VIN
BST
LX
1
2
3
4
5
10
9
SHDN
FB
11
GND
REG
SS
8
7
GND
6
LX
Figure 1. Pin Assignments of FR9855
FR9855-Preliminary 0.3-JAN-2016
1
FR9855
Typical Application Circuit
C5
0.1μF
Power Good
SHDN: >3.5V, Power Saving Mode
VOUT
SHDN: 1.5V~2.5V, PWM
R5
100kΩ
SHDN: 0V, Shutdown
L1
1.5μH
PG
VIN
BST
VIN
LX
VOUT
1.2V
4.5V to 18V
R3
100kΩ
C6
(optional)
C7
R1
12kΩ 1%
FR9855
22μF/6.3V
CERAMIC x 2
FB
SHDN
C1
C2
C3
10μF/25V
CERAMIC
10μF/25V
CERAMIC
0.1μF/50V
CERAMIC
R4
NC
GND
SS
R2
21kΩ 1%
C4
100nF
Figure 2. Application Circuit for SOP-8 Exposed Pad Package
VIN=12V, the recommended BOM list is as below.
VOUT
1.05
1.2
1.8
3.3
5
C1
R1
R2
C2
C6
L1
C7
10μF MLCC
10μF MLCC
10μF MLCC
10μF MLCC
10μF MLCC
7.87kΩ
12kΩ
21kΩ
21kΩ
21kΩ
21kΩ
21kΩ
10μF MLCC
10μF MLCC
10μF MLCC
10μF MLCC
10μF MLCC
5pF~33pF
5pF~33pF
5pF~33pF
5pF~33pF
5pF~33pF
1.5μH
1.5μH
1.5μH
2.2μH
3.3μH
22μF MLCC x2
22μF MLCC x2
22μF MLCC x2
22μF MLCC x2
22μF MLCC x2
28kΩ
69.8kΩ
118kΩ
Table 1. Recommended Component Values
FR9855-Preliminary 0.3-JAN-2016
2
FR9855
Typical Application Circuit (Continued)
C6
0.1μF
VREG
SHDN: >3.5V, Power Saving Mode
SHDN: 1.5V~2.5V, PWM
SHDN: 0V, Shutdown
C4
1uF
L1
1.5μH
REG
VIN
BST
VIN
LX
VOUT
1.2V
4.5V to 18V
R3
100kΩ
C7
(optional)
C8
22μF/6.3V
R1
12kΩ 1%
FR9855
CERAMIC x 2
FB
SHDN
C1
C2
C3
0.1μF/50V
CERAMIC
10μF/25V
CERAMIC
10μF/25V
CERAMIC
R4
NC
GND
SS
R2
21kΩ 1%
C5
100nF
Figure 3. Application Circuit for TDFN-10 Package
VIN=12V, the recommended BOM list is as below.
VOUT
1.05
1.2
1.8
3.3
5
C1
R1
R2
C2
C7
L1
C8
10μF MLCC
10μF MLCC
10μF MLCC
10μF MLCC
10μF MLCC
7.87kΩ
12kΩ
21kΩ
21kΩ
21kΩ
21kΩ
21kΩ
10μF MLCC
10μF MLCC
10μF MLCC
10μF MLCC
10μF MLCC
5pF~33pF
5pF~33pF
5pF~33pF
5pF~33pF
5pF~33pF
1.5μH
1.5μH
1.5μH
2.2μH
3.3μH
22μF MLCC x2
22μF MLCC x2
22μF MLCC x2
22μF MLCC x2
22μF MLCC x2
28kΩ
69.8kΩ
118kΩ
Table 2. Recommended Component Values
FR9855-Preliminary 0.3-JAN-2016
3
FR9855
Functional Pin Description
Pin
Name
Pin No.
(SOP-8EP) (TDFN3x3-10)
Pin No.
Pin Function
This pin includes enable the converter on/off, and select operation mode (The mode setting,
please refer to the following page 11). Connect VIN with a 100kΩ resistor for self-startup and
operate in power saving mode.
