XRP6124ESTR0.5-F - Exar Corporation

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January 2011 Rev. 1.1.0

Exar Corporation www.exar.com

48720 Kato Road, Fremont CA 94538, USA Tel. +1 510 668-7000 – Fax. +1 510 668-7001

GENERAL DESCRIPTION

The XRP6124 is a non synchronous step down

(buck) controller for up to 5Amps point of

loads. A wide 3V to 30V input voltage range

allows for single supply operations from

industry standard 3.3V, 5V, 12V and 24V

power rails.

With a proprietary Constant On-Time (COT)

control scheme, the XRP6124 provides

extremely fast line and load transient response

while the operating frequency remains nearly

constant. It requires no loop compensation

hence simplifying circuit implementation and

reducing overall component count. The

XRP76124 also implements an emulated ESR

circuitry allowing usage of ceramic output

capacitors and insuring stable operations

without the use of extra external components.

Built-in soft start prevents high inrush currents

while under voltage lock-out and output short

protections insure safe operations under

abnormal operating conditions.

The XRP6124 is available in a RoHS compliant,

green/halogen free space-saving 5-pin SOT23

package.

APPLICATIONS

• Point of Load Conversions

• Audio-Video Equipments

• Industrial and Medical Equipments

• Distributed Power Architecture

FEATURES

• 5A Point-of-Load Capable

− Down to 1.2V Output Voltage Conversion

• Wide Input Voltage Range

− 3V to 18V: XRP6124

− 4.5V to 30V: XRP6124HV

• Constant On-Time Operations

− Constant Frequency Operations

− No External Compensation

− Supports Ceramic Output Capacitors

• Built-in 2ms Soft Start

• Short Circuit Protection

• <1µA shutdown current

• RoHS Compliant, Green/Halogen Free

5-pin SOT23 Package

TYPICAL APPLICATION DIAGRAM

Figure 1: XRP6124 Application Diagram

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ABSOLUTE MAXIMUM RATINGS

These are stress ratings only and functional operation of

the device at these ratings or any other above those

indicated in the operation sections of the specifications

below is not implied. Exposure to absolute maximum

rating conditions for extended periods of time may affect

reliability.

VIN (XRP6124) ............................................ -0.3V to 20V

VIN (XRP6124HV) ........................................ -0.3V to 32V

GATE ...................................................... VIN-GATE<8V

FB, EN ..................................................... -0.3V to 5.5V

Storage Temperature .............................. -65°C to 150°C

Power Dissipation ................................ Internally Limited

Lead Temperature (Soldering, 10 sec) ................... 300°C

ESD Rating (HBM - Human Body Model) .................... 2kV

OPERATING RATINGS

Input Voltage Range VIN (XRP6124) ................ 3.0V to 18V

Input Voltage Range VIN (XRP6124HV) ............ 4.5V to 30V

Junction Temperature Range .................... -40°C to 125°C

Thermal Resistance θJA .....................................191°C/W

ELECTRICAL SPECIFICATIONS

Specifications are for an Operating Junction Temperature of TJ = 25°C only; limits applying over the full Operating Junction

Temperature range are denoted by a “•”. Minimum and Maximum limits are guaranteed through test, design, or statistical

correlation. Typical values represent the most likely parametric norm at TJ = 25°C, and are provided for reference purposes

only. Unless otherwise indicated, VIN = 3.0V to 18V, TJ = –40°C to 125°C.

