Fairchild FAN5037 Adjustable switching regulator controller Datasheet

www.fairchildsemi.com
FAN5037
Adjustable Switching Regulator Controller
Features
Description
• High power switch-mode DC-DC controller can provide
in excess of 13A
• Output voltage adjustable from 1.2V to 3.6V
• 85% efficiency
• Cumulative accuracy < 3% over line, load, and
temperature variations
• Overvoltage and short circuit protection
• Built-in soft start
• No overshoot at turn-on
The FAN5037 is a high power, switch-mode DC-DC controller that provides efficient power for all low-voltage applications. This controller has a built-in Soft Start feature which
offers system protection during power-up by reducing both
inrush current and output overshoot. When combined with
the appropriate external circuitry, the FAN5037 can deliver
load currents as high as 13A at efficiencies as high as 88%.
The FAN5037 can generate output voltages from 1.2V up to
3.6V using external resistors.
Applications
The FAN5037 is designed to operate in a constant on-time
control mode under all load conditions. Its accurate low TC
reference eliminates the need for precision external components in order to achieve the tight tolerance voltage regulation required by many applications. Short circuit current
protection is provided through the use of a current sense
resistor, while overvoltage protection is provided internally.
•
•
•
•
I/O and AGP power for desktop computers
High efficiency power for ASICs
High efficiency power for DSPs
Adjustable step-down power supplies
Block Diagram
5V
FAN5037
+12V
+5V
VCCA 2
IFBL 4
Feedback Control
IFBH 3
VCCP 8
CEXT
1
Oscillator
Digital Logic
7
DRV
Vout
1.20V
Reference
VFB 5
GNDP 6
REV. 1.0.3 9/26/01
FAN5037
PRODUCT SPECIFICATION
Pin Assignments
CEXT
1
8
VCCP
VCCA
2
7
DRV
IFBH
3
6
GNDP
IFBL
4
5
VFB
FAN5037
Pin Descriptions
Pin
Name
Pin
Number
CEXT
1
External capacitor. A 180pF capacitor is connected to this pin as part of the constant
on-time pulse width circuit. Careful layout of this pin is critical to system performance.
See Applications Information for details.
VCCA
2
Analog Vcc. Power supply for regulator control circuitry and voltage reference. Connect to
system 5V supply and decouple to ground with 0.1µF ceramic capacitor.
IFBH
3
High side current feedback. Pins 3 and 4 are used as the inputs for the current feedback
control loop and as the short circuit current sense points. Careful layout of the traces from
these pins to the current sense resistor is critical for optimal performance of the short circuit
protection scheme. See Applications Information for details.
IFBL
4
Low side current feedback. See Applications Information for details.
VFB
5
Voltage feedback. Using two external resistors, this pin sets the output voltage level for the
switching regulator.
GNDP
6
Power Ground. Connect to a low impedance ground. See Application Information for
details.
DRV
7
MOSFET driver output. Connect this pin to the gate of the N-channel MOSFET Q1 as
shown in Figure 12. The trace from this pin to the MOSFET gate should be kept as short as
possible (less than 0.5"). See Applications Information for details.
VCCP
8
Power Vcc. Power supply for DRV output driver. Connect to system 12V supply with R-C
filter shown in Figure 12. See Applications Information for details.
Pin Function Description
Absolute Maximum Ratings
Supply Voltages, VCCA
7V
Supply Voltages, VCCP
13V
Junction Temperature, TJ
+150°C
Storage Temperature, TS
-65 to +150°C
Lead Soldering Temperature, 10 seconds
300°C
Thermal Resistance Junction-to-Ambient, ΘJA
163°C/W
Note:
1. Functional operation under any of these conditions is not implied. Performance is guaranteed only if Operating Conditions
are not exceeded.
Operating Conditions
Parameter
Min.
Typ.
Max.
Units
Switching Regulator Supply, VCCA
4.75
5
5.25
V
Ambient Operating Temperature, TA
0
70
°C
12.6
V
Gate Drive Supply, VCCP
2
Conditions
9.5
12
REV. 1.0.3 9/26/01
PRODUCT SPECIFICATION
FAN5037
Electrical Characteristics
(VCCA = 5V, VCCP = 12V, TA = 25°C using circuit of Figure 1, unless otherwise noted)
The • denotes specifications which apply over the full ambient operating temperature range.
