LTC3810-5
23
38105fd
takes over before the INTV
CC
capacitor discharges below
the UV threshold. When the EXTV
CC
regulator is active,
the EXTV
CC
pin can supply up to 50mA RMS. Do not ap-
ply more than 15V to the EXTV
CC
pin. The following list
summarizes the possible connections for EXTV
CC
:
1. EXTV
CC
grounded. This connection will require INTV
CC
to be powered continuously from an external NMOS
from V
IN
resulting in an efficiency penalty as high as
10% at high input voltages.
2. EXTV
CC
connected directly to V
OUT
. This is the normal
connection for 4.7V < V
OUT
< 15V and provides the
highest efficiency. The power supply will start up using
an external NMOS or a bleed resistor until the output
supply is available.
3. EXTV
CC
connected to an output-derived boost network.
If V
OUT
< 4.7V. The low voltage output can be boosted
using a charge pump or flyback winding to greater
than 4.7V.
4. EXTV
CC
connected to INTV
CC
. This is the required con-
nection for EXTV
CC
if INTV
CC
is connected to an external
supply where the external supply is 4.5V < V
EXT
< 15V.
Applications using large MOSFETs with a high input
voltage and high frequency of operation may result in a
large EXTV
CC
pin current. Due to the LTC3810-5 thermally
enhanced package, maximum junction temperature will
rarely be exceeded, however, it is good design practice
to verify that the maximum junction temperature rating
and RMS current rating are within the maximum limits.
Typically, most of the EXTV
CC
current consists of the
MOSFET gates current. In continuous mode operation,
this EXTV
CC
current is:
I
EXTVCC
=
f Q
G(TOP)
+
Q
G(BOTTOM)
(
)
+
3mA
<
50mA
The junction temperature can be estimated from the
equations given in Note 2 of the Electrical Characteristics
as follows:
T
J
= T
A
+ I
EXTVCC
• (V
EXTVCC
– V
INTVCC
)(34°C/W)
The calculated T
J
should be <125°C for E- and I-grade
or < 150°C for H-grade. If absolute maximum ratings are
exceeded, consider using an external supply connected
directly to the INTV
CC
pin.
FEEDBACK LOOP/COMPENSATION
Feedback Loop Types
In a typical LTC3810-5 circuit, the feedback loop con-
sists of the modulator, the output filter and load, and the
feedback amplifier 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 filter 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 first order system. From a
feedback loop point of view, it looks like a linear voltage
controlled current source from I
TH
to V
OUT
and has a gain
equal to (I
MAX
R
OUT
)/1.2V. It has fairly benign AC behavior
at typical loop compensation frequencies with significant
phase shift appearing at half the switching frequency. The
external output capacitor and load cause a first order roll
off at the output at the R
OUT
C
OUT
pole frequency, with
the attendant 90° phase shift. This roll off is what filters
the PWM waveform, resulting in the desired DC output
voltage. The output capacitor also contributes a zero at
the C
OUT
R
ESR
frequency which adds back the 90° phase
and cancels the first 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 LTC3810-5 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 amplifier, 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 filter is a first order system with maxi-
mum of 90° phase shift (at frequencies below f
SW
/4) and
the feedback amplifier 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
crossover frequency, this zero may provide enough phase
boost to achieve the desired phase margin and the only
applicaTions inForMaTion
