LTC1703I 查看數據表（PDF） - Linear Technology

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LTC1703I

Dual 550kHz Synchronous 2-Phase Switching Regulator Controller with 5-Bit VID

Linear Technology 

LTC1703

APPLICATIO S I FOR ATIO

This constant frequency operation brings with it a couple

of benefits. Inductor and capacitor values can be chosen

with a precise operating frequency in mind and the feed-

back loop components can be similarly tightly specified.

Noise generated by the circuit will always be in a known

frequency band with the 550kHz frequency designed to

leave the 455kHz IF band free of interference. Subharmonic

oscillation and slope compensation, common headaches

with constant frequency current mode switchers, are

absent in voltage mode designs like the LTC1703.

During the time that QT is on, its source (the SW pin) is at

VIN. VIN is also the power supply for the LTC1703. How-

ever, QT requires VIN + VGS(ON) at its gate to achieve

minimum RON. This presents a problem for the LTC1703—

it needs to generate a gate drive signal at TG higher than

its highest supply voltage. To accomplish this, the TG

driver runs from floating supplies, with its negative supply

attached to SW and its power supply at BOOST. This allows

it to slew up and down with the source of QT. In combination

with a simple external charge pump (Figure 2), this allows

the LTC1703 to completely enhance the gate of QT without

requiring an additional, higher supply voltage.

The two channels of the LTC1703 run from a common

clock, with the phasing chosen to be 180° from side 1 to

side 2. This has the effect of doubling the frequency of the

switching pulses seen by the input bypass capacitor,

significantly lowering the RMS current seen by the capaci-

tor and reducing the value required (see the 2-Phase

section).

Feedback Amplifier

Each side of the LTC1703 senses the output voltage at

VOUT with an internal feedback op amp (see Block Dia-

gram). This is a real op amp with a low impedance output,

85dB open-loop gain and 25MHz gain-bandwidth product.

The positive input is connected internally to an 800mV

reference, while the negative input is connected to the FB

pin. The output is connected to COMP, which is in turn

connected to the soft-start circuitry and from there to the

PWM generator.

Unlike many regulators that use a resistor divider con-

nected to a high impedance feedback input, the LTC1703

is designed to use an inverting summing amplifier

PVCC BOOST

TG

SW

LTC1703

BG

PGND

VIN

+

DCP

CIN

CCP

1µF

QT LEXT

VOUT

+

QB

COUT

1703 F02

Figure 2. Floating TG Driver Supply

topology with the FB pin configured as a virtual ground.

This allows flexibility in choosing pole and zero locations

not available with simple gm configurations. In particular,

it allows the use of “type 3” compensation, which pro-

vides a phase boost at the LC pole frequency and signifi-

cantly improves loop phase margin (see Figure 3). The

Feedback Loop/Compensation section contains a de-

tailed explanation of type 3 feedback loops. Note that side

1 of the LTC1703 includes R1 and RB internally as part

of the VID DAC circuitry.

MIN/MAX COMPARATORS

Two additional feedback loops keep an eye on the primary

feedback amplifier and step in if the feedback node moves

±5% from its nominal 800mV value. The MAX comparator

(see Block Diagram) activates whenever FB rises more

than 5% above 800mV. It immediately turns the top

MOSFET (QT) off and the bottom MOSFET (QB) on and

keeps them that way until FB falls back within 5% of its

nominal value. This pulls the output down as fast as pos-

sible, preventing damage to the (often expensive) load. If

FB rises because the output is shorted to a higher supply,

QB will stay on until the short goes away, the higher supply

COMP

0.8V

FB

FB

C2

C3

R3

R1

RB

VOUT

R2

C1

1703 F03

Figure 3. “Type 3” Feedback Loop (Side 2 Shown)

1703fa

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