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FAN5037
Adjustable Switching Regulator Controller
Features
* 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
Description
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. 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.
Applications
* * * * 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
VCCA 2 IFBL 4
+12V
+5V
FAN5037
Feedback Control
IFBH 3 VCCP 8
CEXT 1
Oscillator
Digital Logic
7 DRV
Vout 1.20V Reference
VFB 5 GNDP 6
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FAN5037
PRODUCT SPECIFICATION
Pin Assignments
CEXT VCCA IFBH IFBL 1 2 3 4 8 7 6 5 VCCP DRV GNDP VFB
FAN5037
Pin Descriptions
Pin Name CEXT Pin Number 1 Pin Function Description 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. Analog Vcc. Power supply for regulator control circuitry and voltage reference. Connect to system 5V supply and decouple to ground with 0.1F ceramic capacitor. 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. Low side current feedback. See Applications Information for details. Voltage feedback. Using two external resistors, this pin sets the output voltage level for the switching regulator. Power Ground. Connect to a low impedance ground. See Application Information for details. 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. 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.
VCCA IFBH
2 3
IFBL VFB GNDP DRV
4 5 6 7
VCCP
8
Absolute Maximum Ratings
Supply Voltages, VCCA Supply Voltages, VCCP Junction Temperature, TJ Storage Temperature, TS Lead Soldering Temperature, 10 seconds Thermal Resistance Junction-to-Ambient, JA 7V 13V +150C -65 to +150C 300C 163C/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 Switching Regulator Supply, VCCA Ambient Operating Temperature, TA Gate Drive Supply, VCCP Conditions Min. 4.75 0 9.5 12 Typ. 5 Max. 5.25 70 12.6 Units V C V
2
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PRODUCT SPECIFICATION
FAN5037
Electrical Characteristics
(VCCA = 5V, VCCP = 12V, TA = 25C using circuit of Figure 1, unless otherwise noted) The * denotes specifications which apply over the full ambient operating temperature range. Parameter Output Voltage Output Temperature Drift Line Regulation Load Regulation VOUT PSRR Output Ripple, peak-peak Total DC Accuracy1 ILOAD = 5A Open Loop * * * CEXT = 180pF Independent of load ILOAD = 13A * * Efficiency Output Driver Current Short Circuit Threshold Voltage Undervoltage Lockout On Time Pulse Width2 VCCA Supply Current VCCP Supply Current TA = 0C-70C VCCA = 4.75 to 5.25V, ILOAD = 13A ILOAD = 0 to 5A or 5A to 13A VCCA = 4.75 to 5.25V 20MHz BW, ILOAD = 13A * 80 0.5 70 3.5 90 4.0 3.5 5 20 15 25 100 4.5 60 15 55 85 100 Conditions Min. 1.2 40 3 30 5 43 Typ. Max. 3.6 Units V ppm/C mV mV dB mV mV % A mV V s mA 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.
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FAN5037
PRODUCT SPECIFICATION
Typical Operating Characteristics (VCCA = 5V, and TA = +25C using circuit in Figure 1, unless otherwise noted)
Efficiency vs. Output Current 95 94 93 92 91 90 89 88 87 86 85 1 2 3 4 5 6 7 8 9 10 Output Current +1.5 +1.0 VOUT (%) +0.5 Nom -0.5 -1.0 -1.5 0 2 4 6 8 10 Output Current (A) Output Voltage vs. Load
Efficiency (%)
Output Voltage vs. Temperature, IOUT = 10A +0.50 Output Voltage (%) +0.25 Nom. -0.25 -0.50 0 25 50 75 100 125
Transient Response, 0.5 to 5.5A ISW (2A/div) VOUT (50mV/div)
Output Ripple, IOUT = 10A)
Time (100s/division)
VOUT (10mV/division)
Time (2s/division)
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PRODUCT SPECIFICATION
FAN5037
Typical Operating Characteristics (continued)
Output Startup, System Power-Up Pin 7 (DRV), 10A Load
Time (5ms/division)
Time (1s/division)
Pin 7 (DRV), 0.1A Load
Time (1s/division)
Application Circuit
Optional L1 +5V 2.5H +C2 +C4 + C3 1200F 1200F 1200F C8 0.1F
+12V C1 0.1F
R1 47 C5
D2 MMBD4148 D3 1N4735A 1F
FDB6030L Q1 R2 4.7 D1 L2 4.7H R3 5.2m R4 3.48K + VCORE
C7 0.1uF C6 180pF
8 1 7 2 U1 3 FAN5037 6 5 4
...
