LTC3812-5_15 Datasheet, PDF (22/34 Pages) Linear Technology – 60V Current Mode Synchronous Switching Regulator Controller

LTC3812-5
APPLICATIONS INFORMATION
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 LTC3812-5 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)(38°C/W) < 125°C
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 LTC3812-5 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_MAXR_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_OUTC_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
22
the C_OUTR_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 LTC3812-5 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
requirement of the compensation will be to guarantee that
the gain is below zero at frequencies above f_SW/4. If the
ESR zero is above the crossover frequency, the feedback
amplifier 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 10 and 11. When compo-
nent values are chosen properly, these networks provide
C2
IN
R2
C1
R1
FB _–
R_B
V_REF +
–6dB/OCT
GAIN
OUT 0
PHASE
–6dB/OCT
FREQ
–90
–180
–270
–360
38125 F10
Figure 10. Type 2 Schematic and Transfer Function
38125fc