Parameter
Conditions
Min.
Output Voltage
Typ.
1.2
Max.
Units
3.6
V
Output Temperature Drift
TA = 0˚C–70˚C
40
Line Regulation
VCCA = 4.75 to 5.25V, ILOAD = 13A
3
5
mV
Load Regulation
ILOAD = 0 to 5A or 5A to 13A
30
43
mV
VOUT PSRR
VCCA = 4.75 to 5.25V
Output Ripple, peak-peak
20MHz BW, ILOAD = 13A
Total DC
60
Efficiency
ILOAD = 5A
Output Driver Current
Open Loop
dB
15
•
Accuracy1
ppm/˚C
±55
80
mV
±100
85
mV
%
•
0.5
Short Circuit Threshold Voltage
•
70
90
100
mV
Undervoltage Lockout
•
3.5
4.0
4.5
V
On Time Pulse
Width2
CEXT = 180pF
A
3.5
µs
VCCA Supply Current
Independent of load
•
5
15
mA
VCCP Supply Current
ILOAD = 13A
•
20
25
mA
Notes:
1. Total DC accuracy includes setpoint accuracy, temperature drift, line and load regulation.
2. The on-time pulse width of the oscillator is set via external capacitor CEXT.
REV. 1.0.3 9/26/01
3
FAN5037
PRODUCT SPECIFICATION
Typical Operating Characteristics
(VCCA = 5V, and TA = +25°C using circuit in Figure 1, unless otherwise noted)
Output Voltage vs. Load
+1.5
+1.0
VOUT (%)
Efficiency (%)
Efficiency vs. Output Current
95
94
93
92
91
90
89
88
87
86
85
+0.5
Nom
-0.5
-1.0
-1.5
1
2
3
4
5
6
7
8
9
10
0
2
Output Current
4
6
8
10
Output Current (A)
Output Voltage vs. Temperature, IOUT = 10A
Output Voltage (%)
+0.50
+0.25
Nom.
-0.25
-0.50
0
25
50
75
Time (100µs/division)
4
125
Output Ripple, IOUT = 10A)
VOUT (10mV/division)
ISW (2A/div) VOUT (50mV/div)
Transient Response, 0.5 to 5.5A
100
Time (2µs/division)
REV. 1.0.3 9/26/01
PRODUCT SPECIFICATION
FAN5037
Typical Operating Characteristics (continued)
Output Startup, System Power-Up
Pin 7 (DRV), 10A Load
Time (5ms/division)
Time (1µs/division)
Pin 7 (DRV), 0.1A Load
Time (1µs/division)
Application Circuit
Optional
L1
+5V
+C2
+C4
+ C3
1200µF 1200µF 1200µF
2.5µH
+12V
R1
D2
MMBD4148
D3
1N4735A
47Ω C5
1µF
C8
FDB6030L
Q1
R2 4.7Ω
C1
0.1µF
0.1µF
L2
R3
4.7µH
5.2mΩ
D1
8
1
7
2
U1
3 FAN5037 6
5
4
C7
0.1uF
MBRB1545CT
C6
180pF
VCORE
R4
3.48KΩ
+
...
+ C14
1500µF
R5
2KΩ
Figure 1. 13A at 3.3V Application Schematic
REV. 1.0.3 9/26/01
5
FAN5037
PRODUCT SPECIFICATION
Table1. Bill of Materials for a FAN5037 3.3V, 13A Application
Manufacturer
Part Order #
Requirements
and Comments
Qty.
