LTC3810-5
31
38105fd
When load step occurs, V
OUT
immediately shifts by an
amount equal to
D
I
LOAD
(ESR), where ESR is the effective
series resistance of C
OUT
.
D
I
LOAD
also begins to charge or
discharge C
OUT
generating a feedback error signal used by
the regulator to return V
OUT
to its steady-state value. During
this recovery time, V
OUT
can be monitored for overshoot
or ringing that would indicate a stability problem.
Design Example
As a design example, take a supply with the following
specifications: V
IN
= 12V to 60V, V
OUT
= 5V ±5%, I
OUT(MAX)
= 6A, f = 250kHz. First, calculate the timing resistor:
R
ON
=
5V
2.4V • 250kHz • 76pF
=
110k
and choose the inductor for about 40% ripple current at
the maximum V
IN
:
L
=
5V
250kHz • 0.4 • 6A
1
−
5V
60V
⎛
⎝⎜
⎞
⎠⎟
=
7.6
µ
H
With a 7.7µH inductor, ripple current will vary from 1.5A
to 2.4A (25% to 40%) over the input supply range.
Next, choose the bottom MOSFET switch. Since the
drain of the MOSFET will see the full supply voltage 60V
(max) plus any ringing, choose an 60V MOSFET. The
Si7850DP has:
BV
DSS
= 60V
R
DS(ON)
= 31mΩ
(max)/25mΩ
(nom),
δ
= 0.007/°C,
C
MILLER
= (8.3nC – 2.8nC)/30V = 183pF,
V
GS(MILLER)
= 3.8V,
θ
JA
= 22°C/W.
This yields a nominal sense voltage of:
V
SNS(NOM)
= 6A • 1.3 • 0.025Ω
=
195mV
To guarantee proper current limit at worst-case conditions,
increase nominal V
SNS
by at least 50% to 320mV (by tying
V
RNG
to 2V). To check if the current limit is acceptable at
V
SNS
= 320mV, assume a junction temperature of about
55°C above a 70°C ambient (
ρ
125°C
= 1.7):
I
LIMIT
≥
320mV
1.7 • 0.031
Ω
+
1
2
• 2.4A
=
7.3A
and double-check the assumed T
J
in the MOSFET:
P
BOT
=
60V
−
5V
60V
• 7.3A
2
• 1.7 • 0.031
Ω =
2.6W
T
J
= 70°C + 2.6W • 22°C/W = 127°C
Verify that the Si7850DP is also a good choice for the
top MOSFET by checking its power dissipation at current
limit and maximum input voltage, assuming a junction
temperature of 30°C above a 70°C ambient (
ρ
100°C
= 1.5):
P
MAIN
=
5V
60V
• 7.3A
2
1.5 • 0.031
Ω
(
)
+
60V
2
•
7.3A
2
• 2
Ω
• 183pF •
1
5V
−
3.8V
+
1
3.8V
⎛
⎝⎜
⎞
⎠⎟
• 250kHz
=
0.206W
+
1.32W
=
1.53W
T
J
= 70°C + 1.53W • 22°C/W = 104°C
The junction temperature will be significantly less at nomi-
nal current, but this analysis shows that careful attention to
heat sinking on the board will be necessary in this circuit.
Since V
OUT
> 4.7V, the INTV
CC
voltage can be generated
from V
OUT
with the internal LDO by connecting V
OUT
to
the EXTV
CC
pin. A small SOT23 MOSFET such as the
ZXMN10A07F can be used for the pass device if fault
timeout is enabled. Choose R
NDRV
to guarantee that fault
timeout is enabled when power dissipation of M3 exceeds
0.4W (max for 70°C ambient):
I
CC
= 250kHz • 2 • 18nC + 3mA = 12mA
R
NDRV
≤
0.4W / 0.012A – 3V
270µA
=
112k
So, choose R
NDRV
= 100k
.
applicaTions inForMaTion
