WIDE INPUT 2A STEP DOWN CONVERTER
FSP3126
FEATURES
2A Output Current
Up to 95% Efficiency
4.75V to 20V Input Range
8µA Shutdown Supply Current
410kHz Switching Frequency
Adjustable Output Voltage
Cycle-by-Cycle Current Limit Protection
Thermal Shutdown Protection
Frequency Foldback at Short Circuit
Stability with Wide Range of Capacitors,
Including Low ESR Ceramic Capacitors
SOP8L Package
GENERAL DESCRIPTION
The FSP3126 is a current-mode step-down DC/DC
converter that generates up to 2A of output current at
410kHz switching frequency. The device utilizes
ISOBCD20 process for operation with input voltages
up to 20V.
Consuming only 8µA in shutdown mode, the
FSP3126 is highly efficient with peak operating
efficiency at 95%. Protection features include
cycle-by-cycle current limit, thermal shutdown, and
frequency foldback at short circuit.
The FSP3126 is available in a SOP8L package and
requires very few external devices for operation.
Typical Application
TFT LCD Monitors
Portable DVDs
Car-Powered or Battery-Powered Equipments
Set-Top Boxes
Telecom Power Supplies
DSL and Cable Modems and Routers
Termination Supplies
PIN ASSIGNMENT
(Top View)
BS 1
IN
SW
G
2
3
4
8
7
N/C
EN
COMP
FB
FSP3126
6
5
PIN DESCRIPTION
Name
BS
IN
SW
G
FB
COMP
EN
N/C
No.
1
2
3
4
5
6
7
8
Description
Bootstrap. This pin acts as the positive rail for the high-side switch’s gate driver.
Connect a 10nF capacitor between BS and SW.
Input Supply. Bypass this pin to G with a low ESR capacitor. See Input Capacitor
in the Application Information section.
Switch Output. Connect this pin to the switching end of the inductor.
Ground.
Feedback Input. The voltage at this pin is regulated to 1.293V. Connect to the
resistor divider between output and ground to set output voltage.
Compensation Pin. See Stability Compensation in the Application Information
section.
Enable Input. When higher than 1.3V, this pin turns the IC on. When lower than
0.7V, this pin turns the IC off. Output voltage is discharged when the IC is off.
When left unconnected, EN is pulled up to 4.5V tip with a 2µA pull up current.
Not Connected.
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2007-5-28
WIDE INPUT 2A STEP DOWN CONVERTER
FSP3126
ABSOLUTE MAXIMUM RATINGS
Parameter
Value
Unit
IN Supply Voltage
-0.3 to 25
V
SW Voltage
-1 to V
IN
+ 1
V
BS Voltage
V
SW
– 0.3 to V
SW
+ 8
V
EN, FB, COMP Voltage
-0.3 to 6
V
Continuous SW Current
Internally limited
A
Junction to Ambient Thermal Resistance (θ
JA
)
105
°C/W
Maximum Power Dissipation
0.76
W
Operating Junction Temperature
-40 to 150
°C
Storage Temperature
-55 to 150
°C
Lead Temperature (Soldering, 10 sec)
300
°C
(Note: Exceeding these limits may damage the device. Exposure to absolute maximum rating conditions for long
periods may affect device reliability.)
ELECTRICAL CHARACTERISTICS
(V
IN
= 12V, TA= 25°C unless otherwise specified.)
Parameter
Input Voltage
Feedback Voltage
High-Side Switch On
Resistance
Low-Side Switch On
Resistance
SW Leakage
Current Limit
COMP to Current Limit
Transconductance
Error Amplifier
Transconductance
Error Amplifier DC Gain
Switching Frequency
Short Circuit Switching
Frequency
Maximum Duty Cycle
Minimum Duty Cycle
Enable Threshold Voltage
Enable Pull Up Current
Supply Current in Shutdown
IC Supply Current in
Operation
Thermal Shutdown
Temperature
Symbol
V
IN
V
FB
R
ONH
R
ONL
V
EN
= 0
I
LIMT
G
COMP
G
EA
A
VEA
f
SW
V
FB
= 0
D
MAX
V
FB
= 1.1V
V
FB
= 1.4V
Hysteresis = 0.1V
Pin pulled up to 4.5V typically when
left unconnected
V
EN
= 0
V
EN
= 3V, V
FB
= 1.4V
Hysteresis = 10°C
∆I
COMP
= ±10µA
350
2.4
Test Conditions
V
OUT
= 5V, I
LOAD
= 0A to 1A
4.75V
≤
V
IN
≤
20V, V
COMP
= 1.5V
Min.