Parameter Min. Typ. Max. Units Conditions

UVLO Turn-On Threshold 2.5 2.8 3.0 V • XRP6124

UVLO Turn-On Threshold 3.8 4.2 4.5 V • XRP6124HV

UVLO Hysteresis 0.1 V

Operating Input Voltage Range 3.0 18 V • XRP6124

4.5 30 V • XRP6124HV

Shutdown VIN Current 1.5 3 µA EN=0V, VIN=12V

Operating VIN Current 0.5 1 mA VFB=1.2V and after fault

Reference Voltage 0.792 0.8 0.808 V

0.784 0.8 0.816 V •

VSC_TH, Feedback pin Short

Circuit Latch Threshold 0.50 0.55 0.65 V •

TON, Switch On-Time 0.4 0.5 0.6 µs • VIN=12V, XRP6124

TON, Switch On-Time 0.4 0.5 0.6 µs • VIN=24V, XRP6124HV

TOFF_MIN, Minimum Off-Time 250 350 ns • VIN=12V

Soft Start Time 2 ms

EN Turn-On Threshold 2 V

EN Turn-Off Threshold 1 V

EN Bias Current 0.01 0.1 µA

Gate Driver Pull-Down

Resistance 6 9 Ω

Gate Driver Pull-up Resistance 5 8 Ω

tr, gate rise time 45 ns CGATE=1nF

tf, gate fall time 35 ns CGATE=1nF

VIN - GATE voltage difference 5.5 6.4 8 V • VIN=12V

VIN - GATE voltage difference 2.6 V • VIN=3.0V

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BLOCK DIAGRAM

Figure 2: XRP6124 Block Diagram

PIN ASSIGNMENT

1

2

3

5

4

XRP61 24

EN

GND

FB

VI N

GATE

Figure 3: XRP6124 Pin Assignment

PIN DESCRIPTION

Name Pin Number Description

EN 1 Enable Pin. Actively pull high to enable the part.

GND 2 Ground

FB 3 Feedback pin

GATE 4

Gate Pin. Connect to gate of PFET. This pin pulls the gate of the PFET approximately

6V below Vin in order to turn on the FET. For 6V>VIN>3V the gate pulls to within 0.4V

of ground. Therefore a PFET with a gate rating of 2.6V or lower should be used.

VIN 5 Input Voltage

ORDERING INFORMATION

Part Number Temperature

Range Marking Package Packing

Quantity Note 1 Note 2

XRP6124ES0.5-F -40°C≤TJ≤125°C 5-pin SOT23

Bulk Halogen Free 0.5µs/18V max

XRP6124ESTR0.5-F -40°C≤TJ≤125°C 5-pin SOT23

2.5K/Tape & Reel

Halogen Free 0.5µs/18V max

XRP6124HVES0.5-F -40°C≤TJ≤125°C 5-pin SOT23

Bulk Halogen Free 0.5µs/30V max

XRP6124HVESTR0.5-F -40°C≤TJ≤125°C 5-pin SOT23

2.5K/Tape & Reel

Halogen Free 0.5µs/30V max

XRP6124EVB XRP6124 Evaluation Board

“YY” = Year – “WW” = Work Week – “X” = Lot Number

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TYPICAL PERFORMANCE CHARACTERISTICS

All data taken at TJ = TA = 25°C, unless otherwise specified – Curves are based on Schematic and BOM from Application

Information section of this datasheet. Refer to figure 20 for XRP6124 and to figure 21 for XRP6124HV.

Fig. 4: Efficiency versus IOUT, VIN=12V

Fig. 5: Efficiency versus IOUT, VIN=24V

Fig. 6: TON versus VIN

Fig. 7: TON versus VIN

Fig. 8: Load Regulation

Fig. 9: Load Regulation

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Fig. 10: Line Regulation

Fig. 11: Line Regulation

Fig. 12: Steady state, VIN=12V, VOUT=3.3V, IOUT=3A

Fig. 13: Steady state, VIN=24V, VOUT=5.0V, IOUT=3A

Fig. 14: Load step transient response, 1.4A-3A-1.4A

Fig. 15: Load step transient response, 1.4A-3A-1.4A

XRP6124HVES0.5-F

XRP6124ES0.5-F

VOUT

AC coupled

10mV

/

div

LX

10V/div

I

L

2A/div

LX

20V/div

VOUT

AC coupled

20mV

/

div

I

L

2A/div

XRP6124ES0.5-F

1µs/div 2µs/div

XRP6124ES0.5-F XRP6124HVES0.5-F

VOUT

AC coupled

100mV

/

div

VOUT

AC coupled

200mV

/

div

IOUT

1A/div

IOUT

1A/div

10µs/div 20µs/div

90mV 180mV

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Fig. 16: Load step transient response corresponding to a

