LINEAR TECHNOLOGY LTC3878 Technical data

L DESIGN FEATURES

V

OUT

(AC)

50mV/DIV

V

SW

20V/DIV

I

L

10A/DIV

I

LOAD

10A/DIV

5µs/DIV

LOAD STEP 0A TO 10A
VIN = 12V
V

OUT

= 1.2V
MODE = 0V
SW FREQ = 400kHz

V

OUT

(AC)

50mV/DIV

V

SW

20V/DIV

I

L

10A/DIV

I

LOAD

10A/DIV

5µs/DIV

LOAD STEP 10A TO 0A
VIN = 12V
V

OUT

= 1.2V
MODE = 0V
SW FREQ = 400kHz

Compact No R

SENSE

Controllers
Feature Fast Transient Response
and Regulate to Low V
from Wide Ranging V

Introduction

The trend in digital electronics is to
lower voltages and increasing load cur­rents. This puts pressure on DC/DC
converters to produce low voltages
from increasingly voltage-variable
supplies, such as stacked batteries
and unregulated intermediate power
buses, so power converters must be
optimized for low output voltages, low
duty factors, and wide control band­widths. To meet these requirements,
the DC/DC controller IC must offer
high voltage accuracy, good line and
load regulation, and fast transient
response. The constant on-time val­ley current mode architecture used in
the LTC3878 and LTC3879 is ideally
suited to low duty factor operation,
offering a compact solution with excel­lent system performance.

The LTC3878 and LTC3879 are
a new generation of No R
controllers that meet the demanding
requirements of low voltage supplies
for digital electronics. The LTC3878 is
a pin compatible replacement for the
LTC1778 in designs where EXTVCC
is not required. The LTC3879 adds
separate RUN and TRACK/SS pins for
applications requiring voltage track­ing. Both devices offer continuously
programmable current limit, using
the bottom MOSFET VDS voltage to
sense current.

Valley Current Mode
Control Simplifies Loop
Compensation…

There are two common implementa­tions of current mode control. Peak
current mode control regulates the
high side MOSFET on-time, while
valley current mode regulates the
bottom side MOSFET off-time. The
current mode loop bandwidth is in-

18

SENSE

™

Figure 1. Transient response,
positive load step

versely proportional to the on-time for
a peak current controller and inversely
proportional to the off-time for a valley
mode controller. A peak current mode
controller with an on-time of 50ns
must have a closed current loop band­width exceeding 20MHz. For a valley
current mode controller, the current
loop bandwidth is determined by the
typical off-time of 220ns, resulting in a
closed current loop bandwidth require­ment of only 4.5MHz. Consequently,
valley current mode control has less
stringent bandwidth requirements for
the same system performance when
compared to a peak current mode
control in a similar application. This
allows the LTC3878 and LTC3879 to
offer high performance, low duty factor
operation at reasonable current loop
bandwidths.

The constant on-time valley current
mode control of the LTC3878 and
LTC3879 simplifies compensation
design by eliminating the need for
slope compensation. A fixed frequency
valley mode controller requires slope
compensation when operating at less
than 50% duty factor to prevent sub­cycle oscillation. Subcycle oscillation
occurs because the PWM pulse width

IN

OUT

by Terry J. Groom

Figure 2. Transient response,
load release

is not uniquely determined by inductor
current alone. This oscillation cannot
exist in constant-on-time control be­cause the PWM pulse width is uniquely
determined by the internal open loop
pulse generator. True current mode
control and constant on-time combine
to give the LTC3878 and LTC3879
performance advantages over other
constant on-time regulators or fixed
frequency valley current mode control
architectures.

…and Improves Transient
Response Time

In a buck controller, transient response
is largely determined by how quickly
the inductor current responds to loop
disturbances. The most demanding
loop disturbances are load steps and
load releases.

The inherent speed advantage of
a constant on-time architecture lies
in the fact that the regulator is pulse
frequency modulated (PFM) insead
of pulse width modulated (PWM).
Although the switching frequency is
fixed in steady state operation, it can
increase or decrease as required in
response to an output load step or
load release.