1
1
ꢀꢁꢂ
Voltage feedback input pin. Connect FB and VOUT with a resistive voltage divider. This IC
senses feedback voltage via FB and regulates it at 0.765V.
FB
PG
2
3
4
5
6
7
8
9
x
2
x
Open drain power good output.
Soft-start pin. This pin controls the soft-start period. Connect a capacitor from SS to GND to
set the soft-start period.
SS
4
GND
LX
5
Ground pin.
6,7
8
Power switching node. Connect an external inductor to this switching node.
High side gate drive boost pin. A capacitor rating between 10nF~100nF must be connected
from this pin to LX. It can boost the gate drive to fully turn on the internal high side NMOS.
BST
VIN
Power supply input pin. Placed input capacitors as close as possible from VIN to GND to
avoid noise influence.
9,10
11
3
Exposed
Pad
Ground pin. The exposed pad must be soldered to a large PCB area and connected to GND
for maximum power dissipation.
Internal regulator output. Connect a 1uF capacitor to GND to stabilize the internal regulator
voltage.
REG
Block Diagram
VIN
UVLO
&
POR
OTP
VCC
SHDN
Internal
Regulator
VCC
0.5M
Off Time
Generator
BST
REG
(TDFN-10 only)
High-Side
MOSFET
6µA
FB
On Time
Generator
SS
Driver
Logic
Logic
Control
LX
OTP
Vref
UVLO
Low-Side
MOSFET
PG
FB
Cycle by Cycle
Current Limit
(SOP-8 EP only)
0.9 x Vref.
Comparator
LX
OCP
GND
Figure 4. Block Diagram of FR9855
FR9855-Preliminary 0.3-JAN-2016
4
FR9855
(Note 1)
Absolute Maximum Ratings
● Supply Voltage VIN ------------------------------------------------------------------------------------------- -0.3V to +20V
● Enable Voltage ꢀꢁHꢂN -------------------------------------------------------------------------------------
-0.3V to +20V
● LX Voltage VLX ------------------------------------------------------------------------------------------------ -0.3 to VIN+0.3V
● Dynamic LX Voltage in 15ns Duration-------------------------------------------------------------------
-5V to VIN+5V
● BST Pin Voltage VBST --------------------------------------------------------------------------------------- -0.3V to VLX+6.5V
● All Other Pins Voltage -------------------------------------------------------------------------------------- -0.3V to +6V
● Maximum Junction Temperature (TJ) ------------------------------------------------------------------- +150°C
● Storage Temperature (TS) --------------------------------------------------------------------------------- -65°C to +150°C
+260°C
● Lead Temperature (Soldering, 10sec.) -----------------------------------------------------------------
● Package Thermal Resistance, (θJA) (Note 2)
SOP-8 (Exposed Pad) -------------------------------------------------------------------------- +60°C/W
TDFN-10 (3mmx3mm) ------------------------------------------------------------------------- +65°C/W
● Package Thermal Resistance, (θJC) (Note 2)
SOP-8(Exposed Pad) --------------------------------------------------------------------------- +15°C/W
TDFN-10(3mmx3mm) -------------------------------------------------------------------------- +30°C/W
Note 1：Stresses beyond this listed under “Absolute Maximum Ratings" may cause permanent damage to the device.
Note 2：θJA is measured at 25°C ambient with the component mounted on a high effective thermal conductivity 4-layer
board of JEDEC-51-7. θJC is measured at the exposed pad. The thermal resistance greatly varies with layout, copper
thickness, number of layers and PCB size.
Recommended Operating Conditions
● Supply Voltage VIN ------------------------------------------------------------------------------------------ +4.5V to +18V
● Operation Temperature Range --------------------------------------------------------------------------- -40°C to +85°C
FR9855-Preliminary 0.3-JAN-2016
5
FR9855
Electrical Characteristics
(VIN=12V, TA=25°C, unless otherwise specified.)