CCM-DCM transition, 0.05A-1.6A-0.05A

Fig. 17: Load step transient response corresponding to a

CCM-DCM transition, 0.05A-1.6A-0.05A

Fig. 18: Shutdown current versus VIN, VEN=0V

Fig. 19: Shutdown current versus VIN, VEN=0V

XRP6124ES0.5-F XRP6124HVES0.5-F

VOUT

AC coupled

100mV

/

div

VOUT

AC coupled

200mV

/

div

IOUT

1A/div IOUT

1A/div

50µs/div 50µs/div

90mV 180mV

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THEORY OF OPERATION

The XRP6124 utilizes a proprietary Constant

On-Time (COT) control with emulated ESR.

The on-time is internally set and automatically

adjusts during operation, inversely with the

voltage VIN, in order to maintain a constant

frequency. Therefore the switching frequency

is independent of the inductor and capacitor

size, unlike hysteretic controllers. The

emulated ESR ramp allows the use of ceramic

capacitors for output filtering.

At the beginning of each cycle, the XRP6124

turns on the P-Channel FET for a fixed

duration. The on-time is internally set and

adjusted by VIN. At the end of the on-time the

FET is turned off, for a predetermined

minimum off time TOFF-MIN (nominally 250ns).

After the TOFF-MIN has expired the voltage at

feedback pin FB is compared to a voltage

ramp at the feedback comparators positive

input. Once VFB drops below the ramp voltage,

the FET is turned on and a new cycle starts.

This voltage ramp constitutes an emulated

ESR and makes possible the use of ceramic

capacitors, in addition to other capacitors, as

output filter for the buck converter.

VOLTAGE OPTIONS

The XRP6124 is available in two voltage

options as shown in table 1. The low-voltage

and high-voltage options have TON of 0.5µs at

12VIN and 24VIN respectively. Note that TON is

inversely proportional to VIN. The constant of

proportionality K, for each voltage option is

shown in table 1. Variation of TON versus VIN is

shown graphically in figures 6 and 7.

Voltage

rating (V)

Part Number TON (µs) K=TONxVIN

(μs.V)

3-18 XRP6124ES0.5-F 0.5 @ 12VIN

6

4.5-30 XRP6124HVES0.5-F

0.5 @ 24VIN

12

Table 1 : XRP6124 voltage options

For a buck converter the switching frequency

fs can be expressed in terms of VIN, VOUT and

TON as follows:

ONIN

OUT

TV

V

fs ×

=

Since for each voltage option, the product of

VIN and TON is the constant K shown in table 1,

then switching frequency is determined by

VOUT as shown in table 2.

VOUT

Switching frequency fs(kHz)

XRP6124ES0.5-F XRP6124HVES0.5-F

1.2 200 100

1.5 250 125

1.8 300 150

2.5 417 208

3.3 550 275

5.0 833 417

12 --- 1000

Table 2: Switching frequency fs

for the XRP6124 voltage options

Where it is advantageous, the high-voltage

option may be used for low-voltage

applications. For example a 12VIN to 5VOUT

conversion using a low-voltage option will

result in switching frequency of 833kHz as

shown in table 2. If it is desired to increase

the converter efficiency, then switching losses

can be reduced in half by using a high-voltage

option operating at a switching frequency of

417kHz.

VOUT

Maximum Output Current IOUT(A)

XRP6124ES0.5-F XRP6124HVES0.5-F

3.3VIN

5.0VIN 12VIN 18VIN 24VIN

1.2 5 5 4 --- ---

1.5 5 5 4 4 ---

1.8 5 5 4 4 4

2.5 4 4 4 4 4

3.3 --- 4 3 4 4

5.0 --- --- 3 3 3

12 --- --- --- 2 2

Table 3: Maximum recommended IOUT

SHORT-CIRCUIT PROTECTION

The purpose of this feature is to prevent an

accidental short-circuit at the output from

damaging the converter. The XRP6124 has a

short-circuit comparator that constantly

monitors the feedback node, after soft-start is

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finished. If the feedback voltage drops below

0.55V, equivalent to output voltage dropping

below 69% of nominal, the comparator will

trip causing the IC to latch off. In order to

restart the XRP6124, the input voltage has to

be reduced below UVLO threshold and then

increased to its normal operating point.