Reference
Description
3
C1, C7-8
Any
100nF, 25V Capacitor
3
C2-4
Sanyo
10MV1200GX
1200µF, 10V Aluminum Capacitor
1
C5
Any
1µF, 25V Capacitor
1
C6
Any
180pF, 50V Capacitor
C0G
6
C9-14
Sanyo
6MV1500GX
1500µF, 6.3V Aluminum Capacitor
ESR = 44mΩ
IRMS = 2A , See Equation
(2) in Applications
1
R1
Any
47.5Ω
1
R2
Any
4.75Ω
1
R3
N/A
5.2mΩ, 1W Resistor
1
R4
Any
3.48KΩ
1
R5
Any
2KΩ
1
D1
Motorola
MBRB1545CT
15A, 45V Schottky
1
D2
Fairchild
MMBD4148
Signal Diode
1
D3
Motorola
1N4735A
6.2V Zener
1
Q1
Fairchild
FDB6030L
30V, 14mΩ Logic Level MOSFET
Any
2.5µH Inductor
ISAT > 8A
ISAT > 13A
Optional L1
1
L2
Any
4.7µH Inductor
1
U1
Fairchild
FAN5037M
PWM Controller
PCB Trace Resistor, see
Equation (3) Applications
Application Information
High Current Output Drivers
The FAN5037 contains a precision trimmed zero TC voltage
reference, a constant-on-time architecture controller, a high
current output driver, and a low offset error amp. The
detailed block diagram in Figure 1 shows how the FAN5037
works together with external components to achieve a highperformance switching power supply.
The FAN5037 high current output driver (DRV) contains
high speed bipolar power transistors configured in a
push-pull configuration. The output driver is capable of supplying 0.5A of current in less than 100ns. The driver’s power
and ground are separated from the overall chip power and
ground for added switching noise immunity.
Switch-Mode Control Loop
Internal Reference
The main control loop for the switch-mode converter consists
of a current conditioning amplifier and a voltage conditioning
amplifier. The voltage amplifier compares the voltage from the
internal reference with the converter’s output voltage divided
by an external resistor divider. The current amplifier senses the
current by comparing the voltages at the IFBH and IFBL pins,
which are attached to either side of the current sense resistor.
The signals from the voltage and current amplifiers are
summed together, the result being used to control the off-time
of the oscillator. The current feedback signal is also used as
part of the FAN5037 short-circuit protection.
The reference in the FAN5037 is a precision band-gap type
reference. Its temperature coefficient is trimmed to provide a
near zero TC.
6
Constant-On-Time Oscillator
The FAN5037 switch-mode oscillator is designed as a fixed
on-time, variable off-time oscillator. The constant-on-time
oscillator consists of a comparator, an external capacitor, a
fixed current source, a variable current source, and an analog
switch that selects between two threshold voltages for the
comparator. The external timing capacitor is alternately
REV. 1.0.3 9/26/01
PRODUCT SPECIFICATION
FAN5037
charged and discharged through the enabling and disabling
of the fixed current source. The variable current source is
controlled from the error inputs that are received from the
current and voltage feedback signals. The oscillator off-time
is controlled by the amount of current that is available from
the variable current source to charge the external capacitor up
to the high threshold level of the comparator. The on-time is
set by the constant current source that discharges the external
capacitor voltage down to the lower comparator threshold.
+5V
+12V
2
VCCA
4 IFBL
gm
3 IFBH
Constant On-Time Oscillator
gm
IO
8 VCCP
VH
CEXT
1
7 SDRV
VL
VOUT
ION
REF
5 FBSW
6
GNDP
65-5037-07
Figure 2. FAN5037 Detailed Block Diagram
Output Voltage Selection
The FAN5037 precision reference is trimmed to be 1.2V
nominally. When using the FAN5037, the system designer
has complete flexibility in choosing the output voltage for
one regulator from 1.2V to 3.6V. This is done by appropriately selecting the feedback resistors. These could be 0.1%
resistors to realize optimum output accuracy. The following
equations determines the output voltage of the regulator:
R4 + R5
V OUT = 1.2 ×  ---------------------
 R5 
(1)
For example, for 3.3V:
R4 + R5
3.48k + 2.0k
V OUT = 1.2 ×  --------------------- = 1.2 ×  ------------------------------- = 3.3V
 R5 


2.0k
Input Capacitors
The number of input capacitors required for the FAN5037 is
dependent on their ripple current rating, which assures their
rated life. The number required may be determined by
I out * DC – DC
No. Caps = --------------------------------------I rating
2
(2)
where the duty cycle DC = (Vout + Vf,diode) / Vin. For example, with a 1.5V output at 10A, 5V input, and using the
Sanyo capacitors specified in Table 1 which have a 2A ripple
current rating, we have DC = (1.5 + .5)/5 = 0.4, and
REV. 1.0.3 9/26/01
2
10 * 0.4 – 0.4
No. Caps = ----------------------------------- = 2.44
2
so that we need 3 input capacitors.
