LTC3728LEGN-1#TRPBF Datasheet LTC3728LIGN-1#PBF, LTC3728LIUH-1#PBF, LTC3728LEGN-1#TRPBF | P10-P12

LTC3728L-1

3728l1fc

OPERATION

Main Control Loop

The LTC3728L-1 is a constant frequency, current mode

step-down controller with two channels operating 180

degrees out of phase. During normal operation, each top

MOSFET is turned on when the clock for that channel sets

the RS latch, and turned off when the main current com-

parator, I

, resets the RS latch. The peak inductor current

at which I

resets the RS latch is controlled by the voltage

on the I

pin, which is the output of each error ampliﬁ er

EA. The V

OSENSE

pin receives the voltage feedback signal,

which is compared to the internal reference voltage by the

EA. When the load current increases, it causes a slight

decrease in V

OSENSE

relative to the 0.8V reference, which

in turn causes the I

voltage to increase until the average

inductor current matches the new load current. After the

top MOSFET has turned off, the bottom MOSFET is turned

on until either the inductor current starts to reverse, as

indicated by current comparator I

, or the beginning of

the next cycle.

The top MOSFET drivers are biased from ﬂ oating bootstrap

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 V

OUT

, the

loop may enter dropout and attempt to turn on the top

MOSFET continuously. The dropout detector detects 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 cur-

rent source to charge soft-start capacitor C

. When C

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

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 operation.

When both RUN/SS1 and RUN/SS2 are low, all controller

functions are shut down, including the 5V regulator.

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 cur-

rent 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

synchronous MOSFET(s) when the inductor current goes

negative. This combination of requirements will, at low

currents, 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 hyster-

esis 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 having the hysteretic comparator after the error

ampliﬁ er 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 260kHz 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

minimum frequency.

Constant Frequency Operation

When the FCB pin is tied to INTV

, Burst Mode opera-

tion is disabled and the forced minimum output current

requirement is removed. This provides constant frequency,

discontinuous current (preventing reverse inductor cur-

rent) operation over the widest possible output current

range. This constant frequency operation is not as efﬁ cient

(Refer to Functional Diagram)

LTC3728L-1

3728l1fc

OPERATION

(Refer to Functional Diagram)

as Burst Mode operation, but does provide a lower noise,

constant frequency operating mode down to approximately

1% of the designed maximum output current.

Continuous Current (PWM) Operation

Tying the FCB pin to ground will force continuous current

operation. This is the least efﬁ cient 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, the controller will

cause current to ﬂ ow back into the input ﬁ lter capacitor.

If large enough, this element will prevent the input sup-

ply from boosting to unacceptably high levels; see C

OUT

Selection in the Applications Information Section.

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 5V low dropout

linear regulator supplies INTV

power. If EXTV

is taken

above 4.7V, the 5V regulator is turned off and an internal

switch is turned on connecting EXTV

to INTV

. This al-

lows the INTV

power to be derived from a high efﬁ ciency

external source such as the output of the regulator 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.

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

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

current of each switching regulator. Foldback current limit-

ing is activated when the output voltage falls below 70%

of its nominal level. If a short is present, a safe, low output

current is provided due to the internal current foldback

and actual power wasted is low due to the efﬁ cient nature

of the current mode switching regulator.

THEORY AND BENEFITS OF 2-PHASE OPERATION

The LTC1628 and the LTC3728L-1 family of dual high ef-

ﬁ ciency DC/DC controllers brings the considerable beneﬁ ts

of 2-phase operation to portable applications for the ﬁ rst

time. Notebook computers, PDAs, handheld terminals and

automotive electronics will all beneﬁ t from the lower input

ﬁ ltering requirement, reduced electromagnetic interference

(EMI) and increased efﬁ ciency associated with 2-phase

operation.

Why the need for 2-phase operation? Up until the 2-phase

family, constant-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 capacitor and battery. These large amplitude current

pulses increased the total RMS current ﬂ owing 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 signiﬁ cant reduction 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 efﬁ ciency.

LTC3728L-1

3728l1fc

OPERATION

(Refer to Functional Diagram)

Figure 3 compares the input waveforms for a representa-

tive single-phase dual switching regulator to the LTC1628

2-phase dual switching regulator. An actual measurement of

the RMS input current under these conditions 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 fac-

tor of 2.66. The reduced input ripple voltage also means

less power is lost in the input power path, which could

include batteries, switches, trace/connector resistances

and protection circuitry. Improvements 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 opera-

tion 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.

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 LTC1628 2-Phase Regulator Allows

Less Expensive Input Capacitors, Reduces Shielding Requirements for EMI and Improves Efﬁ ciency

3728L1 F03a

5V SWITCH

20V/DIV

3.3V SWITCH

20V/DIV

INPUT CURRENT

5A/DIV

INPUT VOLTAGE

500mV/DIV

IN(MEAS)

= 2.53A

RMS

3728L1 F03b

IN(MEAS)

= 1.55A

RMS

(a) (b)

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

3728L1 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

LTC3728LEGN-1#TRPBF

Mfr. #:

Buy LTC3728LEGN-1#TRPBF

Manufacturer:

Analog Devices / Linear Technology

Description:

Switching Voltage Regulators 2x, 550kHz, 2-PhSync Reg

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