LTC3731 Datasheet, PDF (22/32 Pages) Linear Technology – 3-Phase, 600kHz, Synchronous Buck Switching Regulator Controller

LTC3731
APPLICATIO S I FOR
V_CC
5V
+
ATIO
V_BAT
12V
LTC3731
3731 F10
Figure 10. Automotive Application Protection
converter. Although the IC has a maximum input voltage
of 32V on the SW pins, most applications will be limited to
30V by the MOSFET BV_DSS.
Design Example
As a design example, assume V_CC = 5V, V_IN = 12V(nominal),
V_IN = 20V(max), V_OUT = 1.3V, I_MAX = 45A and f = 400kHz.
The inductance value is chosen first based upon a 30%
ripple current assumption. The highest value of ripple
current in each output stage occurs at the maximum input
voltage.
L
=
V_OUT
f₍∆I₎
⎛⎝⎜_1−
V_OUT
V_IN
⎞
⎠⎟
=
1.3V
(^400kHz)(^30%)(^15A)
⎛⎝⎜_1−
1.3V
20V
⎞⎠⎟
≥ 0.68µH
Using L = 0.6µH, a commonly available value results in
34% ripple current. The worst-case output ripple for the
three stages operating in parallel will be less than 11% of
the peak output current.
R_SENSE1, R_SENSE2 and R_SENSE3 can be calculated by using
a conservative maximum sense current threshold of 65mV
and taking into account half of the ripple current:
R_SENSE =
65mV
15A^{⎛⎝⎜}1+ ^{342%⎞⎠⎟}
= 0.0037Ω
Use a commonly available 0.003Ω sense resistor.
Next verify the minimum on-time is not violated. The
minimum on-time occurs at maximum V_CC:
t_ON(MIN)
=
V_OUT
( ) ^V_IN(MAX) ^f
=
1.3V
20V₍400kHz₎
=
162ns
The output voltage will be set by the resistive divider from
the DIFFOUT pin to SGND, R1 and R2 in the Functional
Diagram. Set R1 = 13.3k and R2 = 11.3k.
The power dissipation on the topside MOSFET can be
estimated. Using a Fairchild FDS6688 for example, R_DS(ON)
= 7mΩ, C_MILLER = 15nC/15V = 1000pF. At maximum input
voltage with T(estimated) = 50°C:
[ ] ^PMAIN
≈
1.8V
20V
(¹⁵)²
1+
(^0.005)(^50°C
−
25°C ₎
0.007Ω
+
(²⁰)²
⎛
⎝⎜
45A ^⎞
(²)(³)⎠⎟
(^2Ω)(^1000pF
)
⎛⎝⎜
5V
1
– 1.8V
+
1
1.8V
⎞⎠⎟
(^400kHz)
=
2.2W
The worst-case power dissipation by the synchronous
MOSFET under normal operating conditions at elevated
ambient temperature and estimated 50°C junction tem-
perature rise is:
P_SYNC
=
20V − 1.3V
20V
(^15A)² (^1.25)(^0.007Ω)
=
1.84W
A short circuit to ground will result in a folded back current
of:
I_SC
≈
25mV
(^{2 + 3})^mΩ
+
1^{⎛ 150ns}(^20V)^⎞
2 _⎝⎜ 0.6µH _⎠⎟
=
7.5A
with a typical value of R_DS(ON) and d = (0.005/°C)(50°C) =
0.25. The resulting power dissipated in the bottom MOSFET
is:
P_SYNC = (7.5A)²(1.25)(0.007Ω) ≈ 0.5W
3731fa
22