LTC3727EG-1#TRPBF Datasheet LTC3727IG-1#PBF, LTC3727EG-1#TRPBF, LTC3727EG#TRPBF | P10-P12

LTC3727/LTC3727-1

3727fc

(Refer to Functional Diagram)

The top MOSFET drivers are biased from floating boot-

strap capacitor C

, which normally is recharged during

each off cycle through an external diode when the top

MOSFET turns off. As V

decreases to a voltage close to

OUT

, the loop may enter dropout and attempt to turn on

the top MOSFET continuously. The dropout detector de-

tects this and forces the top MOSFET off for about 400ns

every tenth cycle to allow C

to recharge.

The main control loop is shut down by pulling the RUN/SS

pin low. Releasing RUN/SS allows an internal 1.2μA

current source to charge soft-start capacitor C

. When

reaches 1.5V, the main control loop is enabled with the

voltage clamped at approximately 30% of its maximum

value. As C

continues to charge, the I

pin voltage is

gradually released allowing normal, full-current opera-

tion. When both RUN/SS1 and RUN/SS2 are low, all

LTC3727/LTC3727-1 controller functions are shut down,

including the 7.5V and 3.3V regulators.

Low Current Operation

The FCB pin is a multifunction pin providing two func-

tions: 1) to provide regulation for a secondary winding by

temporarily forcing continuous PWM operation on

both controllers; and 2) to select between

two

modes of

low current operation. When the FCB pin voltage is below

0.8V, the controller forces continuous PWM current

mode operation. In this mode, the top and bottom

MOSFETs are alternately turned on to maintain the output

voltage independent of direction of inductor current.

When the FCB pin is below V

INTVCC

– 2V but greater than

0.8V, the controller enters Burst Mode operation. Burst

Mode operation sets a minimum output current level

before inhibiting the top switch and turns off the synchro-

nous MOSFET(s) when the inductor current goes nega-

tive. This combination of requirements will, at low cur-

rents, force the I

pin below a voltage threshold that will

temporarily inhibit turn-on of both output MOSFETs until

the output voltage drops. There is 60mV of hysteresis in

the burst comparator B tied to the I

pin. This hysteresis

produces output signals to the MOSFETs that turn them

on for several cycles, followed by a variable “sleep”

interval depending upon the load current. The resultant

output voltage ripple is held to a very small value by

OPERATIO

having the hysteretic comparator follow the error ampli-

fier gain block.

Frequency Synchronization

The phase-locked loop allows the internal oscillator to be

synchronized to an external source via the PLLIN pin. The

output of the phase detector at the PLLFLTR pin is also the

DC frequency control input of the oscillator that operates

over a 250kHz to 550kHz range corresponding to a DC

voltage input from 0V to 2.4V. When locked, the PLL

aligns the turn on of the top MOSFET to the rising edge of

the synchronizing signal. When PLLIN is left open, the

PLLFLTR pin goes low, forcing the oscillator to its mini-

mum frequency.

Continuous Current (PWM) Operation

Tying the FCB pin to ground will force continuous current

operation. This is the least efficient operating mode, but

may be desirable in certain applications. The output can

source or sink current in this mode. When sinking current

while in forced continuous operation, current will be

forced back into the main power supply potentially boost-

ing the input supply to dangerous voltage levels—

BEWARE!

INTV

/EXTV

Power

Power for the top and bottom MOSFET drivers and most

other internal circuitry is derived from the INTV

pin.

When the EXTV

pin is left open, an internal 7.5V low

dropout linear regulator supplies INTV

power. If EXTV

is taken above 7.3V, the 7.5V regulator is turned off and an

internal switch is turned on connecting EXTV

to INTV

This allows the INTV

power to be derived from a high

efficiency external source such as the output of the regu-

lator itself or a secondary winding, as described in the

Applications Information section.

Output Overvoltage Protection

An overvoltage comparator, OV, guards against transient

overshoots (>7.5%) as well as other more serious condi-

tions that may overvoltage the output. In this case, the top

MOSFET is turned off and the bottom MOSFET is turned on

until the overvoltage condition is cleared.

LTC3727/LTC3727-1

3727fc

Power Good (PGOOD) Pin

The PGOOD pin is connected to an open drain of an internal

MOSFET. The MOSFET turns on and pulls the pin low when

either output is not within ±7.5% of the nominal output

level as determined by the resistive feedback divider.

When both outputs meet the ±7.5% requirement, the

MOSFET is turned off within 10μs and the pin is allowed to

be pulled up by an external resistor to a source of up to 7V.

Foldback Current, Short-Circuit Detection

and Short-Circuit Latchoff (LTC3727 Only)

The RUN/SS capacitors are used initially to limit the inrush

current of each switching regulator. After the controller

has been started and been given adequate time to charge

up the output capacitors and provide full load current, the

RUN/SS capacitor is used in a short-circuit time-out

circuit. If the output voltage falls to less than 70% of its

nominal output voltage, the RUN/SS capacitor begins

discharging on the assumption that the output is in an

overcurrent and/or short-circuit condition. If the condi-

tion lasts for a long enough period as determined by the

size of the RUN/SS capacitor, the controller will be shut

down until the RUN/SS pin(s) voltage(s) are recycled.

This built-in latchoff can be overridden by providing a

>5μA pull-up at a compliance of 5V to the RUN/SS pin(s).