Parameter
VIN Quiescent Current
Symbol
IDDQ
ISD
Conditions
ꢀꢁHꢂN=5V, VFB=1V
ꢀꢁHꢂN=0V
Min
Typ
0.6
1
Max
1
Unit
mA
μA
V
VIN Shutdown Supply Current
Feedback Voltage
10
4.5V≦VIN≦18V
VFB=1V
VFB
0.75
0.765
0.01
70
0.78
0.1
Feedback Input Current
High-Side MOSFET RDS(ON)
Low-Side MOSFET RDS(ON)
Current Limit (Note 3)
IFB
μA
mΩ
mΩ
A
RDS(ON)
RDS(ON)
ILIMIT
38
6.8
155
250
4.3
0.35
6
On Time (Note 3)
TON
VIN=12V, VOUT=1.05V
VFB=0.6V
ns
ns
V
Minimum Off Time
TOFF(MIN)
Input Supply Voltage UVLO Threshold
UVLO Threshold Hysteresis
Soft Start Charge Current
VUVLO(Vth) VIN Rising
VUVLO(HYS)
V
ISS
VSS=0V
μA
ꢀꢁHꢂN(Lꢃ
0.5
V
V
ꢁHꢂN Input Low Voltage
ꢁHꢂN Input High Voltage
REG Output Voltage
ꢀꢁHꢂN(Hꢃ
1.5
VREG
IREG
4.1
10
90
85
4
V
6V≦VIN≦18V, TDFN-10
REG Output Current
mA
VREG=4.1V, TDFN-10
VFB Rising, SOP8-EP
VFB Falling, SOP8-EP
VPG=0.5V, SOP8-EP
Power Good Threshold (Note 3)
VPG
%
Power Good Sink Current
IPG
TSD
mA
°C
Thermal Shutdown Threshold (Note 3)
160
30
Thermal Shutdown Hysteresis (Note 3)
THYS
°C
Note 3：Not production tested.
FR9855-Preliminary 0.3-JAN-2016
6
FR9855
Typical Performance Curves
VIN=12V, VOUT=1.2V, C1=10μFx2, C7=22μFx2, L1=1.5μH, TA=+25°C, unless otherwise noted. This is measured by using
FR9855SP.
VOUT=1.05V
VOUT=1.2V
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
VIN=5V
VIN=5V
VIN=12V
VIN=18V
VIN=12V
VIN=18V
0.01
0.1
Load Current (A)
1
10
0.01
0.1
Load Current (A)
1
10
Figure 5. Efficiency vs. Load Current
Figure 6. Efficiency vs. Load Current
VOUT=3.3V
VOUT=5V
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
VIN=12V
VIN=18V
VIN=5V
VIN=12V
VIN=18V
0.01
0.1
Load Current (A)
1
10
0.01
0.1
Load Current (A)
1
10
Figure 7. Efficiency vs. Load Current
Figure 8. Efficiency vs. Load Current
780
775
770
765
760
755
750
780
775
770
765
760
755
750
5
7
9
11
13
15
17
19
-45 -35 -25 -15 -5
5 15 25 35 45 55 65 75 85
Ambient Temperature (°C)
Input Voltage (V)
Figure 9. Feedback Voltage vs. Ambient Temperature
Figure 10. Feedback Voltage vs. Input Voltage
FR9855-Preliminary 0.3-JAN-2016
7
FR9855
Typical Performance Curves (Continued)
VIN=12V, VOUT=1.2V, C1=10μFx2, C7=22μFx2, L1=1.5μH, TA=+25°C, unless otherwise noted. This is measured by using
FR9855SP.