Linear Technology Magazine • June 2009

DESIGN FEATURES L

f Hz

MAX

ON OFF MIN

t t

=

(

)

+

1

( )

( )

f g EA R

I

C

V
V

CGO m C

LIMIT

OUT

REF

OUT

= • •( )

.1 6

1

I

L

5A/DIV

V

OUT

0.5V/DIV

TRACK/SS

0.5V/DIV

20ms/DIV

VIN = 12V
V

OUT

= 1.2V

SW FREQ = 400kHz

+

TRACK/SS

LTC3879

BOOST

16

C

B

0.22µF
M1

RJK0305DPB

C

VCC

4.7µF

C

C1

220pF

C

C2

33pF

D

B

CMDSH-3

L1

0.56µH

C

OUT1

330µF

2.5V
s2

C

OUT2

47µF

6.3V
s2

+

C

IN1

10µF

50V

s3

C

IN2

100µF
50V

V

OUT

1.2V
15A

V

IN

4.5V TO 28V

1

PGOOD

R

PG

100k

R2

80.6k

R

C

27k

R

FB1

10.0k

R1

10.0k
TG

152

V

RNG

SW

143

MODE PGND

134

I

TH

BG

125

SGND INV

CC

116

I

ON

V

IN

107

V

FB

RUN

98

R

ON

432k

R

FB2

10.0k

M2
RJK0330DPB

C

IN1

: UMK325BJ106MM s3

C

OUT1

: SANYO 2R5TPE330M9 s2

C

OUT2

: MURATA GRM31CR60J476M s2

L1: VISHAY IHLP4040DZ-11 0.56µH

C

SS

0.1µF

LOAD CURRENT (A)

30

EFFICIENCY (%)

90

100

20

10

80

50

70

60

40

0.01 1 10 100

0

0.1

CONTINUOUS
MODE

DISCONTINUOUS

MODE

VIN = 12V
V

OUT

= 1.2V

SW FREQ = 400kHz

The maximum frequency in re­sponse to a load step is determined
by the on-time plus the off-time:

In low duty factor applications the
maximum frequency is typically much
greater than the nominal operating fre­quency, producing excellent transient
characteristics.

Figure 1 shows the load step re­sponse of a 12V-to-1.2V converter
operating at 400kHz. In this case the
on-time is equal to 250ns and the
minimum off-time is 220ns. The maxi­mum frequency available to respond
to a load step is 2.12MHz, which is
over five times the nominal switching
frequency. Note the increase in switch­ing frequency of the VSW waveform
in response to the 10A load step. The
increase in switching frequency causes
the inductor current to ramp faster in
constant on-time PFM controllers than
is possible in a true fixed frequency
PWM.

In response to a load release
(Figure 2), the minimum frequency
is effectively zero, since the bottom
gate is held high as long as needed
to ramp the inductor current down
to the internal regulation set point.
In this example, the inductor cur­rent ramps from 11A to –8A in 13µs
as the output recovers from the load
step. For both load transient cases,
variable frequency has an inherent
speed advantage over fixed frequency
in transient recovery.

Start-Up Options

The LTC3878 offers the simplicity of
current limited start-up through the
combined RUN/SS pin. When RUN/SS
is greater than 0.7V all internal bias is
activated. Once RUN/SS exceeds 1.5V,
switching begins. The current limit is
gradually increased as the RUN/SS
pin voltage ramps until reaching full

Figure 3. Start-up into a prebiased output

Transient settling requires both
the large signal ramping of induc­tor current and the stable settling of
the output to the desired regulation
point. Excessive output overshoot or
ringing indicates marginal system
stability likely caused by inadequate
compensation. A rough compensation
check can be made by calculating the
gain crossover frequency, given by the
following equation (where V
for the LTC3878 and V

REF

= 0.8V

REF

= 0.6V for

the LTC3879):

As a rule of thumb, the gain cross­over frequency should be less than
20% of the switching frequency. With
any analog system, transient response
is determined by closed loop band­width. In order to optimize for transient
performance, it is desirable to have a
small inductor and a wide closed loop
bandwidth. A small inductor is desired
for quick output current response,
while the closed loop bandwidth and
phase margin determines how quickly
the output settles after a load step.

output at approximately 3V.

separate RUN and TRACK/SS pins. All
internal bias is activated when RUN
exceeds 0.7V. Switching begins when
RUN exceeds 1.5V. The TRACK/SS
pin can also be used for input volt­age tracking, where the LTC3879’s
output tracks the voltage on the
TRACK/SS pin until it exceeds 0.6V.
Once TRACK/SS exceeds 0.6V the
output regulates to the internal 0.6V
reference. An internal 1µA pull-up cur­rent is available to create a soft-start
voltage ramp when a small capacitor
is connected to TRACK/SS. Together,
RUN and TRACK/SS enable a number

Figure 4. Efficiency for application in Figure 5

The LTC3879 adds the flexibility of

Linear Technology Magazine • June 2009

Figure 5. Wide input range to 1.2V at 15A, operating at 400kHz

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