Short Circuit Considerations
The FAN5037 uses a current sensing scheme to limit the load
current if an output fault condition occurs. The current sense
resistor carries the peak current of the inductor, which is
greater than the maximum load current due to ripple current
flowing in the inductor. The FAN5037 will begin to limit the
output current to the load by reducing the duty cycle of the
top-side MOSFET driver when the voltage across the current-sense resistor exceeds the short circuit comparator
threshold voltage (Vth). When this happens the output voltage will temporarily go out of regulation. As the voltage
across the sense resistor becomes larger, the duty cycle of the
top-side MOSFET will continue to be reduced until the current limit value is reached. At this point, the FAN5037 will
continuously deliver the limit current at a reduced output
voltage level. The short circuit comparator threshold voltage
is typically 90mV, with a tolerance of ±10mV. The ripple
current flowing through the inductor in Figure 1 is 0.6Apeak.
Refer to Application Note AB-23 for detailed discussions.
The sense resistor value can be approximated as follows:
V th,min
V th,min
R SENSE = ---------------- × ( 1 – TF ) = --------------------------------------------- × ( 1 – TF ) (3)
I PK
0.6A + I LOAD,MAX
7
FAN5037
PRODUCT SPECIFICATION
where TF = Tolerance Factor for the sense resistor and 0.6A
accounts for the inductor ripple current.
Since the value of the sense resistor is often less than 10mΩ,
care should be taken in the layout of the PCB. Trace resistance can contribute significant errors. The traces to the
IFBH and IFBL pins of the FAN5037 should be Kelvin connected to the pads of the current-sense resistor. To minimize
the influence of noise, the two traces should be run next to
each other.
Schottky Diode
In Figure 1, MOSFET Q1 and flyback diode D1 are used as
complementary switches in order to maintain a constant current through the output inductor L2. As a result, D1 will have
to carry the full current of the output load when the power
MOSFET is turned off. The power in the diode is a direct
function of the forward voltage at the rated load current during the off time of the FET. The following equation can be
used to estimate the diode power:
P DIODE = I D × V D × ( 1 – DutyCycle )
where ID is the forward current of the diode, VD is the forward voltage of the diode, and DutyCycle is defined the
same as
Vout
Duty Cycle = ------------Vin
For the Motorola MBRB1545CT Rectifier in Figure 1,
P DIODE = 10A × 0.65 × ( 1 – 73.1% ) = 1.75W
Board Design Considerations
MOSFET Placement
Placement of the power MOSFET is critical in the design of
the switch-mode regulator. The MOSFET should be placed
in such a way as to minimize the length of the gate drive path
from the FAN5037 SDRV pin. This trace should be kept
under 0.5" for optimal performance. Excessive lead length
on this trace will cause high frequency noise resulting from
the parasitic inductance and capacitance of the trace. Since
this voltage can transition nearly 12V in around 100nsec, the
resultant ringing and noise would be very difficult to suppress. This trace should be routed on one layer only and kept
well away from the “quiet” analog pins of the device: CEXT,
IFBH, IFBL, and GND. Refer to Figure 2. A 4.7Ω resistor in
series with the MOSFET gate can decrease this layout criticality. Refer to Figure 1.
Inductor and Schottky Diode Placement
The inductor and fly-back Schottky diode need to be placed
close to the source of the power MOSFET for the same reasons stated above. The node connecting the inductor and
Schottky diode will swing between the drain voltage of the
FET and the forward voltage of the Schottky diode. It is recommended that this node be converted to a plane if possible.
This node will be part of the high current path in the design,
and as such it is best treated as a plane in order to minimize
the parasitic resistance and inductance on that node. Since
most PC board manufacturers utilize 1/2 oz copper on the
top and bottom signal layers of the PCB, it is not recommended to use these layers to route the high current portions
of the regulator design. Since it is more common to use 1 oz.
copper on the PCB inner layers, it is recommended to use
those layers to route the high current paths in the design.
