FAIRCHILD RC5037

www.fairchildsemi.com
RC5037
Adjustable Switching Regulator Controller
Features
Description
• High power switch-mode DC-DC controller can provide
in excess of 13A
• Output voltage adjustable from 1.5V to 3.6V
• 85% efficiency
• Cumulative accuracy < 3% over line, load, and
temperature variations
• Overvoltage and short circuit protection
• Built-in soft start
The RC5037 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 RC5037 can deliver load
currents as high as 13A at efficiencies as high as 88%. The
RC5037 can generate output voltages from 1.5V up to 3.6V
using external resistors.
Applications
•
•
•
•
The RC5037 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
+12V
+5V
2
RC5037
4
Feedback Control
3
8
1
Oscillator
Digital Logic
7
Vout
1.5V
Reference
5
6
REV. 1.0.1
RC5037
PRODUCT SPECIFICATION
Pin Assignments
CEXT
1
8
VCCP
VCCA
2
7
DRV
IFBH
3
6
GNDP
IFBL
4
5
VFB
RC5037
Pin Descriptions
Pin
Name
Pin
Number
CEXT
1
External capacitor. A 180pF capacitor is connected to this pin as part of the constant ontime 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
PRODUCT SPECIFICATION
RC5037
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.
1.5
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
Efficiency
ILOAD = 5A
Output Driver Current
Open Loop
Short Circuit Threshold Voltage
On Time Pulse
Width2
60
dB
15
•
Accuracy1
ppm/˚C
±55
80
•
0.5
•
70
CEXT = 180pF
mV
±100
85
mV
%
A
90
100
3.5
mV
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.
3
RC5037
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
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)
10
PRODUCT SPECIFICATION
RC5037
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 RC5037 6
5
4
C7
0.1uF
MBRB1545CT
C6
180pF
VCORE
R4
2.43KΩ
+
...
+ C14
1500µF
R5
2KΩ
Figure 1. 13A at 3.3V Application Schematic
5
RC5037
PRODUCT SPECIFICATION
Table1. Bill of Materials for a RC5037 3.3V, 13A Application
Manufacturer
Part Order #
Requirements
and Comments
Qty.
Reference
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
1
R1
Any
47.5Ω
1
R2
Any
4.75Ω
1
R3
N/A
5.2mΩ, 1W Resistor
1
R4
Any
2.43KΩ
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
Description
1
L2
Any
4.7µH Inductor
1
U1
Fairchild
RC5037M
PWM Controller
IRMS = 2A , See Equation
(2) in Applications
PCB Trace Resistor, see
Eauation (3) Applications
Application Information
High Current Output Drivers
The RC5037 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 RC5037
works together with external components to achieve a highperformance switching power supply.
The RC5037 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 RC5037 short-circuit protection.
The reference in the RC5037 is a precision band-gap type
reference. Its temperature coefficient is trimmed to provide a
near zero TC.
6
Constant-On-Time Oscillator
The RC5037 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
PRODUCT SPECIFICATION
RC5037
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. RC5037 Detailed Block Diagram
Output Voltage Selection
ing, we have DC = 1.5/5 = 0.3, and
The RC5037 precision reference is trimmed to be 1.5V nominally. When using the RC5037, the system designer has
complete flexibility in choosing the output voltage for one
regulator from 1.5V 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 = 1.5 ×  ---------------------
 R5 
(1)
For example, for 3.3V:
R4 + R5
2.43k + 2.0k
V OUT = 1.5 ×  --------------------- = 1.5 ×  ------------------------------- = 3.3V
 R5 


2.0k
Input Capacitors
The number of input capacitors required for the RC5037 is
dependent on their ripple current rating, which assures their
rated life. The number required may be determined by
2
I out * DC – DC
No. Caps = --------------------------------------I rating
(2)
where the duty cycle DC = Vout/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 rat-
2
10*
0.03 – 0.3
No. Caps = ------------------------------------ = 2.29
2
so that we need 3 input capacitors.
Short Circuit Considerations
The RC5037 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 RC5037 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 RC5037 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 0.6A.
Refer to Application Note AM-53 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
RC5037
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 RC5037 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 RC5037 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
= "Quiet" Pins
Figure 3. Examples of good and poor layouts
8
Noisy signal radiates
onto quiet pins and the
trace is too long.
Gate resistor is far away
from the MOSFET.
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 RC5037. 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.
RC5037
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).
9
PRODUCT SPECIFICATION
RC5037
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
–C–
LEAD COPLANARITY
L
ccc C
10
RC5037
PRODUCT SPECIFICATION
Ordering Information
Product Number
RC5037M
Package
8 pin SOIC
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
1/26/00 0.0m 001
Stock#DS30005037
 1998 Fairchild Semiconductor Corporation
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The following are registered and unregistered trademarks Fairchild Semiconductor owns or is authorized to use and is
not intended to be an exhaustive list of all such trademarks.
ISOPLANAR™
MICROWIRE™
POP™
PowerTrench 
QFET™
QS™
Quiet Series™
SuperSOT™-3
SuperSOT™-6
SuperSOT™-8
ACEx™
CoolFET™
CROSSVOLT™
E2CMOSTM
FACT™
FACT Quiet Series™
FAST®
FASTr™
GTO™
HiSeC™
SyncFET™
TinyLogic™
UHC™
VCX™
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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 FAIRCHILD SEMICONDUCTOR CORPORATION.
As used herein:
1. Life support devices or systems are devices or
2. A critical component is any component of a life
support device or system whose failure to perform can
systems which, (a) are intended for surgical implant into
be reasonably expected to cause the failure of the life
the body, or (b) support or sustain life, or (c) whose
support device or system, or to affect its safety or
failure to perform when properly used in accordance
with instructions for use provided in the labeling, can be
effectiveness.
reasonably expected to result in significant injury to the
user.
PRODUCT STATUS DEFINITIONS
Definition of Terms
Datasheet Identification
Product Status
Definition
Advance Information
Formative or
In Design
This datasheet contains the design specifications for
product development. Specifications may change in
any manner without notice.
Preliminary
First Production
This datasheet contains preliminary data, and
supplementary data will be published at a later date.
Fairchild Semiconductor reserves the right to make
changes at any time without notice in order to improve
design.
No Identification Needed
Full Production
This datasheet contains final specifications. Fairchild
Semiconductor reserves the right to make changes at
any time without notice in order to improve design.
Obsolete
Not In Production
This datasheet contains specifications on a product
that has been discontinued by Fairchild semiconductor.
The datasheet is printed for reference information only.
Rev. D