FAIRCHILD FAN8303

FAN8303
2A 23V Non-Synchronous Step-Down DC/DC Regulator
Features
Description
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The FAN8303 is a monolithic, non-synchronous, stepdown (buck) regulator with internal power MOSFETs. It
achieves 2A continuous output current over a wide input
supply range with excellent load and line regulation.
Current-mode operation provides fast transient
response and eases loop stabilization. Fault condition
protection includes cycle-by-cycle current limiting and
thermal shutdown. The regulator draws less than 40µA
shutdown current. FAN8303 requires a minimum
number of readily available standard external components.
2A Output Current
0.22Ω Internal Power MOSFET Switch
Wide 5V to 23V Operating Input Range
Output Adjustable from 0.6 to 20V
Stable with Low ESR Output Ceramic Capacitors
Up to 90% Efficiency
Less than 40µA Shutdown Current
Fixed 370kHz Frequency
Thermal Shutdown with Hysteresis
Cycle-by-Cycle Over-Current Protection
Available in 8-Pin SOIC Package
External compensation, enable, and programmable
soft-start features allow design optimization and
flexibility. Cycle-by-cycle current limit, frequency
foldback, and thermal shutdown provide protection
against shorted outputs.
CBS
10 nF
Applications
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INPUT
5~23V
C IN
10 µF
Set-Top Box
VIN
DSL and Cable Modems
ENABLE
SHUTDOWN
BS
EN
L1
15 µ H
SS
Consumer Appliances (DVD)
2.5V/2A
D1
FAN8303
Distributed Power Systems
OUTPUT
SW
FB
GND
R2
18 k
COMP
COU T
CSS
Auxiliary supplies
22 µF
R3
10 nF
RC
22 k
CC
1nF
5.6 k
CA
OPEN
Figure 1. Typical Application
Ordering Information
Part Number
FAN8303MX
Eco Status
RoHS
Operating Temperature Range
Package
Packing Method
-40°C to +85°C
8-SOIC
Reel
For Fairchild’s definition of “green” Eco Status, please visit: http://www.fairchildsemi.com/company/green/rohs_green.html.
© 2008 Fairchild Semiconductor Corporation
FAN8303 • Rev. 1.0.0
www.fairchildsemi.com
FAN8303 — 2A 23V Non-Synchronous Step-Down DC/DC Regulator
December 2008
Figure 2.
© 2008 Fairchild Semiconductor Corporation
FAN8303 • Rev. 1.0.0
Functional Block Diagram
FAN8303 — 2A 23V Non-Synchronous Step-Down DC/DC Regulator
Internal Block Diagram
www.fairchildsemi.com
2
BS
SS
VIN
EN
SW
COMP
GND
Figure 3.
FB
Pin Configuration (Top View)
Pin Definitions
Name
Pin #
Type
Description
BS
1
Bootstrap
High-Side Drive BOOT Voltage. Connect through capacitor (CBS) to SW.
The IC includes an internal synchronous bootstrap diode to recharge the
capacitor on this pin to VCC when SW is LOW.
VIN
2
Supply Voltage
Power Input. This pin needs to be closely decoupled to the GND pin with a
10µF or greater ceramic capacitor.
SW
3
Switch
Power Switching Output. SW is the switching node that supplies power to
the output.
GND
4
Ground
Power Return and Signal Ground for the IC. All internal control voltages
are referred to this pin. Tie this pin to the ground island / plane through the
lowest impedance connection. This pin is the ground reference for the
regulated output voltage.
FB
5
Feedback
Feedback Input. This pin is the center tap of the external feedback voltage
resistive divider across the output.
COMP
6
Compensation
Compensation Node. Frequency compensation is accomplished at this
node by connecting a series R-C to ground.
EN
7
Enable
Enable Input. EN is a digital input that turns the regulator on or off. Drive
EN HIGH to turn on the regulator, drive it LOW to turn it off. For automatic
startup, leave EN unconnected.
SS
8
Soft Start
External Soft-Start. A capacitor connected between this pin and GND can
be used to set soft-start time.
© 2008 Fairchild Semiconductor Corporation
FAN8303 • Rev. 1.0.0
FAN8303 — 2A 23V Non-Synchronous Step-Down DC/DC Regulator
Pin Configuration
www.fairchildsemi.com
3
Stresses exceeding the absolute maximum ratings may damage the device. The device may not function or be
operable above the recommended operating conditions and stressing the parts to these levels is not recommended.