This current shortens the soft start period but also pre-

vents net discharge of the RUN/SS capacitor(s) during an

overcurrent and/or short-circuit condition. Foldback cur-

rent limiting is also activated when the output voltage falls

below 70% of its nominal level whether or not the short-

circuit latchoff circuit is enabled. Even if a short is present

and the short-circuit latchoff is not enabled, a safe, low

output current is provided due to internal current foldback

and actual power wasted is low due to the efficient nature

of the current mode switching regulator.

PART NUMBER FUNCTION

LTC3727 With Latchoff Function Available

LTC3727-1 Latchoff Always Disabled

THEORY AND BENEFITS OF 2-PHASE OPERATION

The LTC3727 dual high efficiency DC/DC controller brings

the considerable benefits of 2-phase operation to portable

applications. Notebook computers, PDAs, handheld ter-

minals and automotive electronics will all benefit from the

lower input filtering requirement, reduced electromag-

netic interference (EMI) and increased efficiency associ-

ated with 2-phase operation.

Why the need for 2-phase operation? Until recently, con-

stant-frequency dual switching regulators operated both

channels in phase (i.e., single-phase operation). This

means that both switches turned on at the same time,

causing current pulses of up to twice the amplitude of

those for one regulator to be drawn from the input capaci-

tor and battery. These large amplitude current pulses

increased the total RMS current flowing from the input

capacitor, requiring the use of more expensive input

capacitors and increasing both EMI and losses in the input

capacitor and battery.

With 2-phase operation, the two channels of the dual-

switching regulator are operated 180 degrees out of

phase. This effectively interleaves the current pulses

drawn by the switches, greatly reducing the overlap time

where they add together.

The result is a significant reduc-

tion in total RMS input current, which in turn allows less

expen

sive input capacitors to be used, reduces shielding

requirements for EMI and improves real world operating

efficiency.

Figure 3 compares the input waveforms for a representa-

tive single-phase dual switching regulator to the new

LTC3727 2-phase dual switching regulator. An actual

measurement of the RMS input current under these con-

ditions shows that 2-phase operation dropped the input

current from 2.53A

RMS

to 1.55A

RMS

. While this is an

impressive reduction in itself, remember that the power

losses are proportional to I

RMS

, meaning that the actual

power wasted is reduced by a factor of 2.66. The reduced

input ripple voltage also means less power is lost in the

(Refer to Functional Diagram)

OPERATIO

LTC3727/LTC3727-1

3727fc

input power path, which could include batteries, switches,

trace/connector resistances and protection circuitry. Im-

provements in both conducted and radiated EMI also

directly accrue as a result of the reduced RMS input

current and voltage.

Of course, the improvement afforded by 2-phase opera-

tion is a function of the dual switching regulator’s relative

duty cycles which, in turn, are dependent upon the input

voltage V

(Duty Cycle = V

OUT

). Figure 4 shows how

the RMS input current varies for single-phase and 2-phase

operation for 3.3V and 5V regulators over a wide input

voltage range.

It can readily be seen that the advantages of 2-phase

operation are not just limited to a narrow operating range,

but in fact extend over a wide region. A good rule of thumb

for most applications is that 2-phase operation will reduce

the input capacitor requirement to that for just one channel

operating at maximum current and 50% duty cycle.

A final question: If 2-phase operation offers such an

advantage over single-phase operation for dual switching

regulators, why hasn’t it been done before? The answer is

that, while simple in concept, it is hard to implement.

Constant-frequency current mode switching regulators

require an oscillator derived “slope compensation” signal

to allow stable operation of each regulator at over 50%

duty cycle. This signal is relatively easy to derive in single-

phase dual switching regulators, but required the develop-

ment of a new and proprietary technique to allow 2-phase

operation. In addition, isolation between the two channels

becomes more critical with 2-phase operation because

switch transitions in one channel could potentially disrupt

the operation of the other channel.

(b)

(a)

5V SWITCH

20V/DIV

3.3V SWITCH

20V/DIV

INPUT CURRENT

5A/DIV

INPUT VOLTAGE

500mV/DIV

IN(MEAS)

= 1.55A

RMS

3727 F03b

IN(MEAS)

= 2.53A

RMS

3727 F03a

Figure 3. Input Waveforms Comparing Single-Phase (a) and 2-Phase (b) Operation for

Dual Switching Regulators Converting 12V to 5V and 3.3V at 3A Each. The Reduced Input

Ripple with the LTC3727 2-Phase Regulator Allows Less Expensive Input Capacitors,

Reduces Shielding Requirements for EMI and Improves Efficiency

(Refer to Functional Diagram)

OPERATIO

Figure 4. RMS Input Current Comparison

INPUT VOLTAGE (V)

INPUT RMS CURRENT (A)

3.0

2.5

2.0

1.5

1.0

0.5

10 20 30 40

3727 F04

SINGLE PHASE

DUAL CONTROLLER

2-PHASE

DUAL CONTROLLER

= 5V/3A

= 3.3V/3A

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P24 P25-P27 P28-P30 P31-P32

LTC3727EG-1#TRPBF

Mfr. #:

Buy LTC3727EG-1#TRPBF

Manufacturer:

Analog Devices / Linear Technology

Description:

Switching Voltage Regulators Dual, 2-Phase Step-Down Controller

Lifecycle:

New from this manufacturer.

Delivery:

DHL FedEx Ups TNT EMS

Payment:

T/T Paypal Visa MoneyGram Western Union

LTC3727EG-1#TRPBF

LTC3727EG-1#TRPBF

Products related to this Datasheet