1
0.8
0.6
0.4
0.2
0
1
0.8
0.6
0.4
0.2
0
5
7
9
11
13
15
17
19
-45 -35 -25 -15 -5
5 15 25 35 45 55 65 75 85
Ambient Temperature (°C)
Input Voltage (V)
Figure 11. Quiescent Current vs. Ambient Temperature
Figure 12. Quiescent Current vs. Input Voltage
1
1
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
5
7
9
11
13
15
17
19
-45 -35 -25 -15 -5
5 15 25 35 45 55 65 75 85
Ambient Temperature (°C)
Input Voltage (V)
Figure 13. Shutdown Current vs. Ambient Temperature
VIN=12V, V ꢀꢁꢂ=2V VOUT=1V, L=1.5μH
Figure 14. Shutdown Current vs. Input Voltage
VIN=12V, V ꢀꢁꢂ=2V VOUT=5V, L=3.3μH
700
800
750
700
650
600
550
500
450
400
650
600
550
500
450
400
350
300
1
1.5
2
2.5
Load Current (A)
Figure 16. Switch Frequency vs. Load Current
3
3.5
4
4.5
5
5.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
Load Current (A)
Figure 15. Switch Frequency vs. Load Current
FR9855-Preliminary 0.3-JAN-2016
8
FR9855
Typical Performance Curves (Continued)
VIN=12V, VOUT=1.2V, C1=10μFx2, C7=22μFx2, L1=1.5μH, TA=+25°C, unless otherwise noted. This is measured by using
FR9855SP.
IOUT=5.5A
IOUT=0A
VOUT 50mV/div.
VOUT 50mV/div.
VLX 5V/div.
VLX 5V/div.
IL 5A/div.
IL
0.5A/div.
10ms/div.
Figure 17. DC Ripple Waveform
2μs/div.
Figure 18. DC Ripple Waveform
IOUT=5.5A
IOUT=0A
VIN 5V/div.
VIN 5V/div.
VOUT 0.5V/div.
VOUT 0.5V/div.
IL
1A/div.
IL
2A/div.
VLX 5V/div.
VLX 5V/div.
20ms/div.
20ms/div.
Figure 19. Startup Through Power Supply Waveform
Figure 20. Startup Through Power Supply Waveform
IOUT=0A
IOUT=5.5A
VIN 5V/div.
VIN 5V/div.
VOUT
1V/div.
VOUT 1V/div.
IL 5A/div.
IL 5A/div.
VLX 5V/div.
40ms/div.
VLX 5V/div.
40ms/div.
Figure 22. Shutdown Through Power Supply Waveform
Figure 21. Shutdown Through Power Supply Waveform
FR9855-Preliminary 0.3-JAN-2016
9
FR9855
Typical Performance Curves (Continued)
VIN=12V, VOUT=1.2V, C1=10μFx2, C7=22μFx2, L1=1.5μH, TA=+25°C, unless otherwise noted. This is measured by using
FR9855SP.
IOUT=0A
IOUT=5.5A
ꢃ
5V/div.
ꢃ
5V/div.
1V/div.
ꢀꢁꢂ
ꢀꢁꢂ
VOUT 1V/div.
IL 1A/div.
VOUT
IL 5A/div.
VLX 5V/div.
VLX 5V/div.
20ms/div.
20ms/div.
Figure 23. Startup Through ꢁHꢂN Waveform
IOUT=0A
Figure 24. Startup Through ꢁHꢂN Waveform
IOUT=5.5A
ꢃ
5V/div.
ꢃ
5V/div.
ꢀꢁꢂ
ꢀꢁꢂ
VOUT 1V/div.
VOUT 1V/div.
IL
5A/div.
IL
5A/div.
VLX 5V/div.
VLX 5V/div.
4ms/div.
4ms/div.
Figure 26. Shutdown Through ꢁHꢂN Waveform
Figure 25. Shutdown Through ꢁHꢂN Waveform
IOUT=0.1A~5.5A
VOUT
50mV/div.
IL 2A/div.
200μs /div.
Figure 27. Load Transient Waveform
FR9855-Preliminary 0.3-JAN-2016
10
FR9855
Function Description
The FR9855 is a synchronous step-down DC/DC
converter with fast constant on time (FCOT) mode
control. It has integrated high-side (70mΩ, typꢃ and
low-side (38mΩ, typ) power switches, and provides
5.5A continuous load current. It regulates input
voltage from 4.5V to 18V, and down to an output
voltage as low as 0.765V. Using FCOT control
scheme provides fast transient response, which can
minimize the component size without additional
external compensation network.