In addition, extended exposure to stresses above the recommended operating conditions may affect device
reliability. The absolute maximum ratings are stress ratings only. All voltage values, except differential voltages, are
given with respect to the network ground terminal. Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device
Symbol
Parameter
VIN
Supply Voltage, VIN to GND
VSW
Switch Voltage, SW to GND
VBS
Boost Voltage
VFB
Feedback Voltage
VEN
Min.
-0.3
Max.
Unit
25
V
VIN+0.3
V
VSW + 6
V
6.0
V
-0.3
Enable Voltage
-0.3
6.0
V
Compensation Voltage
-0.3
6.0
V
VSS
Soft-Start Voltage
-0.3
6.0
V
ΘJA
Thermal Resistance, Junction-Air
105
°C/W
ΘJC
Thermal Resistance, Junction-Case
40
°C/W
+125
°C
+260
°C
+150
°C
VCOMP
TJ
Operating Junction Temperature
TL
Lead Temperature (Soldering, 5 Seconds)
-40
TSTG
Storage Temperature Range
-65
Human Body Model, JEDEC JESD22-A114
Electrostatic Discharge
Charged Device Model, JEDEC JESD22Protection Level
C101
3.0
ESD
kV
2.5
Recommended Operating Conditions
FAN8303 — 2A 23V Non-Synchronous Step-Down DC/DC Regulator
Absolute Maximum Ratings
The Recommended Operating Conditions table defines the conditions for actual device operation. Recommended
operating conditions are specified to ensure optimal performance to the datasheet specifications. Fairchild does not
recommend exceeding them or designing to absolute maximum ratings.
Symbol
Parameter
VIN
Supply Voltage
TA
Operating Ambient Temperature
© 2008 Fairchild Semiconductor Corporation
FAN8303 • Rev. 1.0.0
Min
Max.
Unit
5
23
V
-40
+85
°C
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4
VIN=12V, TA= -40 to +85°C, unless otherwise noted.
Symbol
VFB
Parameter
Feedback Voltage
Condition
Min.
Typ.
Max.
Unit
25°C, 5V<VIN<23V
0.58
0.60
0.62
V
RON_H
Upper Switch On Resistance
0.22
Ω
RON_L
Lower Switch On Resistance
4
Ω
ILKG
Upper Switch Leakage Current
IPK
Peak Inductor Current
fOSC
Oscillator Frequency
VFB>0.3V
315
370
435
kHz
VUVLO
Under-Voltage Lockout
Rising VIN
4.2
4.6
5.0
V
fSHORT
Short Circuit Frequency
VFB<0.3V
25
45
55
kHz
DMAX
Maximum Duty Cycle
90
%
TON_MIN
Minimum On Time
210
ns
VEN
Enable Threshold
VEN_H
VEN=0V,VSW =0V
0
10
3.5
1.2
Enable Threshold Hysteresis
1.6
µA
A
2.0
150
V
mV
IOFF
Supply Current (Shutdown)
VEN=0V
10
40
µA
IQ
Supply Current (Quiescent)
VEN>1.6V; VFB=0.8V
1.0
2.0
mA
GCS
Current Sense Gain
GEA
AVEA
ISS
TSD
2
A/V
Error Amplifier Transconductance
380
µA/V
Error Amplifier Voltage Gain
400
V/V
6
µA
155
°C
Soft-Start Current
Thermal Shutdown Temperature
© 2008 Fairchild Semiconductor Corporation
FAN8303 • Rev. 1.0.0
FAN8303 — 2A 23V Non-Synchronous Step-Down DC/DC Regulator
Electrical Characteristics
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5
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VIN = 12V, VOUT = 5V, L1 = 15μH, CIN = 10μF, COUT = 22μF, TA = +25 C, unless otherwise noted.
CH2
CH1(VO) : 2V, 500µs/div.
CH2(EN) : 4V, 500µs/div.
CH3(SW) : 6V, 500µs/div.
CH4(IL) : 1A, 500µs/div.
CH1(VO) : 2V, 50µs/div.
CH2(EN) : 4V, 50µs/div.
