www.fairchildsemi.com

REV. 1.0.1

Features

· 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

Applications

· I/O and AGP power for desktop computers
· High efficiency power for ASICs
· High efficiency power for DSPs
· Adjustable step-down power supplies

Description

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.

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 compo-
nents in order to achieve the tight tolerance voltage regula-
tion required by many applications. Short circuit current
protection is provided through the use of a current sense
resistor, while overvoltage protection is provided internally.

Block Diagram

Feedback Control

Oscillator

Digital Logic

RC5037

1.5V

Reference

Vout

+12V

+5V

RC5037

Adjustable Switching Regulator Controller

RC5037

PRODUCT SPECIFICATION

Pin Assignments

Absolute Maximum Ratings

Note:

1. Functional operation under any of these conditions is not implied. Performance is guaranteed only if Operating Conditions

are not exceeded.

Operating Conditions

Pin Descriptions

Pin

Name

Pin

Number

Pin Function Description

CEXT

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

Analog V

Power supply for regulator control circuitry and voltage reference. Connect to

system 5V supply and decouple to ground with 0.1µF ceramic capacitor.

IFBH

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

Low side current feedback.

See Applications Information for details.

VFB

Voltage feedback.

Using two external resistors, this pin sets the output voltage level for the

switching regulator.

GNDP

Power Ground.

Connect to a low impedance ground. See Application Information for

details.

DRV

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

Power V

Power supply for DRV output driver. Connect to system 12V supply with R-C

filter shown in Figure 12. See Applications Information for details.

Supply Voltages, VCCA

Supply Voltages, VCCP

13V

Junction Temperature, T

+150°C

Storage Temperature, T

-65 to +150°C

Lead Soldering Temperature, 10 seconds

300°C

Thermal Resistance Junction-to-Ambient,

163°C/W

Parameter

Conditions

Min.

Typ.

Max.

Units

Switching Regulator Supply, VCCA

4.75

5.25

Ambient Operating Temperature, T

°C

Gate Drive Supply, VCCP

9.5

12.6

CEXT

VCCA

IFBH

VCCP

DRV

GNDP

IFBL

VFB

RC5037

PRODUCT SPECIFICATION

RC5037

Electrical Characteristics

(VCCA = 5V, VCCP = 12V, T

= 25

C using circuit of Figure 1, unless otherwise noted)

The

denotes specifications which apply over the full ambient operating temperature range.

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 C

EXT

Parameter

Conditions

Min.

Typ.

Max.

Units

Output Voltage

1.5

3.6

Output Temperature Drift

= 0°C70°C

ppm/°C

Line Regulation

VCCA = 4.75 to 5.25V, I

LOAD

= 13A

Load Regulation

LOAD

= 0 to 5A or 5A to 13A

OUT

PSRR

VCCA = 4.75 to 5.25V

Output Ripple, peak-peak

20MHz BW, I

LOAD

= 13A

Total DC Accuracy

±55

±100

Efficiency

LOAD

= 5A

Output Driver Current

Open Loop

0.5

Short Circuit Threshold Voltage

100

On Time Pulse Width

EXT

= 180pF

3.5

µs

VCCA Supply Current

Independent of load

VCCP Supply Current

LOAD

= 13A

RC5037

PRODUCT SPECIFICATION

Typical Operating Characteristics

(VCCA = 5V, and T

= +25

C using circuit in Figure 1, unless otherwise noted)

Output Current (A)

Output Voltage vs. Load

OUT

(%)

Output Voltage vs. Temperature, I

OUT

= 10A

Nom.

+0.25

+0.50

-0.25

-0.50

Output Voltage (%)

+1.0

+0.5

Nom

-0.5

-1.0

-1.5

+1.5

100

125

Transient Response, 0.5 to 5.5A

(2A/div)

OUT

(50mV/div)

Time (100

s/division)

Output Ripple, I

OUT

= 10A)

OUT

(10mV/division)

Time (2

s/division)

Efficiency vs. Output Current

95
94
93
92
91
90
89
88
87
86
85

Efficiency (%)

Output Current

RC5037

PRODUCT SPECIFICATION

Table1. Bill of Materials for a RC5037 3.3V, 13A Application

Qty.

Reference

Manufacturer

Part Order #

Description

Requirements

and Comments

C1, C7-8

Any

100nF, 25V Capacitor

C2-4

Sanyo
10MV1200GX

1200µF, 10V Aluminum Capacitor

RMS

= 2A , See Equation

(2) in Applications

Any

1µF, 25V Capacitor

Any

180pF, 50V Capacitor

C0G

C9-14

Sanyo
6MV1500GX

1500µF, 6.3V Aluminum Capacitor

ESR = 44m

Any

47.5

Any

4.75

N/A

5.2m

, 1W Resistor

PCB Trace Resistor, see
Eauation (3) Applications

Any

2.43K

Any

Motorola
MBRB1545CT

15A, 45V Schottky

Fairchild
MMBD4148

Signal Diode

Motorola
1N4735A

6.2V Zener

Fairchild
FDB6030L

30V, 14m

Logic Level MOSFET

Optional

Any

2.5µH Inductor

SAT

> 8A

Any

4.7µH Inductor

SAT

> 13A

Fairchild
RC5037M

PWM Controller

Application Information

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 high-
performance switching power supply.

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 RC5037 short-circuit protection.

