LTC1267CG-5 - Linear Technology

09/95/114

The LTC1267 Dual Switching Regulator Controller Operates

from High Input Voltages – Design Note 114

Randy G. Flatness and Craig Varga

Introduction

The LTC

1267 dual switching regulator controller is the

latest addition to Linear Technology’s family of better than

90% efficient step-down DC/DC converters. The LTC1267

features an extremely wide 4V to 40V input operating

voltage range and reduced supply currents. The quiescent

current is a low 320µA and current in Shutdown mode

drops to less than 20µA. The combination of low supply

currents and high input voltage capability is ideal for

battery-powered applications that require high voltage AC

wall adapters.

Linear Technology offers three versions of the LTC1267, all

in the space-saving 28-lead SSOP package. The LTC1267

provides fixed output voltages of 3.3V and 5V with indi-

vidual shutdown capability. The LTC1267-ADJ provides

two user-programmable output voltages set by external

resistive dividers. The LTC1267-ADJ5 is configured with a

fixed 5V output and a programmable output set by an

external resistor divider.

High Efficiency with Dual Output Voltages

To boost efficiency, a unique EXT V

pin on the LTC1267

(also present on the single output LTC1159) allows the

MOSFET drivers and control circuitry to be powered from

an external source, such as the output of the regulator

itself. Obtaining control and driver power from V

OUT

im-

proves efficiency at high input voltages, since the resulting

current drawn from V

is scaled by the duty cycle of the

regulator. During start-up and short-circuit conditions,

operating power is supplied by an internal 4.5V low drop-

out regulator. This regulator automatically turns off when

the EXT V

pin rises above 4.5V.

This 28-pin controller uses Linear Technology’s high per-

formance, current mode architecture and Burst Mode

operation. The LTC1267 automatically switches to Burst

Mode operation at low output currents to maintain high

efficiency over two decades of load current range. The wide

operating range is illustrated by the typical efficiency curve

of Figure 1. Battery life is extended by providing high

OUTPUT CURRENT

1mA

EFFICIENCY (%)

100

10mA 100mA 1A 3A

DN114 • F01

= 12V

5V SECTION

3.3V SECTION

efficiencies at load currents from a few milliamps (when the

device is in standby or sleep modes) to amps (under full

power conditions).

Description

Both regulator blocks in the LTC1267 use a constant off

time, current mode architecture. This results in a power

supply that has very high efficiency over a wide load current

range, fast transient response and very low dropout. The

LTC1267 is ideal for applications that require 3.3V and 5V

to be implemented with the highest conversion efficiencies

over a wide load current range in a small board space. The

LTC1267-ADJ has two externally adjustable outputs which

allow remote load sensing and user customized output

voltages.

Each regulator section employs a pair of external, comple-

mentary MOSFETs and a user programmable current sense

resistor to set the operation current level to optimize

performance for each application. A master Shutdown pin

turns off both main outputs and the 4.5V LDO. Both outputs

in the LTC1267 and LTC1267-ADJ5 have individual Shut-

down capability, whereas the LTC1267-ADJ has a Shut-

down pin for only one of its two outputs.

, LTC and LT are registered trademarks of Linear Technology Corporation.

Burst Mode is a trademark of Linear Technology Corporation.

Figure 1. LTC1267 Efficiency vs Output Current

of the Figure 2 Circuit

 LINEAR TECHNOLOGY CORPORATION 1995

LT/GP 0995 190K • PRINTED IN THE USA

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7487

(408) 432-1900

●

FAX

: (408) 434-0507

●

TELEX

: 499-3977

For literature on our DC/DC Converters,

call 1-800-4-LINEAR. For applications help,

call (408) 432-1900, Ext. 361

The higher input voltage capability of the LTC1267 is

required by battery-powered systems that use many cells

in series to provide more power and longer battery life. For

applications requiring 12 cells or more, the AC adapter

voltage can be as high as 30V, well below the 40V maxi-

mum of the LTC1267, allowing operation directly from the

AC adapter. At low input voltages, the internal 4.5V low

dropout regulator stays in regulation with an input of only

5V, extracting the maximum possible energy from the

battery pack.

All members of the LTC1142/LTC1267 family are capable

of 100% duty cycle, providing very low dropout operation

(lower than that of most linear low dropout regulators) and

all have built-in current limiting. As the input voltage on the

LTC1267 drops, the loop extends the on time for the

P-channel switch (off time is constant) thereby keeping the

inductor ripple current constant. Eventually the on time

extends so far that the P-channel MOSFET is on at DC or

100% duty cycle. Load and line regulation are excellent for

a wide variety of conditions, including making the transi-

tion from Burst Mode operation to continuous mode opera-

tion.

Fixed Output 3.3V and 5V Converter

A fixed LTC1267 application circuit creating 3.3V/3A and

5V/3A is shown in Figure 2. The operating efficiency shown

in Figure 1 exceeds 90% for both the 3.3V and 5V sections.

The 3.3V section of the circuit in Figure 2 comprises the

main switch Q1, synchronous switch Q2, inductor L1 and

current shunt R

SENSE3

The 5V section is similar and comprises Q3, Q4, L2 and

SENSE5

. Each current sense resistor (R

SENSE

) monitors

the inductor current and is used to set the output current

according to the formula I

OUT

= 100mV/R

SENSE

. Advan-

tages of current control include excellent line and load

transient rejection, inherent short-circuit protection and

controlled start-up currents. Peak inductor currents for L1

and L2 are limited to 150mV/R

SENSE

or 4.5A. The EXT V

pin is connected to the 5V output, increasing efficiency at

high input voltages. The maximum input voltage is limited

by the external MOSFETs and should not exceed 28V.

Conclusion

The LTC1267 adds even more versatility to Linear

Technology’s family of high efficiency step-down regulator

controllers. Providing for up to 40V input voltage, the

LTC1267 allows the use of higher voltage wall adapters.

The 28-lead SSOP package and associated external com-

ponents make dual output voltage, high efficiency DC/DC

conversion feasible in the extremely small board space

available in today’s portable electronics.

1000pF

PGATE 3

PDRIVE 3

SENSE

–

3SENSE

–

SHDN3

PDRIVE 5

SHDN5

NDRIVE 3

PGND3 SGND3 C

TH3

TH5

SGND5 PGND5

NGATE 5

SENSE

PGATE 5

CC3

CAP3 CAP5MSHDN V

CC5

EXT V

LTC1267



270pF

7 11 9 10 15 16 20 22



3300pF C



3300pF



270pF



0.15µF0.15µF

1N4148

262722128

OUT5

5V

OUT5

220µF

10V

× 3

SENSE5

0.033Ω

Q3

P-CH

Si9435DY

L2

20µH

D5

MBRS140T3

Q4

N-CH

Si9410DY

IN5

100µF

50V

3.3µF

0.1µF

IN3

100µF

50V Q1

P-CH

Si9435DY

Q2

N-CH

Si9410DY

0V = RUN

>2V = SHDN 0V = RUN

>2V = SHDN

D1

MBRS140T3

OUT3

220µF

10V

× 2

L1

15µH

SENSE3

0.033Ω

OUT3

3.3V

5.5V < V

< 25V

SENSE

: KRL SL-C1-1/2R033

L1: COILTRONICS CTX15-4

L2: COILTRONICS CTX20-4

KRL (603) 668-3210

COILTRONICS (407) 241-7876

DN114 • F02

3.3µF

1N4148

Figure 2. LTC1267 Dual Output 3.3V and 5V High Efficiency Regulator