CH3(SW) : 6V, 50µs/div.
CH4(IL) : 1A, 50µs/div.
CH2
CH1
CH1
CH3
CH3
CH4
CH4
Figure 4. EN Startup with 2A Load
Figure 5. EN Turn-off with 2A Load
CH1
CH1
CH2
CH2
CH3
CH3
CH1(VO) : 2V, 1ms/div.
CH2(VIN) : 4V, 1ms/div.
CH3(SW) : 6V, 1ms/div.
CH4(Io) : 1A, 1ms/div.
CH1(VO) : 2V, 200µs/div.
CH2(VIN) : 4V, 200µs/div.
CH3(SW) : 6V, 200µs/div.
CH4(Io) : 1A, 200µs/div.
CH4
CH4
Figure 6. Power-on with 2A Load
Figure 7. Power-off with 2A Load
CH1(VO) : 5V offset
200mV, 50µs/div.
CH2(COMP) : 300mV, 50µs/div.
CH3(SW) : 10V, 50µs/div.
CH4(Io) : 1A, 50µs/div.
CH1
FAN8303 — 2A 23V Non-Synchronous Step-Down DC/DC Regulator
Typical Performance Characteristics
CH1
△Vo = 240mV
CH1(VO) : 5V offset
200mV, 50µs/div.
CH2(COMP) : 300mV, 50µs/div.
CH3(SW) : 10V, 50µs/div.
CH4(Io) : 1A, 50µs/div.
△Vo = 204mV
Slew Rate( 2.5A/µs)
Slew Rate( 2.5A/µs)
CH4
CH4
CH3
CH3
CH2
CH2
Figure 8. Load Transient Response (0.5A to 1.5A)
© 2008 Fairchild Semiconductor Corporation
FAN8303 • Rev. 1.0.0
Figure 9. Load Transient Response (1.5A to 0.5A)
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o
VIN = 12V, VOUT = 5V, L1 = 15μH, CIN = 10μF, COUT = 22μF, TA = +25 C, unless otherwise noted.
Δf=45kHz
CH1
CH1
CH2
CH2
CH1(VO) : 2V, 20µ/div.
CH2(VIN) : 4V, 20µs/div.
CH3(SW) : 6V, 20µs/div.
CH4(IL) : 2A, 20µs/div.
CH4
CH1(VO) : 2V, 20µ/div.
CH2(VIN) : 4V, 20µs/div.
CH3(SW) : 6V, 20µs/div.
CH4(IL) : 2A, 20µs/div.
CH4
Figure 10. Hard-Short at Output (OCP)
Figure 11. Overload at Output (OCP)
1
95
5.0Vo
0.5
3.3Vo
0
85
Vout [%]
Efficiency [%]
90
2.5Vo
80
-0.5
-1
1.8Vo
75
-1.5
-40
70
0
0.5
1
1.5
Load Current [A]
10
35
60
85
2
Temperature [℃]
Figure 13. Normalized Output Voltage vs.
Temperature
Figure 12. Efficiency Curve
4
380
Load Current [A]
370
Frequency [kHz]
-15
FAN8303 — 2A 23V Non-Synchronous Step-Down DC/DC Regulator
Typical Performance Characteristics (Continued)
360
350
340
330
320
3.5
3
2.5
2
-40
-15
10
35
60
85
0
Temperature [℃]
40
60
80
100
Duty [%]
Figure 14. Oscillator Frequency vs. Temperature
© 2008 Fairchild Semiconductor Corporation
FAN8303 • Rev. 1.0.0
20
Figure 15. Current Limited Level vs. Duty Ratio
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The FAN8303 is a monolithic, non-synchronous,
current-mode, step-down regulator with internal power
MOSFETs. It achieves 2A continuous output current
over a wide input supply range from 5V to 23V with
excellent load and line regulation. The output voltage
can be regulated as low as 0.6V. The FAN8303 uses
current-mode operation that provides fast transient
response and eases loop stabilization. The FAN8303
requires a minimum number of readily available
standard external components.
Inductor Selection
A higher inductor value lowers ripple current. The
inductor value can be calculated as:
L=
VOUT
fS ⋅ ΔIL
⎛ VOUT
⎜1 −
⎜
VIN
⎝
⎞
⎟
⎟
⎠
(1)
where:
fs is the switching frequency;
Current Mode PWM Control Loop
VOUT is the output voltage;
FAN8303 uses current-mode PWM control scheme.