High Current Output Drivers

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.

Internal Reference

The reference in the RC5037 is a precision band-gap type
reference. Its temperature coefficient is trimmed to provide a
near zero TC.

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.

Figure 2. RC5037 Detailed Block Diagram

Constant On-Time Oscillator

VOUT

+5V

VCCA

IFBL

IFBH

VCCP

FBSW

SDRV

GNDP

CEXT

65-5037-07

+12V

REF

Output Voltage Selection

The RC5037 precision reference is trimmed to be 1.5V nom-
inally. 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% resis-
tors to realize optimum output accuracy. The following
equations determines the output voltage of the regulator:

For example, for 3.3V:

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

where the duty cycle DC = V

out

. For example, with a

1.5V output at 10A, 5V input, and using the Sanyo capaci-
tors specified in Table 1 which have a 2A ripple current rat-

ing, we have DC = 1.5/5 = 0.3, and

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 cur-
rent-sense resistor exceeds the short circuit comparator
threshold voltage (V

). When this happens the output volt-

age 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 cur-
rent 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:

OUT

1.5

---------------------

(1)

OUT

1.5

---------------------

1.5

2.43k

2.0k

-------------------------------

3.3V

No. Caps

out

rating

----------------------------------------

(2)

No. Caps

0.03

0.3

10*

------------------------------------

2.29

SENSE

th,min

----------------

(

)

th,min

0.6A

LOAD,MAX

---------------------------------------------

(

)

(3)

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 10m

care should be taken in the layout of the PCB. Trace resis-
tance can contribute significant errors. The traces to the
IFBH and IFBL pins of the RC5037 should be Kelvin con-
nected 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 cur-
rent 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 dur-
ing the off time of the FET. The following equation can be
used to estimate the diode power:

where I

is the forward current of the diode, V

is the for-

ward voltage of the diode, and DutyCycle is defined the
same as

For the Motorola MBRB1545CT Rectifier in Figure 1,

It is recommended that the diode T0-220 package be
attached to a heatsink.

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 par-
asitic 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 sup-
press. 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 criti-
cality. 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 rea-
sons 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 rec-
ommended 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 recom-
mended 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.

DIODE

DutyCycle

(

)

Duty Cycle

Vout

Vin

-------------

DIODE

10A

0.65

73.1%

(

)

1.75W

Figure 3. Examples of good and poor layouts

= "Quiet" Pins

Example of

a Good Layout

Example of

a Problem Layout

Noisy signal radiates
onto quiet pins and the
trace is too long.
Gate resistor is far away
from the MOSFET.

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.

PRODUCT SPECIFICATION

RC5037

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 47

resis-

tor 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 (V

), and therefore reduces

DS(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
R

DS(ON)

MOSFET Gate Bias

Figure 4. 12V Gate Bias Configuration

OUT

+5V

VCCP

SENSE

BULK

GNDP

+12V

PRODUCT SPECIFICATION

RC5037

Mechanical Dimensions

8 Lead SOIC Package

ccc C

LEAD COPLANARITY

SEATING
PLANE

h x 45

.053

.069

1.35

1.75

Symbol

Inches

Min.

Max.

Min.

Max.

Millimeters

Notes

.004

.010

0.10

0.25

.020

0.51

.013

0.33

.008

.010

0.20

0.25

.150

.158

3.81

4.01

.228

.244

5.79

6.20

.010

.020

0.25

0.50

.050 BSC

1.27 BSC

.016

.050

0.40

1.27

ccc

.004

0.10

.189

.197

4.80

5.00

Notes:

Dimensioning and tolerancing per ANSI Y14.5M-1982.

"D" and "E" do not include mold flash. Mold flash or
protrusions shall not exceed .010 inch (0.25mm).

"L" is the length of terminal for soldering to a substrate.

Terminal numbers are shown for reference only.

"C" dimension does not include solder finish thickness.

Symbol "N" is the maximum number of terminals.

RC5037

PRODUCT SPECIFICATION

1/26/00 0.0m 001

Stock#DS30005037

1998 Fairchild Semiconductor Corporation

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

Ordering Information

Product Number

Package

RC5037M

8 pin SOIC

TRADEMARKS

ACExTM
CoolFETTM
CROSSVOLTTM
E

CMOS

FACTTM
FACT Quiet SeriesTM
FAST

FASTrTM
GTOTM
HiSeCTM

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.

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
systems which, (a) are intended for surgical implant into
the body, or (b) support or sustain life, or (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 significant injury to the
user.

2. A critical component is 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.

PRODUCT STATUS DEFINITIONS

Definition of Terms

Datasheet Identification

Product Status

Definition

Advance Information

Preliminary

No Identification Needed

Obsolete

This datasheet contains the design specifications for
product development. Specifications may change in
any manner without notice.

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.

This datasheet contains final specifications. Fairchild
Semiconductor reserves the right to make changes at
any time without notice in order to improve design.

This datasheet contains specifications on a product
that has been discontinued by Fairchild semiconductor.
The datasheet is printed for reference information only.

Formative or
In Design

First Production

Full Production

Not In Production

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.

SyncFETTM
TinyLogicTM
UHCTM
VCXTM

ISOPLANARTM
MICROWIRETM
POPTM
PowerTrench
QFETTM
QSTM
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SuperSOTTM-3
SuperSOTTM-6
SuperSOTTM-8

Rev. D

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