Input Under Voltage Lockout
When the FR9855 is power on, the internal circuits
are held inactive until VIN voltage exceeds the input
UVLO threshold voltage. And the regulator will be
disabled when VIN is below the input UVLO
threshold voltage. The hysteretic of the UVLO
comparator is 350mV (typ).
Over Current Protection
The FR9855 over current protection function is
implemented using cycle-by-cycle current limit
architecture. The inductor current is monitored by
Low-side MOSFET. When the load current
increases, the inductor current also increases.
When the valley inductor current reaches the current
limit threshold, the output voltage starts to drop.
When the over current condition is removed, the
output voltage returns to the regulated value.
Enable/Mode
The FR9855 ꢁHꢂN pin includes enable and mode
function. Enable function provides digital control to
turn on/off the converter. When the voltage of ꢁHꢂN
exceeds the threshold voltage, the converter starts
the soft start function. If the ꢁHꢂN pin voltage is
below than the shutdown threshold voltage, the
converter will turn into the shutdown mode and the
shutdown current will be smaller than 1μA. The
mode function can be selected in PWM or power
saving mode. The mode setting can refer to
following table.
Short Circuit Protection
The FR9855 provides short circuit protection
function to prevent the device damage from short
condition. When the short condition occurs and the
feedback voltage drops lower than 0.33V, the
oscillator frequency will be reduced naturally and
hiccup mode will be triggered to prevent the inductor
current increasing beyond the current limit. Once the
short condition is removed, the frequency will return
to normal.
ꢀꢁꢂ
>3.5V
Mode
Power Saving Mode
PWM
1.5V~2.5V
0V
Shutdown
For auto start-up operation, connect ꢁHꢂN to VIN
through a 100kΩ resistor, and the converter can
automatically enter power saving mode.
Over Temperature Protection
The FR9855 incorporates an over temperature
protection circuit to protect itself from overheating.
When the junction temperature exceeds the thermal
shutdown threshold temperature, the regulator will
be shutdown. And the hysteretic of the over
temperature protection is 30°C (typ).
Soft Start
The FR9855 employs adjustable soft start function
to reduce input inrush current during start up.
When the device turns on, a 6μA current begins
charging the capacitor which is connected from SS
pin to GND. The equation for the soft start time is
shown as below:
Power Good Signal Output (PG)
PG pin is an open-drain output and requires a pull
up resistor. PG is actively held low in soft-start,
standby and shutdown. It is released when the
output voltage rises above 90% of nominal
regulation point.
ꢀ
ꢁ
Cꢁꢁ nF ꢅꢀFꢆ
ꢀ
ꢁ
Tꢁꢁ ms ꢄ
ꢀ
ꢁ
ꢇꢁꢁ μA
The VFB voltage is 0.765V and the ISS current is 6μA.
If a 100nF capacitor is connected from SS pin to
GND, the soft-start time will be 12.75ms.
FR9855-Preliminary 0.3-JAN-2016
11
FR9855
Application Information
Output Voltage Setting
A low ESR capacitor is required to keep the noise
minimum. Ceramic capacitors are better, but
tantalum or low ESR electrolytic capacitors may also
suffice. When using tantalum or electrolytic
capacitors, a 0.1μF ceramic capacitor should be
placed as close to the IC as possible.
The output voltage VOUT is set using a resistive
divider from the output to FB. The FB pin regulated
voltage is 0.765V. Thus the output voltage equation
is:
R1
ꢀOꢈTꢄ0.ꢉ6ꢊꢀꢅ ꢂ1ꢋ
ꢃ
Output Capacitor Selection
R2
The output capacitor is used to keep the DC output
voltage and supply the load transient current.
When operating in constant current mode, the
output ripple is determined by four components:
Table 3 lists recommended values of R1 and R2 for
most used output voltage.