The peak inductor current is modulated in each
switching cycle by an internal op-amp output signal to
achieve the output voltage regulation. An internal slope
compensation circuit is included to avoid sub-harmonic
oscillation at duty cycle greater than 50%. Currentmode control provides cycle-by-cycle current limit
protection and superior regulation control loop response
compared to the traditional voltage-mode control.
VIN is the input supply voltage; and
ΔIL Is the inductor ripple current.
Considering worst case, the equation is changed to:
L=
In normal operation, the high-side MOSFET is turned on
at the beginning of each switching cycle, which causes
the current in the inductor to build up. The currentcontrol loop senses the inductor current by sensing the
voltage across the high-side senseFET during on time.
The output of the current-sense amplifier is summed
with the slope compensation signal and the combined
signal is compared with the error amplifier output to
generate the PWM signal. As the inductor current
ramps up to the controlled value, the high-side
MOSFET is turned off and the inductor current reaches
zero through a freewheeling diode. In light-load
condition, the high-side switch may be kept off for
several cycles to improve efficiency.
⎛
⎜ 1 − VOUT
⎜ VIN ,MAX
⎝
⎞
⎟
⎟
⎠
(2)
Input Capacitor Selection
To prevent high-frequency switching current passing to
the input, the input capacitor impedance at the
switching frequency must be less than input source
impedance. High-value, small, inexpensive, lower-ESR
ceramic capacitors are recommended. 10µF ceramic
capacitors should be adequate for 2A applications.
Output Capacitor Selection
A larger output capacitor value keeps the output ripple
voltage smaller. The formula of output ripple ΔVOUT is:
⎛
1
ΔVOUT ≅ ΔIL ⎜⎜ ESR +
⋅
⋅ fS
8
C
OUT
⎝
Short-Circuit Protection
The FAN8303 protects output short circuit by switching
frequency fold-back. The oscillator frequency is reduced
to about 45kHz when the output is shorted to ground.
This frequency fold-back allows the inductor current
more time to decay to prevent potential run-away
condition. The oscillator frequency switches to 370kHz
as VOUT rises gradually from 0V back to regulated level.
⎞
⎟
⎟
⎠
(3)
where COUT is the output capacitor and ESR is the
equivalent series resistance of the output capacitor.
Output Voltage Programming
The output voltage is set by a resistor divider, according
to the following equation:
Slope Compensation and Inductor Peak
Current
R2 ⎞
⎛
VOUT = 0.6⎜1 +
⎟
R3 ⎠
⎝
The slope compensation provides stability in constant
frequency architecture by preventing sub-harmonic
oscillations at high duty cycles. It is accomplished
internally by adding a compensating ramp to the
inductor current signal at duty cycles in excess of 50%.
(4)
Freewheeling Diode
An output freewheeling diode carries load current when
the high-side switch is turned off. Therefore, use a
Schottky diode to reduce loss due to diode forward
voltage and recovery time. The diode should have at
least 2A current rating and a reverse blocking voltage
greater than the maximum input voltage. The diode
should be close to the SW node to keep traces short
and reduce ringing.
Maximum Load Current at Low VIN
The FAN8303 is able to operate with input supply
voltage as low as 5V, although the maximum allowable
output current is reduced as a function of duty cycle
(see Figure 15). Additionally, at this low input voltage; if
the duty cycle is greater than 50%, slope compensation
reduces allowable output current.
© 2008 Fairchild Semiconductor Corporation
FAN8303 • Rev. 1.0.0
VOUT
fS ⋅ ΔI L,MAX
FAN8303 — 2A 23V Non-Synchronous Step-Down DC/DC Regulator
Functional Description
www.fairchildsemi.com
8
A capacitor, CSS, connected between the SS pin and
GND helps control the rate of rise on the output voltage.
When EN is HIGH and VIN is within the operating range,
a trimmed bias current charges the capacitor connected
to the SS pin, causing the voltage to rise.
The first step of the compensation design is choosing
the compensation resistor (RC) to set the crossover
frequency by the following equation:
The time it takes this voltage to reach 0.6V and the
PWM output to reach regulation is given by:
tRISE ( ms ) ≈ 0.1 • CSS
RC =
(5)
where CSS is in nF.
