IRU3138

Rev. 1.0

01/29/04

www.irf.com

DESCRIPTION

The IRU3138 controller IC is designed to provide a low

cost synchronous Buck regulator for voltage tracking

applications such DDR memory and general purpose

on-board DC to DC converter. Modern micro processors

combined with DDR memory, need high-speed bandwidth

data bus which requires a particular bus termination volt-

age. This voltage will be tightly regulated to track the

half of chipset voltage for best performance. The IRU3138

together with two N-channel MOSFETs, provide a low

cost solution for such applications. This device features

a programmable frequency set from 200KHz to 400KHz,

under-voltage lockout for both Vcc and Vc supplies, an

external programmable soft-start function as well as

output under-voltage detection that latches off the de-

vice when an output short is detected.

1A Peak Output Drive Capability

0.8V Precision Reference Voltage Available

Shuts off both drivers at shorted output

and shutdown

200KHz to 400KHz operation set by an external

resistor

Soft-Start Function

Uncommitted Error Amplifier available for DDR

voltage tracking application

Protects the output when control FET is shorted

Synchronous Controller in 14-Pin Package

PACKAGE ORDER INFORMATION

FEATURES

SYNCHRONOUS PWM CONTROLLER

FOR TERMINATION POWER SUPPLY APPLICATIONS

(癈)

DEVICE PACKAGE

0 To 70

IRU3138CS 14-Pin Plastic SOIC NB (S)

Data Sheet No. PD94701

Figure 1 - Typical application of IRU3138 when V

tracks the V

DDQ

APPLICATIONS

DDR memory source sink Vtt application

Graphic Card

Low cost on-board DC to DC such as

5V to 3.3V, 2.5V or 1.8V

TYPICAL APPLICATION

IRU3138

Vcc

HDrv

LDrv

Gnd

Comp

SS/SD

C1
0.1uF

C2
1uF

0.1uF

3300pF

13K

Q1
IRF7460

Q2
IRF7456

3.3uH

1uH

C3
2x 47uF

C4
47uF

Vtt

1.25V @ 10A

C6
2x 150uF
40m

12V

PGnd

REF

R1
1K

R2
1K

DDR

Memory

DDQ

(2.5V)

D1
BAT54

Rev. 1.0

01/29/04

IRU3138

www.irf.com

ELECTRICAL SPECIFICATIONS

Unless otherwise specified, these specifications apply over V

=5V, V

=12V and T

=0 to 70癈. Typical values refer

to T

=25癈. Low duty cycle pulse testing is used which keeps junction and case temperatures equal to the ambient

temperature.

ABSOLUTE MAXIMUM RATINGS

Supply Voltage .................................................. -0.5V - 25V

Supply Voltage .................................................... -0.5V - 25V

Storage Temperature Range ...................................... -65癈 To 150癈

Operating Junction Temperature Range ..................... 0癈 To 125癈

CAUTION: Stresses above those listed in "Absolute Maximum Ratings" may cause permanent damage to the device.

PARAMETER

SYM TEST CONDITION

MIN TYP MAX

UNITS

Reference Voltage
V

REF

Voltage

Fb Voltage Line Regulation
UVLO
UVLO Threshold - V

UVLO Hysteresis - V

UVLO Threshold - V

UVLO Hysteresis - V

UVLO Threshold - Fb
UVLO Hysteresis - Fb
Supply Current
V

Dynamic Supply Current

Static Supply Current

Soft-Start Section
Charge Current
Oscillator
Frequency

Ramp Amplitude

5<V

<12

Supply Ramping Up

Fb Ramping Down
Note 1

Freq=200KHz, C

=3000pF

Freq=200KHz, C

=3000pF

SS=0V
SS=0V

SS=0V

Rt=Open
Rt=Gnd
Note 1

0.784

4.0

3.0

0.3

180
360

0.8

4.25
0.25

3.5

0.25

0.4

0.02

6.5

2.5

200
400

1.25

0.816

1.6

4.5

3.65

0.5

6
4

220
440

V
V
V
V
V
V

mA
mA
mA
mA

KHz

=88

C/W

REG

UVLO Vcc

UVLO Vc

UVLO Fb

Dyn Icc

Dyn Ic

CCQ

Freq

RAMP

PACKAGE INFORMATION

14-PIN PLASTIC SOIC NB (S)

Vcc

LDrv

Gnd

PGnd

HDrv

12 Comp

14 NC

REF

SS/SD

IRU3138

Rev. 1.0

01/29/04

www.irf.com

PARAMETER

SYM TEST CONDITION

MIN TYP MAX

UNITS

This pin is connected directly to the output of the switching regulator via resistor divider to

provide feedback to the Error amplifier.

Non-inverting input of error amplifier.

Reference Voltage.

This pin provides biasing for the internal blocks of the IC as well as power for the low side

driver. A minimum of 1

F, high frequency capacitor must be connected from this pin to

ground to provide peak drive current capability.

No Connection.

Output driver for the synchronous power MOSFET.

Analog ground for internal reference and control circuitry. Connect to PGnd with a short

trace.

This pin serves as the separate ground for MOSFET's drivers and should be connected to

system's ground plane. A high frequency capacitor (0.1

F to 1

F) must be connected

from V

and V

pins to this pin for noise free operation.

Output driver for the high side power MOSFET. This pin should not go negative (below

ground), this may cause problem for the gate drive circuit. It can happen when the inductor

current goes negative (Source/Sink), soft-start at no load and for the fast load transient

from full load to no load. To prevent negative voltage at gate drive, a low forward voltage

drop diode might be connected between this pin and ground.