Table 3 Recommended Resistance Values
VOUT
5V
R1
R2
ꢀ ꢁ ꢀ ꢁ
ꢀRꢇPPLꢍ t ꢄꢀRꢇPPLꢍ C t ꢋꢀRꢇPPLꢍ ꢍꢁR t
ꢁ
ꢀ ꢁ
ꢀ
ꢁ
ꢀ
118kΩ
69.8kΩ
28kΩ
21kΩ
21kΩ
21kΩ
21kΩ
21kΩ
ꢀ ꢁ
ꢋꢀRꢇPPLꢍ(ꢍꢁLꢃ t ꢋꢀNOꢇꢁꢍ
ꢀ ꢁ
t
3.3V
1.8V
1.2V
1.05V
The following figures show the form of the ripple
contributions.
12kΩ
VRIPPLE(ESR)(t)
7.87kΩ
Place resistors R1 and R2 close to FB pin to prevent
stray pickup.
Input Capacitor Selection
(t)
+
The use of the input capacitor is filtering the input
voltage ripple and the MOSFETS switching spike
voltage. Because the input current to the step-down
converter is discontinuous, the input capacitor is
required to supply the current to the converter to
keep the DC input voltage. The capacitor voltage
rating should be 1.25 to 1.5 times greater than the
maximum input voltage. The input capacitor ripple
current RMS value is calculated as:
VRIPPLE(ESL) (t)
(t)
(t)
+
VRIPPLE(C) (t)
ꢀ
ꢁ
ꢇCꢇN(RMꢁꢃꢄꢇOꢈTꢅꢄꢂꢅ 1ꢌꢂ
ꢀOꢈT
ꢂꢄ
+
VNOISE (t)
ꢀꢇN
Where D is the duty cycle of the power MOSFET.
This function reaches the maximum value at D=0.5
and the equivalent RMS current is equal to IOUT/2.
=
The
following
diagram
is
the
graphical
VRIPPLE(t)
representation of above equation.
3
5.5A
3A
2.5
2
1.5
1
(t)
1A
0.5
0
10 20 30 40 50 60 70 80 90
D (%)
FR9855-Preliminary 0.3-JAN-2016
12
FR9855
Application Information (Continued)
ꢀOꢈT
ꢀ
That will lower ripple current and result in lower
output ripple voltage. The ΔꢇL is inductor
peak-to-peak ripple current:
ꢀRꢇPPLꢍ(ꢍꢁRꢃ
ꢄ
ꢅ ꢂ1ꢆ OꢈTꢃ ꢅꢍꢁR
FOꢁCꢅL
ꢀꢇN
ꢍꢁL
ꢀRꢇPPLꢍ(ꢍꢁLꢃ
ꢄ
ꢅꢀꢇN
ꢀOꢈT
ꢀ
ꢅ ꢂ1ꢆ OꢈTꢃ
ꢀꢇN
LꢋꢍꢁL
ꢀOꢈT
ꢎꢅFOꢁC2ꢅLꢅCOꢈT
ꢏꢇLꢄ
FOꢁCꢅL
ꢀ
ꢅ ꢂ1ꢆ OꢈTꢃ
ꢀꢇN
ꢀRꢇPPLꢍ(Cꢃ
ꢄ
A good compromise value between size and
efficiency is to set the peak-to-peak inductor ripple
current ΔꢇL equal to 30% of the maximum load
current. But setting the peak-to-peak inductor ripple
current ΔꢇL between 20%~50% of the maximum load
current is also acceptable. Then the inductance can
be calculated with the following equation:
Where FOSC is the switching frequency, L is the
inductance value, VIN is the input voltage, ESR is the
equivalent series resistance value of the output
capacitor, ESL is the equivalent series inductance
value of the output capacitor and the COUT is the
output capacitor.
ꢏꢇLꢄ0.3ꢅꢇOꢈT(MAꢐꢃ
Low ESR capacitors are preferred to use.
Ceramic, tantalum or low ESR electrolytic capacitors
can be used depending on the output ripple
requirement. When using the ceramic capacitors,
the ESL component is usually negligible.
