LTC3735 Datasheet, PDF (23/32 Pages) Linear Technology – 2-Phase, High Efficiency DC/DC Controller for Intel Mobile CPUs

LTC3735
APPLICATIONS INFORMATION
load current. When a load step occurs, V_OUT shifts by an
amount equal to ∆I_LOAD(ESR), where ESR is the effective
series resistance of C_OUT. ∆I_LOAD also begins to charge or
discharge C_OUT generating the feedback error signal that
forces the regulator to adapt to the current change and
return V_OUT to its steady-state value. During this recovery
time V_OUT can be monitored for excessive overshoot or
ringing, which would indicate a stability problem. The
availability of the I_TH pin not only allows optimization of
control loop behavior but also provides a DC coupled and
AC filtered closed loop response test point. The DC step,
rise time, and settling at this test point truly reflects the
closed loop response. Assuming a predominantly second
order system, phase margin and/or damping factor can be
estimated using the percentage of overshoot seen at this
pin. The bandwidth can also be estimated by examining the
rise time at the pin. The I_TH external components shown
in the Figure 1 circuit will provide an adequate starting
point for most applications.
The I_TH series R_C-C_C filter sets the dominant pole-zero
loop compensation. The values can be modified slightly
(from 0.2 to 5 times their suggested values) to optimize
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 first because the various types and values determine
the loop gain and phase. An output current pulse of 20%
to 80% of full-load current having a rise time of <1µs 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 result-
ing 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.
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,
including 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 10 is the most straightfor-
ward approach to protect a DC/DC converter from the
ravages of an automotive power 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 LT3735 has a maximum input
voltage of 32V, most applications will be limited to 30V
by the MOSFET BV_DSS.
V_BAT
12V
PV_CC
+
PV_CC
LTC3735
3735 F10
Figure 10. Automotive Application Protection
3735fa
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