LT1777 Linear Technology, LT1777 Datasheet - Page 8

• Manufacturer Part Number: LT1777
• Description: Low Noise Step-Down Switching Regulator
• Manufacturer: Linear Technology

OPERATIO
LT1777
The LT1777 is a current mode step-down switcher regu-
lator IC designed for low noise operation. The Block
Diagram shows an overall view of the system. The indi-
vidual blocks are straightforward and similar to those
found in traditional designs, including: Internal Bias Regu-
lator, Oscillator, Logic, and Feedback Amplifier. The novel
portion includes a specialized Output Switch section in-
cluding circuits to limit the dI/dt and dV/dt switching rates.
The LT1777 operates much the same as traditional current
mode switchers, the major difference being its specialized
output switch section. Due to space constraints, this
discussion will not reiterate the basics of current mode
switcher/controllers and the “buck” topology. A good
source of information on these topics is Application Note
AN19.
A straightforward output stage is provided by current
source I
collector of Q2 in turn drives the base of NPN output device
Q1. The considerable base/collector capacitance of PNP
Q2 acts to limit dV/dt rate during switch turn-on. However,
when the switch is to be turned off, the only natural limit
to voltage slew rate would be the collector/base capaci-
tance of Q1 providing drive for the same device. While
dependent upon output load level and Q1’s , the turn-off
voltage slew rate would be typically much faster than the
turn-on rate. To limit the voltage slew rate on switch turn-
off, an extra function is supplied. This is denoted by the
block labeled “– dV/dt Limiter.”
The details of the – dV/dt Limiter can be seen in the Output
Stage Simplified Schematic. Transistors Q3 and Q4 are
connected in a Darlington configuration whose input is
8
1
driving the base of PNP transistor Q2. The
U
coupled with small-valued capacitor C1 to the V
rail. The product of negative voltage slew rate times this
capacitor value equals current, and when this current
through emitter/base resistor R1 exceeds a diode drop, Q3
and then Q4 turn on supplying base drive to output device
Q1 to limit – dV/dt rate.
In addition to voltage rates, the current slew rate also
needs to be controlled for reduced noise behavior. This is
provided by the section in the Block Diagram labeled
“ dI/dt Limiter.” The details of this circuit can be seen in
the Output Stage Simplified Schematic. Note that an extra,
small-valued inductor, termed the “sense inductor” has
been added to the classic buck topology. As this inductor
is external to the LT1777, its value can be chosen by the
user allowing for optimization on a per application basis.
Operation of the current slew limiter is as follows: The
product of the sense inductor times the dI/dt through it
generates a voltage according to the well known formula
V = (L)(dI/dt). The remainder of the circuit is configured
such that when the voltage across the sense inductor
reaches 2V
as necessary to limit current slew rate. The actual sensing
is performed between the output node labeled V
new node labeled V
In the case of switch turn-on, current drive is provided by
PNP Q2. If the voltage at V
V
drive from Q1’s base. Similarly for turn-off, as the V
node goes 2V
drive Q1’s base as needed. The net effect is that of limiting
the switch node dI/dt in both directions at a rate inversely
proportional to the external sense inductor value.
D
, transistor Q5 turns on and removes a portion of Q2’s
BE
BE
, drive current will be supplied or removed
below V
D
.
D
, transistor Q6 then turns on to
SW
reaches 2V
BE
above that at
IN
SW
supply
and a
SW