LTC3716 Datasheet, PDF (23/28 Pages) Linear Technology – 2-Phase, 5-Bit VID, Current Mode, High Efficiency, Synchronous Step-Down Switching Regulator

LTC3716
APPLICATIO S I FOR ATIO
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 opera-
tion. 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 alternator
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 9 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,
50A I_PK RATING
V_IN
12V
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
LTC3716
3716 F09
Figure 9. Automotive Application Protection
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 LT3716 has a maximum input
voltage of 36V, most applications will be limited to 30V by
the MOSFET BV_DSS.
Design Example
Asadesignexample,assumeV_IN =5V(nominal),V_IN␣ =␣ 5.5V
(max), V_OUT = 1.2V, I_MAX = 20A, T_A = 70°C and f␣ =␣ 300kHz.
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. Tie the FREQSET pin
to the INTV_CC pin for 300kHz operation. The minimum
inductance for 30% ripple current is:
L
≥
V_OUT
f₍∆L₎
_1−
V_OUT
V_IN


≥
1.2V
(^300kHz)(^30%)(^10A)
_1−
15..25VV 
≥ 1.04µH
A 1µH inductor will produce 31% ripple current. The peak
inductor current will be the maximum DC value plus one
half the ripple current, or 11.5A. The minimum on-time
occurs at maximum V_IN:
t_ON(MIN)
=
V_OUT
V_IN f
=
1.2V
(^5.5V)(^300kHz)
=
0.73µs
The R_SENSE resistors value can be calculated by using the
maximum current sense voltage specification with some
accomodation for tolerances:
R_SENSE
=
50mV
11.5A
≈
0.004Ω
The power dissipation on the topside MOSFET can be
easily estimated. Using a Siliconix Si4420DY for example;
R_DS(ON) = 0.013Ω, C_RSS = 300pF. At maximum input
voltage with T_J (estimated) = 110°C at an elevated ambient
temperature:
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