LTC1702 查看數據表（PDF） - Linear Technology
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LTC1702 Datasheet PDF : 36 Pages
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LTC1702
APPLICATIONS INFORMATION
amp of continuous current with peak currents up to 5A to
slew large MOSFET gates quickly. The external MOSFETs
are connected with the drain of QT attached to the input
supply and the source of QT at the switching node SW. QB
is the synchronous rectifier with its drain at SW and its
source at PGND. SW is connected to one end of the
inductor, with the other end connected to VOUT. The output
capacitor is connected from VOUT to PGND.
When a switching cycle begins, QB is turned off and QT is
turned on. SW rises almost immediately to VIN and the
inductor current begins to increase. When the PWM pulse
finishes, QT turns off and one nonoverlap interval later, QB
turns on. Now SW drops to PGND and the inductor current
decreases. The cycle repeats with the next tick of the
master clock. The percentage of time spent in each mode
is controlled by the duty cycle of the PWM signal, which in
turn is controlled by the feedback amplifier. The master
clock generates a 1VP-P, 550kHz sawtooth waveform and
turns QT once every 1.8µs. In a typical application with a
5V input and a 1.6V output, the duty cycle will be set at 1.6/
5 × 100% or 32% by the feedback loop. This will give
roughly a 575ns on-time for QT and a 1.22µs on-time for
QB.
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 LTC1702.
During the time that QT is on, its source (the SW pin) is at
VIN. VIN is also the power supply for the LTC1702. How-
ever, QT requires VIN + VGS(ON) at its gate to achieve
minimum RON. This presents a problem for the LTC1702—
it needs to generate a gate drive signal at TG higher than
its highest supply voltage. To get around this, the TG driver
runs from floating supplies, with its negative supply at-
tached to SW and its power supply at BOOST. This allows
it to slew up and down with the source of QT. In combina-
tion with a simple external charge pump (Figure 2), this
allows the LTC1702 to completely enhance the gate of QT
without requiring an additional, higher supply voltage.
The two channels of the LTC1702 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, sig-
nificantly lowering the RMS current seen by the capacitor
and reducing the value required (see the 2-Phase section).
PVCC BOOST
TG
SW
LTC1702
BG
PGND
VIN
+
DCP
CIN
CCP
1µF
QT LEXT
VOUT
+
QB
COUT
1702 F02
Figure 2. Floating TG Driver Supply
Feedback Amplifier
Each side of the LTC1702 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 LTC1702
is designed to use an inverting summing amplifier topol-
ogy 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 provides
a phase boost at the LC pole frequency and significantly
1702fa
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