SOFT-START

To limit in-rush current the XRP6124 has an

internal soft-start. The nominal soft-start time

is 2ms and commences when VIN exceeds the

UVLO threshold. As explained above, the

short-circuit comparator is enabled as soon as

soft-start is complete. Therefore if the input

voltage has a very slow rising edge such that

at the end of soft-start the output voltage has

not reached 69% of its final value then the

XRP6124 will latch-off.

ENABLE

By applying a logic-level signal to the enable

pin EN the XRP6124 can be turned on and off.

Pulling the enable below 1V shuts down the

controller and reduces the VIN leakage current

to 1.5µA nominal as seen in figure 18. Enable

signal should always be applied after the input

voltage or concurrent with it. Otherwise

XRP6124 will latch up. In applications where

an independent enable signal is not available,

a Zener diode can be used to derive VEN from

VIN.

DISCONTINUOUS CONDUCTION MODE, DCM

Because XRP6124 is a non-synchronous

controller, when load current IOUT is reduced to

less than half of peak-to-peak inductor current

ripple ΔIL, the converter enters DCM mode of

operation. The switching frequency fs is now

IOUT dependent and no longer governed by the

relationship shown in table 2. As IOUT is

decreased so does fs until a minimum

switching frequency, typically in the range of

few hundred Hertz, is reached at no load. This

contributes to good converter efficiency at

light load as seen in figures 4 and 5. The

reduced fs corresponding to light load,

however, increases the output voltage ripple

and causes a slight increase in output voltage

as seen in figures 8 and 9. Another effect of

reduced fs at light load is slow down of

transient response when a load step

transitions from a high load to a light load.

This is shown in figures 16 and 17.

APPLICATION INFORMATION

SETTING THE OUTPUT VOLTAGE

Use an external resistor divider to set the

output voltage. Program the output voltage

from:

⎟

⎠

⎞

⎜

⎝

⎛−×= 1

8.0

21 OUT

V

RR

where:

R1 is the resistor between VOUT and FB

R2 is the resistor between FB and GND

(nominally 2kΩ)

0.8V is the nominal feedback voltage.

FEED-FORWARD CAPACITOR CFF

CFF, which is placed in parallel with R1,

provides a low-impedance/high-frequency

path for the output voltage ripple to be

transmitted to FB. It also helps get an

optimum transient response. An initial value

for CFF can be calculated from:

11.02

1

Rfs

CFF ××××

=

π

where:

fs is the switching frequency from table 2

This value can be adjusted as necessary to

provide an optimum load step transient

response.

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OUTPUT INDUCTOR

Select the output inductor L1 for inductance L,

DC current rating IDC and saturation current

rating ISAT. IDC should be larger than regulator

output current. ISAT, as a rule of thumb, should

be 50% higher than the regulator output

current. Calculate the inductance from:

()

⎟

⎠

⎞

⎜

⎝

⎛

××Δ

−=

INL

OUT

OUTIN VfsI

V

VVL

Where:

ΔIL is peak-to-peak inductor current ripple

nominally set to 30% of IOUT

fS is nominal switching frequency from table 2

OUTPUT CAPACITOR COUT

Select the output capacitor for voltage rating,

capacitance COUT and Equivalent Series

Resistance ESR. The voltage rating, as a rule

of thumb, should be twice the output voltage.

When calculating the required capacitance,

usually the overriding requirement is current

load-step transient. If the unloading transient

requirement (i.e., when IOUT transitions from a

high to a low current) is met, then usually the

loading transient requirement (when IOUT

transitions from a low to a high current) is met

as well. Therefore calculate the COUT

capacitance based on the unloading transient

requirement from:

()

⎟

⎠

⎞

⎜

⎝

⎛

−+

−

×= 22

22

OUTtransientOUT

LOWHigh

OUT VVV

II

LC

Where:

L is the inductance calculated in the preceding

step

IHigh is the value of IOUT prior to unloading. This

is nominally set equal to regulator current

rating.