This pin is connected to a voltage that must be at least 4V higher than the bus voltage of

the switcher (assuming 5V threshold MOSFET) and powers the high side output driver. A

minimum of 1

F, high frequency capacitor must be connected from this pin to ground to

provide peak drive current capability.

The switching frequency can be Programmed between 200KHz and 400KHz by connect-

ing a resistor between Rt and Gnd. Floating the pin set the switching frequency to 200KHz

and grounding the pin set the switching frequency to 400KHz.

Compensation pin of the error amplifier. An external resistor and capacitor network is

typically connected from this pin to ground to provide loop compensation.

This pin provides soft-start for the switching regulator. An internal current source charges

an external capacitor that is connected from this pin to ground which ramps up the output

of the switching regulator, preventing it from overshooting as well as limiting the input

current. The converter can be shutdown by pulling this pin below 2.8V.

Error Amp
Fb Voltage Input Bias Current
Fb Voltage Input Bias Current
V

Voltage Range

Transconductance
Output Drivers
Rise Time
Fall Time
Dead Band Time
Max Duty Cycle
Min Duty Cycle

SS=3V, Fb=1V
SS=0V, Fb=1V

LOAD

=3000pF

LOAD

=3000pF

Fb=0.7V, Freq=200KHz
Fb=1.5V

0.7

500

0.1

800

35
35

100

mho

ns
ns
ns

%
%

REF

LDrv

Gnd

PGnd

HDrv

Comp

SS / SD

FB1

FB2

Ton

Toff

1.5

1000

70
70

Note 1: Guaranteed by design, but not tested in production.

PIN DESCRIPTIONS

PIN# PIN SYMBOL PIN DESCRIPTION

IRU3138

Rev. 1.0

01/29/04

www.irf.com

THEORY OF OPERATION

Introduction

The IRU3138 is a fixed frequency, voltage mode syn-

chronous controller and consists of a precision refer-

ence voltage, an error amplifier, an internal oscillator, a

PWM comparator, 1A peak gate driver, soft-start and

shutdown circuits (see Block Diagram). The output volt-

age of the synchronous converter is set and controlled

by the output of the error amplifier; this is the amplified

error signal from the sensed output voltage and the refer-

ence voltage. This voltage is compared to a fixed fre-

quency linear sawtooth ramp and generates fixed fre-

quency pulses of variable duty-cycle, which drives the

two N-channel external MOSFETs.The timing of the IC

is provided through an internal oscillator circuit which

uses on-chip capacitor. The oscillation frequency is pro-

grammable between 200KHz to 400KHz by using an

external resistor. Figure 4A shows switching frequency

vs. external resistor.

Soft-Start

The IRU3138 has a programmable soft-start to control

the output voltage rise and limit the current surge at the

start-up. To ensure correct start-up, the soft-start se-

quence initiates when the Vc and Vcc rise above their

threshold (3.5V and 4.25V respectively) and generates

the Power On Reset (POR) signal. Soft-start function

operates by sourcing an internal current to charge an

external capacitor to about 3V. Initially, the soft-start func-

tion clamps the E/A's output of the PWM converter and

disables the short circuit protection. During the power

up, the output starts at zero and voltage at Fb is below

0.4V. The feedback UVLO is disabled during this time

by injecting a current (64

A) into the Fb. This generates

a voltage about 1.6V (64

25K) across the negative

input of E/A and positive input of the feedback UVLO

comparator (see Fig3).

Figure 3 - Soft-start circuit for IRU3138.

The magnitude of this current is inversely proportional to

the voltage at soft-start pin.

The 20

A current source starts to charge up the exter-

nal capacitor. In the mean time, the soft-start voltage

ramps up, the current flowing into Fb pin starts to de-

crease linearly and so does the voltage at the positive

pin of feedback UVLO comparator and the voltage nega-

tive input of E/A.

When the soft-start capacitor is around 1V, the current

flowing into the Fb pin is approximately 32

A. The volt-

age at the positive input of the E/A is approximately:

The E/A will start to operate and the output voltage starts

to increase. As the soft-start capacitor voltage contin-

ues to go up, the current flowing into the Fb pin will keep

decreasing. Because the voltage at pin of E/A is regu-

lated to reference voltage 0.8V, the voltage at the Fb is:

The feedback voltage increases linearly as the injecting

current goes down. The injecting current drops to zero

when soft-start voltage is around 2V and the output volt-

age goes into steady state.

As shown in Figure 4, the positive pin of feedback UVLO

comparator is always higher than 0.4V, therefore, feed-

back UVLO is not functional during soft-start.

Figure 4 - Theoretical operational waveforms

during soft-start.

25K = 0.8V

= 0.8-25K

(Injected Current)

20uA

64uA
Max

POR

Error Amp

64uA

25K=1.6V

When SS=0

POR

0.4V

Feeback
UVLO Comp

SS/SD

Comp

25K

0.8V

25K

HDrv

LDrv

Soft-Start

Voltage

Voltage at negative input

of Error Amp and Feedback

UVLO comparator

Voltage at Fb pin

Current flowing

into Fb pin

64uA

0uA

0.8V

1.6V

0.8V

Output of UVLO

POR

协谢械泻褌褉芯薪薪褘泄 泻芯屑锌芯薪械薪褌: IRU3138

协谢械泻褌褉芯薪薪褘泄泻芯屑锌芯薪械薪褌: IRU3138