7
1.267
Typ.
1.293
0.20
4.7
0
2.85
1.8
550
4000
410
50
90
0.7
1
2
8
0.7
160
20
0
1.3
470
10
Max.
20
1.319
Unit
V
V
Ω
Ω
µA
A
A/V
µA/V
V/V
kHz
kHz
%
%
V
µA
µA
mA
°C
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2007-5-28
WIDE INPUT 2A STEP DOWN CONVERTER
FSP3126
FUNCTIONAL BLOCK DIAGRAM
IN
ENABLE
REGULATOR
&
REFERENCE
EN
BS
CURRENT SENSE
AMPLIFIER
COMP
ERROR
AMPLIFIER
+
–
1.298V
FB
+
+
–
PWM
COMPARATOR
–
+
–
0.2O
HIGH-SIDE
POWER
SWITCH
SW
FOLDBACK
CONTROL
OSCILLATOR
&
RAMP
LOGIC
4.7O LOW-SIDE
POWER SWITCH
THERMAL
SHUTDOWN
G
FUNCTIONAL DESCRIPTION
As seen in the above Figure, Functional Block Diagram, the FSP3126 is a current mode pulse width modulation
(PWM) converter. The converter operates as follows:
A switching cycle starts when the rising edge of the Oscillator clock output causes the High-Side Power Switch to turn
on and the Low-Side Power Switch to turn off. With the SW side of the inductor now connected to IN, the inductor
current ramps up to store energy in the magnetic field. The inductor current level is measured by the Current Sense
Amplifier and added to the Oscillator ramp signal. If the resulting summation is higher than the COMP voltage, the
output of the PWM Comparator goes high. When this happens or when Oscillator clock output goes low, the
High-Side Power Switch turns off and the Low-Side Power Switch turns on. At this point, the SW side of the inductor
swings to a diode voltage below ground, causing the inductor current to decrease and magnetic energy to be
transferred to output. This state continues until the cycle starts again.
The High-Side Power Switch is driven by logic using BS as the positive rail. This pin is charged to V
SW
+ 6V when the
Low-Side Power Switch turns on.
The COMP voltage is the integration of the error between FB input and the internal 1.293V reference. If FB is lower
than the reference voltage, COMP tends to go higher to increase current to the output. Current limit happens when
COMP reaches its maximum clamp value of 2.55V.
The Oscillator normally switches at 410kHz. However, if FB voltage is less than 0.7V, then the switching frequency
decreases until it reaches a minimum of 50kHz at V
FB
= 0.5V.
Shutdown control
The FSP3126 has an enable input EN for turning the IC on or off. When EN is less than 0.7V, the IC is in 8µA low
current shutdown mode and output is discharged through the Low-Side Power Switch. When EN is higher than 1.3V,
the IC is in normal operation mode. EN is internally pulled up with a 2µA current source and can be left unconnected
for always-on operation. Note that EN is a low voltage input with a maximum voltage of 6V; it should never be directly
connected to IN.
Thermal Shutdown
The FSP3126 automatically turns off when its junction temperature exceeds 160°C.
3/9
2007-5-28
WIDE INPUT 2A STEP DOWN CONVERTER
FSP3126
APPLICATION INFORMATION
Output Voltage Setting
Figure1. Output Voltage Setting
Figure 1 shows the connections for setting the output voltage. Select the proper ratio of the two feedback resistors
RFB1 and RFB2 based on the output voltage. Typically, use RFB2
≈
10kΩ and determine RFB1 from the following
equation:
⎞
⎛
V
R
FB1
=
R
FB2
⎜
OUT
−
1
⎟
⎟
⎜
1.293 V
⎠
(1)
⎝
Inductor Selection
The inductor maintains a continuous current to the output load. This inductor current has a ripple that is dependent on
the inductance value: higher inductance reduces the peak-to-peak ripple current. The trade off for high inductance
value is the increase in inductor core size and series resistance, and the reduction in current handling capability. In
general, select an inductance value L based on the ripple current requirement:
V
OUT
•
( V
IN
−
V
OUT
)
L
=
V
IN
f
SW
I
OUTMAX
K
RIPPLE
(2)
where V
IN
is the input voltage, V
OUT
is the output voltage, f
SW
is the switching frequency, I
OUTMAX
is the maximum
output current, and K
RIPPLE
is the ripple factor. Typically, choose K
RIPPLE
= 30% to correspond to the peak-to-peak
ripple current being 30% of the maximum output current.