ILow is the value of IOUT after unloading. This is

nominally set equal to 50% of regulator

current rating.

Vtransient is the maximum permissible voltage

transient corresponding to the load step

mentioned above. Vtransient is typically specified

from 3% to 5% of VOUT.

ESR of the capacitor has to be selected such

that the output voltage ripple requirement

VOUT(ripple), nominally 1% of VOUT, is met.

Voltage ripple VOUT(ripple) is composed mainly of

two components: the resistive ripple due to

ESR and capacitive ripple due to COUT charge

transfer. For applications requiring low voltage

ripple, ceramic capacitors are recommended

because of their low ESR which is typically in

the range of 5mΩ. Therefore VOUT(ripple) is

mainly capacitive. For ceramic capacitors

calculate the VOUT(ripple) from:

fsC

I

OUT

L

××

Δ

=8

V)OUT(ripple

Where:

COUT is the value calculated above

If tantalum or electrolytic capacitors are used

then VOUT(ripple) is essentially a function of ESR:

ESRIL

×

Δ

=

V )OUT(ripple

INPUT CAPACITOR CIN

Select the input capacitor for voltage rating,

RMS current rating and capacitance. The

voltage rating, as a rule of thumb, should be

50% higher than the regulator’s maximum

input voltage. Calculate the capacitor’s current

rating from:

()

DDIOUT −××= 1 I RMSCIN,

Where:

IOUT is regulator’s maximum current

D is duty cycle (D=VOUT/VIN)

Calculate the CIN capacitance from:

()

ININ

OUTINOUTOUT

VVfs

VVVI

Δ××

−×

×

=2

IN

C

Where:

ΔVIN is the permissible input voltage ripple,

nominally set to 1% of VIN.

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TYPICAL APPLICATIONS

12V TO 3.3V / 3A CONVERSION

XRP6124ES

VIN

GND

GATEFB

EN

D1

MBRA340

L1, 4.7uH

DR74-4R7-R

M1

IRF9335

R1, 1%

6.34k

R2, 1%

2k

CFF

1nF

1

2

34

5

COUT, X5R

2x22uF, 10V

CIN, X5R

22uF, 25V

VIN 6V to 18V

VOUT 3.3V/3A

Fig. 20: 12V to 3.3V/3A regulator

24V TO 5V / 3A CONVERSION

XRP6124HVES

VIN

GND

GATEFB

EN

D1

MBRA340

L1, 8.2uH

HCM0730

M1

DMP4050SSS

R1, 1%

10.5k

R2, 1%

2k

CFF

0.47nF

1

2

34

5

COUT, X5R

2x22uF, 16V

CIN, X5R

10uF, 50V

VIN 8V to 30V

VOUT 5.0V/3A

Fig. 21: 24V to 5V/3A regulator

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PACKAGE SPECIFICATION

5-PIN SOT23

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REVISION HISTORY

Revision Date Description

1.0.0 01/26/2011 Initial release of datasheet

1.1.0 01/31/2011 Corrected typo (changed V to I) on formula under Input Capacitor CIN paragraph

FOR FURTHER ASSISTANCE

Email: customersupport@exar.com

Exar Technical Documentation: http://www.exar.com/TechDoc/default.aspx?

EXAR CORPORATION

HEADQUARTERS AND SALES OFFICES

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Fremont, CA 94538 – USA

Tel.: +1 (510) 668-7000

Fax: +1 (510) 668-7030

www.exar.com

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design, performance or reliability. EXAR Corporation assumes no responsibility for the use of any circuits described herein,

conveys no license under any patent or other right, and makes no representation that the circuits are free of patent

infringement. Charts and schedules contained here in are only for illustration purposes and may vary depending upon a

user’s specific application. While the information in this publication has been carefully checked; no responsibility, however,

is assumed for inaccuracies.

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