It is recommended that the diode T0-220 package be
attached to a heatsink.
Example of
a Good Layout
Noisy signal is routed
away from quiet pins and the
trace length is kept under 0.5in.
The gate resistor is as close
as possible to the MOSFET.
Example of
a Problem Layout
5
4
5
4
6
3
6
3
7
2
7
2
8
1
8
1
Noisy signal radiates
onto quiet pins and the
trace is too long.
Gate resistor is far away
from the MOSFET.
= "Quiet" Pins
Figure 3. Examples of good and poor layouts
8
REV. 1.0.3 9/26/01
PRODUCT SPECIFICATION
Power and Ground Connections
The connection of VCCA to the 5V power supply plane
should be short and bypassed with a 0.1µF directly at the
VCCA pin of the FAN5037. The ideal connection would be a
via down to the 5V power plane. A similar arrangement
should be made for the VCCP pin that connects to +12V.
Each ground should have a separate via connection to the
ground plane below.
A 12V power supply is used to bias the VCCP. A 47Ω resistor is used to limit the transient current into VCCP. A 1uF
capacitor filter is used to filter the VCCP supply and source
the transient current required to charge the MOSFET gate
capacitance. This method provides sufficiently high gate bias
voltage to the MOSFET (VGS), and therefore reduces
RDS(ON) of the MOSFET and its power loss.
FAN5037
MOSFET Gate Bias
+5V
+12V
47Ω
VCCP
Q1
L2
1µF
D1
RSENSE
VOUT
CBULK
GNDP
Figure 4. 12V Gate Bias Configuration
Figure 4 provides about 5V of gate bias which works well
when using typical logic-level MOSFETs. Non-logic-level
MOSFETs should not be used because of their higher
RDS(ON).
REV. 1.0.3 9/26/01
9
PRODUCT SPECIFICATION
FAN5037
Mechanical Dimensions
8 Lead SOIC Package
Inches
Symbol
Min.
A
A1
B
C
D
E
e
H
h
L
N
α
ccc
Millimeters
Max.
Min.
Max.
.053
.069
.004
.010
.013
.020
.008
.010
.189
.197
.150
.158
.050 BSC
1.35
1.75
0.10
0.25
0.33
0.51
0.20
0.25
4.80
5.00
3.81
4.01
1.27 BSC
.228
.010
.016
5.79
0.25
0.40
.244
.020
.050
8
6.20
0.50
1.27
8
0°
8°
0°
8°
—
.004
—
0.10
8
Notes:
Notes
1. Dimensioning and tolerancing per ANSI Y14.5M-1982.
2. "D" and "E" do not include mold flash. Mold flash or
protrusions shall not exceed .010 inch (0.25mm).
3. "L" is the length of terminal for soldering to a substrate.
4. Terminal numbers are shown for reference only.
5
2
2
5. "C" dimension does not include solder finish thickness.
6. Symbol "N" is the maximum number of terminals.
3
6
5
E
1
H
4
h x 45°
D
C
A1
A
α
SEATING
PLANE
e
B
REV. 1.0.3 9/26/01
–C–
LEAD COPLANARITY
L
ccc C
10
FAN5037
PRODUCT SPECIFICATION
Ordering Information
Product Number
FAN5037M
Package
8 pin SOIC
DISCLAIMER
FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO
ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION OR DESIGN. FAIRCHILD DOES NOT ASSUME
ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN;
NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS.
LIFE SUPPORT POLICY
FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES
OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR
CORPORATION. As used herein:
1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body, or
(b) support or sustain life, and (c) whose failure to perform
when properly used in accordance with instructions for use
provided in the labeling, can be reasonably expected to
result in a significant injury of the user.
2. A critical component in any component of a life support
device or system whose failure to perform can be reasonably expected to cause the failure of the life support device
or system, or to affect its safety or effectiveness.
www.fairchildsemi.com
9/26/01 0.0m 001
Stock#DS30005037
 2001 Fairchild Semiconductor Corporation
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