2π ⋅ COUT ⋅ fC ⋅ VOUT
GCS ⋅ GEA ⋅ VFB
(10)
Loop Compensation
where VFB is reference voltage and GCS is the current
sense gain, which is roughly the output current divided
by the voltage at COMP (2A/V).
The goal of the compensation design is to shape the
converter frequency response to achieve high DC gain
and fast transient, while maintaining loop stability.
FAN8303 employs peak current-mode control for fast
transient response and to help simplify the loop to a
one-pole and one-zero system.
The next step is choosing the compensation capacitor
(CC) to achieve the desired phase margin. For
applications with typical inductor values, setting the
compensation zero, fZ2, to below one fourth of the
crossover frequency provides sufficient phase margin.
Determine the (CC) value by the following equation:
The system pole is calculated by the equation:
fP1 =
1
2π ⋅ COUT ⋅ RL
CC =
(6)
The system zero is due to the output capacitor and its
ESR system zero is calculated by following equation:
fz1 =
2π ⋅ COUT ⋅ ESR
1
f
< S
2π ⋅ COUT ⋅ ESR 2
(7)
CA =
The pole is calculated by the following equation:
GEA
2π ⋅ CC ⋅ AVEA
(12)
If required, add the second compensation capacitor
(CA) to set the pole fP3 at the location of the ESR zero.
Determine the (CA) value by the equation:
The characteristics of the control system are controlled
by a series capacitor and resistor network connected to
the COMP pin to set the pole and zero.
fp 2 =
(11)
Determine if the second compensation capacitor (CA) is
required. It is required if the ESR zero of the output
capacitor is located at less than half of the switching
frequency.
where RL is the load resistor value (VOUT/IOUT).
1
2
π ⋅ RC ⋅ fC
(8)
COUT ⋅ ESR
RC
FAN8303 — 2A 23V Non-Synchronous Step-Down DC/DC Regulator
The system crossover frequency (fC), where the control
loop has unity gain, is recommended for setting the
1/10th of switching frequency. Generally, higher fC
means faster response to load transients, but can result
in instability if not properly compensated.
Soft-Start
(13)
SW
FAN8303
VO
where:
_
GEA is the error amplifier transconductance (380µA/V);
PWM
modulator
AVEA is the error amplifier voltage gain (400V/V); and
CC is the compensation capacitor.
1
2π ⋅ CC ⋅ RC
RC
CA
CC
(9)
where RC is compensation resistor.
© 2008 Fairchild Semiconductor Corporation
FAN8303 • Rev. 1.0.0
0.6V
COMP
Zero is due to the compensation capacitor (CC) and
resistor (RC) calculated by the following equation:
f z2 =
+
FB
Figure 16.
Block Diagram of Compensation
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9
Layout Consideration
Assume the VIN voltage is 12V with a 10% tolerance.
The maximum load current is 2A and the output voltage
is set to 2.5V at 2A maximum load. Calculate the
inductor value from the following formula:
As with all switching power supplies, careful attention to
PCB layout is important to the design. A few design
rules should be implemented to ensure good layout:
VOUT
L=
fOSC ⋅ ΔI L,MAX
⎛
⎜1 − VOUT
⎜ VIN ,MAX
⎝
⎞
⎟
⎟
⎠
(14)
Substituting VOUT=2.5V, VIN,MAX=12V, Δ IL,MAX=0.4A, and
fS = 370kHz in the formula gives:
2.5 ⎞
2.5
⎛
L=
⎜1 −
⎟ = 13 μH
370kHz(0.4 A ) ⎝
12 ⎠
ƒ
Keep the high-current traces and load connections
as short as possible.
ƒ
Place the input capacitor, the inductor, the
freewheeling diode, and the output capacitor as
close as possible to the IC terminals.
ƒ
Keep the loop area between the SW node,
freewheeling diode, inductor, and output capacitor
as small as possible. Minimizing ground loops
reduces EMI issues.
ƒ
Route high-dV/dt signals, such as SW node, away
from the error amplifier input/output pins. Keep
components connected to these pins close to the
pins.
ƒ
To effectively remove heat from the MOSFETs, use
wide land areas with appropriate thermal vias.
(15)
A 15µH inductor is chosen for this application.
