LTC1628IG-PG#PBF Datasheet LTC1628CG-PG#TRPBF, LTC1628IG#TRPBF, LTC1628IG-PG#TRPBF | P10-P12

LTC1628/LTC1628-PG

1628fb

(Refer to Functional Diagram)

Main Control Loop

The LTC1628 uses a constant frequency, current mode

step-down architecture with the two controller channels

operating 180 degrees out of phase. During normal opera-

tion, each top MOSFET is turned on when the clock for that

channel sets the RS latch, and turned off when the main

current comparator, I

, resets the RS latch. The peak

inductor current at which I

resets the RS latch is con-

trolled by the voltage on the I

pin, which is the output of

each error amplifier 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 in-

creases, 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 compara-

tor I

, or the beginning of the next cycle.

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 500ns

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

LTC1628 controller functions are shut down, and the

STBYMD pin determines if the standby 5V and 3.3V

regulators are kept alive.

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

controller 1 (or both controllers depending upon the

FLTCPL pin); and 2) select between

two

modes of low

current operation. When the FCB pin voltage is below

0.800V, 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

0.80V, 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

having the hysteretic comparator after the error amplifier

gain block.

Constant Frequency Operation

When the FCB pin is tied to INTV

, Burst Mode operation

is disabled and the forced minimum output current re-

quirement is removed. This provides constant frequency,

discontinuous (preventing reverse inductor current) cur-

rent operation over the widest possible output current

range. This constant frequency operation is not as efficient

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

constant frequency operating mode down to approxi-

mately 1% of designed maximum output current.

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!

OPERATIO

LTC1628/LTC1628-PG

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Frequency Setting

The FREQSET pin provides frequency adjustment of the

internal oscillator from approximately 140kHz to 310kHz.

This input is nominally biased through an internal resistor

to the 1.19V reference, setting the oscillator frequency to

approximately 220kHz. This pin can be driven from an

external AC or DC signal source to control the instanta-

neous frequency of the oscillator.

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 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.

Standby Mode Pin

The STBYMD pin is a three-state input that controls

common circuitry within the IC as follows: When the

STBYMD pin is held at ground, both controller RUN/SS

pins are pulled to ground providing a single control pin to

shut down both controllers. When the pin is left open, the

internal RUN/SS currents are enabled to charge the

RUN/SS capacitor(s), allowing the turn-on of either con-

troller and activating necessary common internal biasing.

When the STBYMD pin is taken above 2V, both internal

linear regulators are turned on independent of the state on

the RUN/SS pins of the two switching regulator control-

lers, providing an output power source for “wake-up”

circuitry. Decouple the pin with a small capacitor (0.01µF)

to ground if the pin is not connected to a DC potential.

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.

Fault Coupling Pin

The FLTCPL pin (LTC1628 only) controls two functions

that can operate individually (FLTCPL = 0V) or unilaterally

(FLTCPL = INTV

) between the two controllers. When the

FLTCPL pin is grounded (internally tied default mode for

the LTC1628-PG), 1) the FCB input forces continuous

operation only on the first controller when the applied

voltage drops below 0.8V and 2) the short-circuit latchoff

function only latches off the controller having the shorted

output. When the FLTCPL pin is tied to INTV

, 1) the FCB

input forces continuous operation on

both

controllers

when the applied voltage drops below 0.8V and 2) the

short-circuit latchoff function latches off

both

controllers

when either has a shorted output.

Power Good (PGOOD) Pin

The PGOOD pin (LTC1628-PG only) is connected to an

open drain of an internal MOSFET. The MOSFET turns on

and pulls the pin low when both the outputs are not within

±7.5% of their nominal output levels as determined by

their resistive feedback dividers. 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

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 condition

lasts for a long enough period as determined by the size of

the RUN/SS capacitor, the controller (or both controllers

as determined by the FLTCPL pin, LTC1628 only) 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

(Refer to Functional Diagram)

OPERATIO

LTC1628/LTC1628-PG

1628fb

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.

THEORY AND BENEFITS OF 2-PHASE OPERATION

The LTC1628 dual high efficiency DC/DC controller brings

the considerable benefits of 2-phase operation to portable

applications for the first time. Notebook computers, PDAs,

handheld terminals and automotive electronics will all

benefit from the lower input filtering requirement, reduced

electromagnetic interference (EMI) and increased effi-

ciency associated with 2-phase operation.

Why the need for 2-phase operation? Up until the LTC1628,

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 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 reduction in

total RMS input current, which in turn allows less expen-

sive input capacitors to be used, reduces shielding re-

quirements 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

LTC1628 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

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

(b)

(a)

5V SWITCH

20V/DIV

3.3V SWITCH

20V/DIV

INPUT CURRENT

5A/DIV

INPUT VOLTAGE

500mV/DIV

IN(MEAS)

= 1.55A

RMS

DC236 F03b

IN(MEAS)

= 2.53A

RMS

DC236 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 LTC1628 2-Phase Regulator Allows Less Expensive

Input Capacitors, Reduces Shielding Requirements for EMI and Improves Efficiency

(Refer to Functional Diagram)

OPERATIO

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

LTC1628IG-PG#PBF

Mfr. #:

Buy LTC1628IG-PG#PBF

Manufacturer:

Analog Devices / Linear Technology

Description:

Switching Voltage Regulators Dual 2-phase Step-dn + Pgood

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LTC1628IG-PG#PBF

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