ꢀ
ꢁ
ꢀꢇNꢆꢀOꢈT ꢅꢀOꢈT
Lꢄ
ꢀꢇNꢅFOꢁCꢅꢏꢇL
External Diode Selection
For 5V input applications, it is recommended to add
an external boost diode. This helps improving the
efficiency. The boost diode can be a low cost one
such as 1N4148.
It is important to use the proper method to eliminate
high frequency noise when measuring the output
ripple. The figure shows how to locate the probe
across the capacitor when measuring output ripple.
Removing the scope probe plastic jacket in order to
expose the ground at the tip of the probe. It gives a
very short connection from the probe ground to the
capacitor and eliminating noise.
D1
1N4148
VIN
VIN
5V
BST
LX
FR9855
C3
Probe Ground
REG Capacitor Selection
Connect a 1uF ceramic capacitor between the REG
and GND, This helps stabilize the internal regulator
voltage.
VOUT
GND
Ceramic Capacitor
Inductor Selection
The output inductor is used for storing energy and
filtering output ripple current. But the trade-off
condition often happens between maximum energy
storage and the physical size of the inductor. The
first consideration for selecting the output inductor is
to make sure that the inductance is large enough to
keep the converter in the continuous current mode.
FR9855-Preliminary 0.3-JAN-2016
13
FR9855
Application Information (Continued)
PCB Layout Recommendation
L1
VIN
LX
VOUT
The device’s performance and stability is
dramatically affected by PCB layout. It is
recommended to follow these general guidelines
shown as below:
C5
6
5
8
1
7
C3
C7
C1 C2
GND
Exposed
Pad
1. Place the input capacitors and output capacitors
as close to the device as possible. Trace to these
capacitors should be as short and wide as
possible to minimize parasitic inductance and
resistance.
GND
4
2
3
C4
R5
R3
R2
2. Place feedback resistors close to the FB pin.
R1
C6
3. Keep the sensitive signal (FB) away from the
switching signal (LX).
Figure 28. Recommended PCB Layout Diagram for
SP Package
4. The exposed pad of the package should be
soldered to an equivalent area of metal on the
PCB. This area should connect to the GND
plane and have multiple via connections to the
back of the PCB as well as connections to
intermediate PCB layers. The GND plane area
connecting to the exposed pad should be
maximized to improve thermal performance.
L1
VIN
LX
VOUT
C3
C2
C1
C8
9
7
6
5
10
1
8
Exposed
Pad
GND
GND
5. Multi-layer PCB design is recommended.
2
3
4
C5
C4
R3
R2
R1
C7
Figure 29. Recommended PCB Layout Diagram for
DA Package
FR9855-Preliminary 0.3-JAN-2016
14
FR9855
Outline Information
SOP-8 (Exposed Pad) Package (Unit: mm)
DIMENSION IN MILLIMETER
SYMBOLS
UNIT
MIN
1.25
0.00
1.25
0.31
4.80
3.04
3.80
2.15
1.20
5.80
0.40
MAX
1.70
0.15
1.55
0.51
5.00
3.50
4.00
2.41
1.34
6.20
1.27
A
A1
A2
B
D
D1
E
E1
e
H
L
Note：Followed From JEDEC MO-012-E.
Carrier Dimensions
Life Support Policy
Fitipower’s products are not authorized for use as critical components in life support devices or other medical systems.
FR9855-Preliminary 0.3-JAN-2016
15
FR9855
Outline Information (Continued)
TDFN-10 3mm x 3mm (pitch 0.5 mm) Package (Unit: mm)
DIMENSION IN MILLIMETER
SYMBOLS
UNIT
MIN
0.70
0.00
0.18
2.95
2.95
0.30
0.18
0.45
2.20
1.40
MAX
0.80
0.05
0.25
3.05
3.05
0.50
0.30
0.55
2.70
1.75
A
A1
A2
D
E
a
b
e
D1
E1
Note：Followed From JEDEC MO-229F.
Carrier Dimensions
Life Support Policy
Fitipower’s products are not authorized for use as critical components in life support devices or other medical systems.
FR9855-Preliminary 0.3-JAN-2016
16
相关型号：
©2020 ICPDF网 联系我们和版权申明