LTC3773 Datasheet, PDF (25/32 Pages) Linear Technology – Triple Output Synchronous 3-Phase DC/DC Controller with Up/Down Tracking

LTC3773
APPLICATIONS INFORMATION
The I_TH series R_C-C_C filter sets the dominant pole-zero
loop compensation. The values can be modified slightly
to maximize transient response once the final PC layout
is done and the particular output capacitor type and value
have been determined. The output capacitors need to be
decided upon because the various types and values deter-
mine the loop feedback factor gain and phase. An output
current pulse of 20% to 80% of full load current having
a rise time of <2μs will produce output voltage and I_TH
pin waveforms that will give a sense of the overall loop
stability without breaking the feedback loop. The initial
output voltage step, resulting from the step change in
output current, may not be within the bandwidth of the
feedback loop, so this signal cannot be used to determine
phase margin. This is why it is better to look at the I_TH
pin signal which is in the feedback loop and is the filtered
and compensated control loop response. The gain of the
loop will be increased by increasing R_C and the bandwidth
of the loop will be increased by decreasing C_C. If R_C is
increased by the same factor that C_C is decreased, the
zero frequency will be kept the same, thereby keeping
the phase the same in the most critical frequency range
of the feedback loop. The output voltage settling behavior
is related to the stability of the closed-loop system and
will demonstrate the actual overall supply performance.
For a detailed explanation of optimizing the compensation
components, including a review of control loop theory,
refer to Application Note 76.
Automotive Considerations: Plugging into the
Cigarette Lighter
As battery-powered devices go mobile, there is a natural
interest in plugging into the cigarette lighter in order to
conserve or even recharge battery packs during operation.
But before you connect, be advised: you are plugging into
the supply from hell. The main battery line in an automobile
is the source of a number of nasty potential transients, in-
cluding load dump, reverse battery and double battery.
Load dump is the result of a loose battery cable. When the
cable breaks connection, the field collapse in the alterna-
tor can cause a positive spike as high as 60V which takes
several hundred milliseconds to decay. Reverse battery is
just what it says, while double battery is a consequence of
tow-truck operators finding that a 24V jump start cranks
cold engines faster than 12V.
The network shown in Figure 8 is the most straightforward
approach to protect a DC/DC converter from the ravages
of an automotive battery line. The series diode prevents
current from flowing during reverse battery, while the
transient suppressor clamps the input voltage during
load dump. Note that the transient suppressor should not
conduct during double-battery operation, but must still
clamp the input voltage below breakdown of the converter.
Although the IC has a maximum input voltage of 36V on
the SW pins, most applications will be limited to 30V by
the MOSFET B_VDSS.
V_CC
5V
V_BAT
12V
+
TG
LTC3773
SW
BG
PGND
3773 F08
Figure 8. Automotive Application Protection
Design Example
As a design example for one channel, assume V_IN = 12V
(nominal), V_IN = 22V(max), V_OUT = 1.8V, I_MAX = 15A, and
f = 220kHz.
The inductance value is chosen first based on a 30%
ripple current assumption. The highest value of ripple
current occurs at the maximum input voltage. Short the
PLLFLTR pin to ground to program for 220kHz operation.
The minimum inductance for 30% ripple current is:
L
=
V_OUT
(f)(I_L
)


1–
V_OUT
V_IN


=
1.8V
(220k)(30%)(15A)


1

1.8V
22V


=
1.67μH
Using L = 1.5μH, a commonly available value results in
30% ripple current. The peak inductor current will be the
maximum DC value plus one half the ripple current, or
17.3A. Increasing the ripple current will also help ensure
3773fb
25