With this inductor value, the peak inductor current is I
OUT
• (1 + K
RIPPLE
/ 2). Make sure that this peak inductor current is
less that the 3A current limit. Finally, select the inductor core size so that it does not saturate at 3A. Typical inductor
values for various output voltages are shown in Table 1.
V
OUT
1.5V 1.8V 2.5V 3.3V 5V
L
6.8µH 6.8µH 10µH 15µH 22µH
Table 1. Typical Inductor Values
Input Capacitor
The input capacitor needs to be carefully selected to maintain sufficiently low ripple at the supply input of the
converter. A low ESR capacitor is highly recommended. Since large current flows in and out of this capacitor during
switching, its ESR also affects efficiency.
The input capacitance needs to be higher than 10µF. The best choice is the ceramic type; however, low ESR tantalum
or electrolytic types may also be used provided that the RMS ripple current rating is higher than 50% of the output
current. The input capacitor should be placed close to the IN and G pins of the IC, with the shortest traces possible. In
the case of tantalum or electrolytic types, they can be further away if a small parallel 0.1µF ceramic capacitor is
placed right next to the IC.
Output Capacitor
The output capacitor also needs to have low ESR to keep low output voltage ripple. The output ripple voltage is:
V
IN
+
28
•
f
SW 2
LC
OUT
V
RIPPLE
=
I
OUTMAX
K
RIPPLE
R
ESR
(3)
where I
OUTMAX
is the maximum output current, K
RIPPLE
is the ripple factor, R
ESR
is the ESR of the output capacitor, f
SW
is
the switching frequency, L is the inductor value, and C
OUT
is the output capacitance. In the case of ceramic output
capacitors, R
ESR
is very small and does not contribute to the ripple. Therefore, a lower capacitance value can be used
for ceramic capacitors. In the case of tantalum or electrolytic capacitors, the ripple is dominated by R
ESR
multiplied by
the ripple current. In that case, the output capacitor is chosen to have sufficiently low ESR.
4/9
2007-5-28
WIDE INPUT 2A STEP DOWN CONVERTER
For ceramic output capacitors, typically choose a capacitance of about 22µF. For tantalum or electrolytic capacitors,
choose a capacitor with less than 50mΩ ESR.
Rectifier Diode
Use a Schottky diode as the rectifier to conduct current when the High-Side Power Switch is off. The Schottky diode
must have a current rating higher than the maximum output current and a reverse voltage rating higher than the
maximum input voltage.
Stability Compensation
FSP3126
Figure 2. Stability Compensation
The feedback loop of the IC is stabilized by the components at the COMP pin, as shown in Figure 2. The DC loop gain
of the system is determined by the following equation:
1. 3V
A
VDC
=
A
VEA
G
COMP
I
OUT
(4)
The dominant pole P1 is due to C
COMP
:
G
EA
f
P1
=
2
π
A
VEA
C
COMP
(5)
The second pole P2 is the output pole:
I
OUT
f
P 2
=
2
π
V
OUT
C
OUT
(6)
The first zero Z1 is due to R
COMP
and C
COMP
:
1
f
Z1
=
2
π
R
COMP
C
COMP
(7)
And finally, the third pole is due to R
COMP
and C
COMP2
(if C
COMP2
is used):
1
f
P 3
=
2
π
R
COMP
C
COMP 2
(8)
The following steps should be used to compensate the IC:
STEP1. Set the crossover frequency at 1/10 of the switching frequency via RCOMP:
2
π
V
OUT
C
OUT
f
SW
R
COMP
=
10 G
EA
G
COMP
•
1. 3V
(9)
but limit RCOMP to 15kΩ maximum.
STEP2. Set the zero fZ1 at 1/4 of the crossover frequency. If RCOMP is less than 15kΩ, the equation for CCOMP is:
C
COMP
=
1 . 8
×
10
−
5
R
COMP
(F )
C
COMP2
is needed only for high ESR output capacitor
(10)
If RCOMP is limited to 15kΩ, then the actual crossover frequency is 3.4 / (VOUTCOUT). Therefore:
C
COMP
=
1.2
×
10
−
5
V
OUT
C
OUT
(F )
(11)
STEP3. If the output capacitor’s ESR is high enough to cause a zero at lower than 4 times the crossover frequency,
an additional compensation capacitor CCOMP2 is required. The condition for using CCOMP2 is:
⎛
1 . 1
×
10
−
6
,0. 012
•
V
OUT
≥
Min
⎜
⎜
C
OUT
⎝
⎞
⎟
⎟
⎠
(
Ω
)
R
ESRCOUT
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(12)
2007-5-28
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