LTC3810EG LINER [Linear Technology], LTC3810EG Datasheet - Page 24

• Manufacturer Part Number: LTC3810EG
• Description: 100V Current Mode Synchronous Switching Regulator Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC3810
FEEDBACK LOOP/COMPENSATION
Feedback Loop Types
In a typical LTC3810 circuit, the feedback loop consists
of the modulator, the output fi lter and load, and the
feedback amplifi er with its compensation network. All of
these components affect loop behavior and must be ac-
counted for in the loop compensation. The modulator and
output fi lter consists of the internal current comparator,
the output MOSFET drivers and the external MOSFETs,
inductor and output capacitor. Current mode control
eliminates the effect of the inductor by moving it to the
inner loop, reducing it to a fi rst order system. From a
feedback loop point of view, it looks like a linear voltage
controlled current source from I
equal to (I
at typical loop compensation frequencies with signifi cant
phase shift appearing at half the switching frequency. The
external output capacitor and load cause a fi rst order roll
off at the output at the R
the attendant 90° phase shift. This roll off is what fi lters
the PWM waveform, resulting in the desired DC output
voltage. The output capacitor also contributes a zero at
the C
and cancels the fi rst order roll off.
So far, the AC response of the loop is pretty well out of the
user’s control. The modulator is a fundamental piece of
the LTC3810 design and the external output capacitor is
usually chosen based on the regulation and load current
requirements without considering the AC loop response.
The feedback amplifi er, on the other hand, gives us a
handle with which to adjust the AC response. The goal is
to have 180° phase shift at DC (so the loop regulates), and
something less than 360° phase shift (preferably about
300°) at the point that the loop gain falls to 0dB, i.e., the
crossover frequency, with as much gain as possible at
frequencies below the crossover frequency. Since the
modulator/output fi lter is a fi rst order system with maxi-
mum of 90° phase shift (at frequencies below f
the feedback amplifi er adds another 90° of phase shift,
some phase boost is required at the crossover frequency
to achieve good phase margin. If the ESR zero is below the
24
OUT
R
MAX
ESR
R
frequency which adds back the 90° phase
OUT
)/1.2V. It has fairly benign AC behavior
OUT
C
OUT
TH
to V
pole frequency, with
OUT
and has a gain
SW
/4) and
crossover frequency, this zero may provide enough phase
boost to achieve the desired phase margin and the only
requirement of the compensation will be to guarantee that
the gain is below zero at frequencies above f
ESR zero is above the crossover frequency, the feedback
amplifi er will probably be required to provide phase boost.
For most LTC3810 applications, Type 2 compensation will
provide enough phase boost; however some applications
where high bandwidth is required with low ESR ceramics
and lots of bulk capacitance, Type 3 compensation may
be necessary to provide additional phase boost.
The two types of compensation networks, “Type 2” and
“Type 3” are shown in Figures 11 and 12. When compo-
nent values are chosen properly, these networks provide
a “phase bump” at the crossover frequency. Type 2 uses
a single pole-zero pair to provide up to about 60° of phase
boost while Type 3 uses two poles and two zeros to provide
up to 150° of phase boost.
IN
IN
R1
R
R1
R
B
V
B
FB
REF
R3
V
Figure 11. Type 2 Schematic and Transfer Function
Figure 12. Type 3 Schematic and Transfer Function
REF
FB
C3
+
–
R2
–
+
R2
C2
C2
C1
C1
OUT
OUT
0
0
PHASE
GAIN
GAIN
–6dB/OCT
–6dB/OCT
+6dB/OCT
PHASE
–6dB/OCT
SW
–6dB/OCT
/4. If the
3810 F11
FREQ
–90
–180
–270
–360
3810fb
3810 F12
FREQ
–90
–180
–270
–360