MBRB1545CT
+ C14 1500F
R5 2K
Figure 1. 13A at 3.3V Application Schematic REV. 1.0.3 9/26/01
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FAN5037
PRODUCT SPECIFICATION
Table1. Bill of Materials for a FAN5037 3.3V, 13A Application
Qty. 3 3 1 1 6 1 1 1 1 1 1 1 1 1 Reference C1, C7-8 C2-4
C5 C6
Manufacturer Part Order # Any Sanyo 10MV1200GX Any Any Sanyo 6MV1500GX Any Any N/A Any Any Motorola MBRB1545CT Fairchild MMBD4148 Motorola 1N4735A Fairchild FDB6030L Any Any Fairchild FAN5037M
Description 100nF, 25V Capacitor 1200F, 10V Aluminum Capacitor 1F, 25V Capacitor 180pF, 50V Capacitor 1500F, 6.3V Aluminum Capacitor 47.5 4.75 5.2m, 1W Resistor 3.48K 2K 15A, 45V Schottky Signal Diode 6.2V Zener 30V, 14m Logic Level MOSFET 2.5H Inductor 4.7H Inductor PWM Controller C0G
Requirements and Comments IRMS = 2A , See Equation (2) in Applications
C9-14 R1 R2 R3 R4 R5 D1 D2 D3 Q1
ESR = 44m
PCB Trace Resistor, see Equation (3) Applications
Optional L1 1 1 L2 U1
ISAT > 8A ISAT > 13A
Application Information
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.
High Current Output Drivers
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
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.
Internal Reference
The reference in the FAN5037 is a precision band-gap type reference. Its temperature coefficient is trimmed to provide a near zero TC.
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
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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 VCCA
2
4 IFBL
gm
3 IFBH
Constant On-Time Oscillator
gm CEXT IO
8 VCCP
VH
1
7 SDRV
VL ION REF
5 FBSW 6
VOUT
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 x -------------------- R5 10 * 0.4 - 0.4 No. Caps = ---------------------------------- = 2.44 2
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 = --------------- x ( 1 - TF ) = --------------------------------------------- x ( 1 - TF ) (3) I PK 0.6A + I LOAD,MAX
(1)
For example, for 3.3V:
R4 + R5 3.48k + 2.0k V OUT = 1.2 x -------------------- = 1.2 x ------------------------------ = 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
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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 10m, 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.
Board Design Considerations
MOSFET Placement
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 x V D x ( 1 - DutyCycle )
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 4.7 resistor in series with the MOSFET gate can decrease this layout criticality. Refer to Figure 1.
Inductor and Schottky Diode Placement
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 x 0.65 x ( 1 - 73.1% ) = 1.75W
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
Example of a Problem Layout
5 6 7 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. 8
4 3 2 1
5 6 7 8
4 3 2 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
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PRODUCT SPECIFICATION
FAN5037
Power and Ground Connections
MOSFET Gate Bias
+5V +12V
The connection of VCCA to the 5V power supply plane should be short and bypassed with a 0.1F 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 47 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. 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).
47
VCCP
Q1 L2 RSENSE VOUT CBULK
1F GNDP
D1
Figure 4. 12V Gate Bias Configuration
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PRODUCT SPECIFICATION
FAN5037
Mechanical Dimensions
8 Lead SOIC Package
Inches Min. A A1 B C D E e H h L N ccc Max. Millimeters Min. Max. 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.
Symbol
.053 .069 .004 .010 .013 .020 .008 .010 .189 .197 .150 .158 .050 BSC .228 .010 .016 8 0 -- 8 .004 .244 .020 .050
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 5.79 0.25 0.40 8 0 -- 8 0.10 6.20 0.50 1.27
3 6
8
5
E
H
1
4
D A1 A SEATING PLANE B -C- LEAD COPLANARITY ccc C e
h x 45 C
L
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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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