If the VOUT voltage is 2.5V, choose R2=18kΩ(1%), and
R3 can be calculated from:
⎛ 0.6 ⎞
R3 = 18kΩ⎜
⎟ = 5.68kΩ
⎝ 2.5 − 0.6 ⎠
(16)
Choose R3=5.6kΩ (1%).
In this application, with the desired crossover frequency
at 30kHz, RC value is calculated as follows:
RC =
2π ⋅ 22μF ⋅ 30kHz ⋅ 2.5V
2 A / V ⋅ 380 μA / V ⋅ 0.6V
(17)
If RC=22.72kΩ , choose 22kΩ for the design.
If RC=22kΩ , use the following equation to get CC:
CC =
2
(18)
π ⋅ 22kΩ ⋅ 30kHz
CC= 0.965nF, choose 1nF for the design.
FAN8303 — 2A 23V Non-Synchronous Step-Down DC/DC Regulator
Design example
Table 1. Recommended Compensation Values
(VIN=12V)
VO
L
1.8V
10µH
COUT
2.5V
15µH
22µF
3.3V
15µH
MLCC
5V
22µH
R2
18kΩ
© 2008 Fairchild Semiconductor Corporation
FAN8303 • Rev. 1.0.0
R3
RC
CC
9kΩ
16kΩ
1.5nF
5.6kΩ
22kΩ
1nF
4kΩ
27kΩ
820pF
2.45kΩ
43kΩ
560pF
Figure 17. Recommended PCB Layout
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10
FAN8303 — 2A 23V Non-Synchronous Step-Down DC/DC Regulator
Physical Dimensions
5.00
4.80
A
0.65
3.81
5
8
B
6.20
5.80
PIN ONE
INDICATOR
1.75
4.00
3.80
1
5.60
4
1.27
(0.33)
0.25
M
1.27
C B A
LAND PATTERN RECOMMENDATION
0.25
0.10
SEE DETAIL A
1.75 MAX
R0.10
0.25
0.19
C
0.10
0.51
0.33
0.50 x 45°
0.25
C
OPTION A - BEVEL EDGE
GAGE PLANE
R0.10
OPTION B - NO BEVEL EDGE
0.36
NOTES: UNLESS OTHERWISE SPECIFIED
8°
0°
0.90
0.406
A) THIS PACKAGE CONFORMS TO JEDEC
MS-012, VARIATION AA, ISSUE C,
B) ALL DIMENSIONS ARE IN MILLIMETERS.
C) DIMENSIONS DO NOT INCLUDE MOLD
FLASH OR BURRS.
D) LANDPATTERN STANDARD: SOIC127P600X175-8M.
E) DRAWING FILENAME: M08AREV13
SEATING PLANE
(1.04)
DETAIL A
SCALE: 2:1
Figure 18. 8-Lead, Small Outline Integrated Circuit (SOIC-8)
Dimensions
Symbol
A
A1
b
c
D
E
e
F
H
L
θ˚
Min.
Millimeter
Typ.
1.346
0.101
Max.
Min.
1.752
0.254
0.053
0.004
Inch
Typ.
0.406
0.203
4.648
3.810
0.016
0.008
4.978
3.987
0.183
0.150
1.270
0.381X45˚
5.791
0.406
0˚
Max.
0.069
0.010
0.196
0.157
0.050
0.015X45˚
6.197
1.270
8˚
0.228
0.016
0˚
0.244
0.050
8˚
Package drawings are provided as a service to customers considering Fairchild components. Drawings may change in any manner
without notice. Please note the revision and/or date on the drawing and contact a Fairchild Semiconductor representative to verify
or obtain the most recent revision. Package specifications do not expand the terms of Fairchild’s worldwide terms and conditions,
specifically the warranty therein, which covers Fairchild products.
Always visit Fairchild Semiconductor’s online packaging area for the most recent package drawings:
http://www.fairchildsemi.com/packaging/.
© 2008 Fairchild Semiconductor Corporation
FAN8303 • Rev.1.0.0
www.fairchildsemi.com
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FAN8303 — 2A 23V Non-Synchronous Step-Down DC/DC Regulator
© 2008 Fairchild Semiconductor Corporation
FAN8303 • Rev.1.0.0
www.fairchildsemi.com
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