LTC3828_15 Datasheet, PDF (24/32 Pages) Linear Technology – Dual, 2-Phase Step-Down Controller with Tracking

LTC3828
APPLICATIONS INFORMATION
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 LTC3828 have a maximum input
voltage of 30V, most applications will also be limited to
30V by the MOSFET BVD_SS.
50A I_PK RATING
V_IN
12V
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
LTC3828
3828 F11a
Figure 11a. Automotive Application Protection
For applications where the main input power is 5V, tie the
V_IN, INTV_CC and DRV_CC pins together and tie the combined
pins to the 5V input with a 1Ω or 2.2Ω resistor as shown
in Figure 11b to minimize the voltage drop caused by the
gate charge current. This will override the INTV_CC regula-
tor and will prevent INTV_CC from dropping too low due to
the dropout voltage. Make sure the INTV_CC voltage is at
or exceeds the R_DS(ON) test voltage for the MOSFET which
is typically 4.5V for logic-level devices.
V_IN
LTC3828
INTV_CC
DRV_CC*
*LTC3828EUH ONLY
R_VIN
1Ω
5V
+ CINTVCC
4.7μF
C_IN
3828 F11b
Figure 11b. Setup for a 5V Input
Design Example
As a design example for one channel, assume V_IN =
12V(nominal), V_IN = 22V(max), V_OUT = 1.8V, I_MAX = 5A
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 PLLFLTR
pin to a resistive divider from the INTV_CC pin, generating
0.7V for 300kHz operation. The minimum inductance for
30% ripple current is:
ΔI_L
=
V_OUT
(f)(L)
⎛⎝⎜_1–
V_OUT
V_IN
⎞
⎠⎟
A 4.7μH inductor will produce 23% ripple current and a
3.3μH will result in 33%. The peak inductor current will
be the maximum DC value plus one half the ripple cur-
rent, or 5.84A, for the 3.3μH value. Increasing the ripple
current will also help ensure that the minimum on-time
of 100ns is not violated. The minimum on-time occurs at
maximum V_IN:
t_ON(MIN)
=
V_OUT
V_IN(MAX)f
=
1.8V
22V(300kHz)
=
273ns
The R_SENSE resistor value can be calculated by using the
maximum current sense voltage specification with some
accommodation for tolerances:
R_SENSE
≤
60mV
5.84A
≈
0.01Ω
Since the output voltage is below 2.4V the output resis-
tive divider will need to be sized to not only set the output
voltage but also to absorb the SENSE pin’s specified input
current.
R1_(MAX)
=
⎛
24k_⎝⎜
0.8V
2.4V – V_OUT
⎞
⎠⎟
=
⎛
24k_⎝⎜
0.8V ^⎞
2.4V – 1.8V⎠⎟
=
32k
Choosing 1% resistors: R1 = 25.5k and R2 = 32.4k yields
an output voltage of 1.816V.
The power dissipation on the topside MOSFET can be easily
estimated. Choosing a Fairchild FDS6982S dual MOSFET
results in: R_DS(ON) = 0.035Ω/0.022Ω, C_MILLER = 215pF. At
maximum input voltage with T(estimated) = 50°C:
3828fc
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