LTC3879_15 Datasheet, PDF (20/28 Pages) Linear Technology – Fast, Wide Operating Range No RSENSE Step-Down Controller

LTC3879
APPLICATIONS INFORMATION
C_SS
0.1μF
R1 R2
10.0k 80.6k
1 _{TRACK/SS BOOST} 16
R_PG
100k
2
LTC3879
PGOOD
TG ¹⁵
D_B
CMDSH-3
C_B
0.22μF
C_C1
220pF ^RC
27k
C_C2
33pF
3 _VRNG
4 _MODE
5 _ITH
6 _SGND
SW ¹⁴
PGND ¹³
BG ¹²
INV_CC ¹¹
C_VCC
4.7μF
R_FB1
10.0k
R_FB2 R_ON
10.0k 432k
7 _ION
8 _VFB
V_IN ¹⁰
RUN ⁹
C_IN1
10μF
50V
s3
M1
RJK0305DPB
L1
0.56μH
+ C_IN2
100μF
50V
M2
C_{OUT1 +}
330μF
RJK0330DPB _2.5V
s2
C_OUT2
47μF
6.3V
s2
V_IN
4.5V TO 28V
V_OUT
1.2V
15A
C_IN1: UMK325BJ106MM s3
C_OUT1: SANYO 2R5TPE330M9 s2
C_OUT2: MURATA GRM31CR60J476M s2
L1: VISHAY IHLP4040DZ-11 0.56μH
3879 F10
Figure 10. Design Example: 1.2V/15A at 400kHz
at 25°C) is required to account for variation in MOSFET
on-resistance with temperature. Choosing an RJK0330
(R_DS(ON) = 2.8mΩ (nominal) 3.9mΩ (maximum), V_GS =
4.5V, θ_JA = 40°C/W) yields a drain source voltage of:
( ) V_DS
=
⎛
⎝⎜
I_LIMIT
–
1
2
I_RIPPLE
⎞
⎠⎟
3.9mΩ
(ρτ)
V_RNG sets current limit by fixing the maximum peak V_DS
voltage on the bottom MOSFET switch. As a result, the
average DC current limit includes significant temperature
and component variability. Design to guarantee that the
average DC current limit will always exceed the rated oper-
ating output current by assuming worst-case component
tolerance and temperature.
The worst-case minimum INTV_CC is 5.15V. The bottom
MOSFET worst-case R_DS(ON) is 3.9mΩ and the junction
temperature is 80°C above a 70°C ambient with ρ_150°C =
1.5. Set T_ON equal to the minimum specification of 15%
low and the inductor 15% high.
By setting I_LIMIT equal to 15A we get 79mV for peak V_DS
voltage which corresponds to a V_RNG equal to 592mV:
V_DS
=
⎛
⎝⎜
15A
–
1
2
• 5.1A
•
0.85 ⎞
1.15 ⎠⎟
3.9mΩ
5.15V
• 1.5
5.3V
V_RNG = 7.5 • V_DS
20
Verify that the calculated nominal T_J is less than the
assumed worst-case T_J in the bottom MOSFET:
P_BOT
=
28V – 1.2V
28V
(15A
)²
•
1.5
•
3.9mΩ
=
1.25W
T_J = 70°C + 1.25W • 40°C/W = 120°C
Because the top MOSFET is on for a short time, an
RJK0305DPB (R_DS(ON) = 10mΩ (nominal) 13mΩ (maxi-
mum), C_MILLER = Q_GD/10V = 150pF, V_BOOST = 5V, V_GS =
4.5V, V_MILLER = 3V, θ_JA = 40°C/W) is sufficient. Checking its
power dissipation at current limit with = ρ_100°C = 1.4:
P_TOP
=
1.2V
28V
(15A)²
•
1.4
•
13mΩ
+
(28V
)²
⎛
⎝⎜
15A
2
⎞
⎠⎟
(150pF
)
⎛
⎝⎜
2.5Ω
5V – 3V
+
1.2Ω
3V
⎞
⎠⎟
400kHz
= 0.18W + 0.58W = 0.76W
T_J = 70°C + 0.76W • 40°C/W = 100°C
The junction temperatures will be significantly less at
nominal current, but this analysis shows that careful
attention to heat sinking will be necessary.
Select C_IN to give an RMS current rating greater than 4A
at 85°C. The output capacitor C_OUT1 is chosen for a low
ESR of 4.5mΩ to minimize output voltage changes due to
3879f