LTC1704 Linear Technology, LTC1704 Datasheet - Page 7

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LTC1704

Manufacturer Part Number
LTC1704
Description
550kHz Synchronous Switching Regulator Controller Plus Linear Regulator Controller
Manufacturer
Linear Technology
Datasheet

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TYPICAL PERFOR A CE CHARACTERISTICS
PI FU CTIO S
TG (Pin 1): Switcher Controller Top Gate Drive. The TG pin
drives the gate of the top N-channel MOSFET, QT. The TG
driver draws power from the BOOST pin and returns it to
the SW pin, providing true floating drive to QT. TG is de-
signed to typically drive up to 10,000pF of gate capacitance.
SW (Pin 2): Switcher Controller Switching Node. Connect
SW to the switching node of the main converter. The TG
driver ground returns to SW, providing floating gate drive
to the top N-channel MOSFET, QT. The voltage at SW is
compared to I
the bottom MOSFET, QB is on. The Burst comparator
(BURST, see Block Diagram) monitors the potential at SW
and switches to Burst Mode operation under light load
conditions.
I
I
the switcher controller. If the voltage drop across the
bottom MOSFET, QB, exceeds the magnitude of the volt-
age at I
The I
allowing the current threshold to be set with a single
external resistor to PGND. Kelvin connect this current
setting resistor to the source of QB. Refer to the Current
Limit Programming section for more information on choos-
ing R
MAX
MAX
U
MAX
(Pin 3): Switcher Controller Current Limit Set. The
pin sets the current limit comparator threshold for
IMAX
MAX
pin has an internal 10 A current source pull-up,
.
U
, the switcher controller enters current limit.
MAX
4.5
1.5
6.0
3.0
0
–50
by the current limit comparator while
Supply Current vs Temperature
V
TG, BG FLOAT
CC
U
–25
= PV
CC
0
W
TEMPERATURE ( C)
= BOOST = 5V
25
U
I
I
BOOST
I
PVCC
VCC
50
75
100
1704 G26
125
RUN/SS (Pin 4): Switcher Controller Soft-Start. A capaci-
tor from RUN/SS to GND controls the turn-on time and
rate of rise of the switcher output voltage at power up. An
internal 3 A current source pull-up at RUN/SS sets the
turn-on time at approximately 300ms/ F. If both RUN/SS
and REGILM are pulled low, the LTC1704 enters shutdown
mode.
COMP (Pin 5): Switcher Controller Loop Compensation.
The COMP pin is connected directly to the output of the
switcher controller’s error amplifier and the input to the
PWM comparator. Use an RC network between the COMP
pin and the FB pin to compensate the feedback loop for
optimum transient response.
FB (Pin 6): Switcher Controller Feedback Input. FB should
be connected through a resistor divider network to V
to set the switcher output voltage. Also, connect the
switcher loop compensation network to FB.
REGDR (Pin 7): Linear Regulator Controller Driver Output.
Connect REGDR to the base of the external NPN Pass
transistor. The REGILM pin input current controls the
linear regulator controller maximum driving capability.
GND (Pin 8): Signal Ground. All internal low power cir-
cuitry returns to the GND pin. Connect to a low impedance
ground, separated from the PGND node. All feedback,
35
30
20
10
25
15
0
5
I
0
PVCC
T
PV
A
CC
= 25 C
2000
, I
= BOOST = 5V
BOOST
TG, BG LOAD (pF)
4000
vs Driver Load
LTC1704/LTC1704B
6000
8000
1704 G27
10000
OUTSW
1704bfa
7

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