LTC1702 Linear Technology, LTC1702 Datasheet - Page 9

• Manufacturer Part Number: LTC1702
• Description: Dual 550kHz Synchronous 2-Phase Switching Regulator Controller
• Manufacturer: Linear Technology

APPLICATIONS
remember that the power losses are proportional to I
meaning that the actual power wasted is reduced by a
factor of 3.75. 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.
Small Footprint
The LTC1702 operates at a 550kHz switching frequency,
allowing it to use low value inductors without generating
excessive ripple currents. Because the inductor stores
less energy per cycle, the physical size of the inductor can
be reduced without risking core saturation, saving PCB
board space. The high operating frequency also means
less energy is stored in the output capacitors between
cycles, minimizing their required value and size. The
remaining components, including the 150mil SSOP-24
LTC1702, are tiny, allowing an entire dual-output LTC1702
circuit to be constructed in 1.5in
this space is generally located right next to the micropro-
cessor or in some similarly congested area, where PCB
real estate is at a premium. The fact that the LTC1702 runs
off the 5V supply, often available from a power plane, is an
added benefit in portable systems —it does not require a
dedicated supply line running from the battery.
Fast Transient Response
The LTC1702 uses a fast 25MHz GBW op amp as an error
amplifier. This allows the compensation network to be
designed with several poles and zeros in a more flexible
configuration than with a typical g
The high bandwidth of the amplifier, coupled with the high
switching frequency and the low values of the external
inductor and output capacitor, allow very high loop cross-
over frequencies. The low inductor value is the other half
of the equation—with a typical value on the order of 1 H,
the inductor allows very fast di/dt slew rates. The result is
superior transient response compared with conventional
solutions.
U
INFORMATION
U
2
of PCB space. Further,
m
W
feedback amplifier.
U
RMS
2
,
High Efficiency
The LTC1702 uses a synchronous step-down (buck)
architecture, with two external N-channel MOSFETs per
output. A floating topside driver and a simple external
charge pump provide full gate drive to the upper MOSFET.
The voltage mode feedback loop and MOSFET V
limit sensing remove the need for an external current
sense resistor, eliminating an external component and a
source of power loss in the high current path. Properly
designed circuits using low gate charge MOSFETs are
capable of efficiencies exceeding 90% over a wide range
of output voltages.
ARCHITECTURE DETAILS
The LTC1702 dual switching regulator controller includes
two identical, independent regulator channels. The two
sides of the chip and their corresponding external compo-
nents act independently of each other with the exception
of the common input bypass capacitor and the FCB and
FAULT pins, which affect both channels. In the following
discussions, when a pin is referred to without mentioning
which side is involved, that discussion applies equally to
both sides.
Switching Architecture
Each half of the LTC1702 is designed to operate as a
synchronous buck converter (Figure 1). Each channel
includes two high power MOSFET gate drivers to control
external N-channel MOSFETs QT and QB. These drivers
have 0.5 output impedances and can carry well over an
LTC1702
PGND
Figure 1. Synchronous Buck Architecture
SW
BG
TG
V
IN
QT
QB
L
EXT
+
+
C
C
LTC1702
IN
OUT
1702 F01
V
DS
OUT
current
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