LTC3816_15 Datasheet, PDF (34/44 Pages) Linear Technology – Single-Phase Wide VIN Range DC/DC Controller for Intel IMVP-6/IMVP-6.5 CPUs

LTC3816
APPLICATIONS INFORMATION
and operating frequency. Given a specified limit for ripple
current, the inductor value can be obtained using the fol-
lowing equation:
L
=
f_OSC
V_OUT
• ∆I_L(MAX)



1–
V_OUT
V_IN(MAX)



Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite cores. Ferrite
designs have very low core loss and are thus preferred
at high switching frequencies. Ferrite core materials
saturate hard, which means that inductance collapses
abruptly when the peak design current is exceeded. This
results in an abrupt increase in inductor ripple current and
consequent output voltage ripple. Do not allow the core to
saturate! A variety of inductors designed for high current,
low voltage applications are available from manufacturers
such as Vishay, Sumida, Pulse, Wurth Elektronik, Vitec
and Toko.
Automotive Considerations
Before you connect an LTC3816 converter to an automo-
tive cigarette lighter supply, 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 19 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
LTC3816
TG
SW
BG
BSOURCE
V_IN
Q_T
Q_B
L
D⁺
V_OUT
C_OUT
3816 F19
V_BAT
12V
Figure 19. Automotive Application Protection
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 40V on
the SW pins, most applications will be limited to 30V by
the MOSFET BV_DSS.
Checking Transient Response
For all new LTC3816 PCB circuits, transient tests need to
be performed to verify the proper feedback loop operation.
The regulator loop response can be checked by looking at
the load current transient response. Switching regulators
take several cycles to respond to a step in DC (resistive)
load current. When a load step occurs, V_OUT shifts by an
amount equal to ∆V_AVP . ∆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.
Measuring transient response presents challenges in two
respects: obtaining an accurate measurement and generat-
ing a suitable transient to use to test the circuit. Output
measurements should be taken with a scope probe directly
across the output capacitor. Proper high frequency probing
techniques should be used. In particular, don’t use the 6"
ground lead that comes with the probe! Use an adapter that
fits on the tip of the probe and has a short ground clip to
ensure that inductance in the ground path doesn’t cause
a bigger spike than the transient signal being measured.
3816f
34