LTC3810EG LINER [Linear Technology], LTC3810EG Datasheet - Page 32

• Manufacturer Part Number: LTC3810EG
• Description: 100V Current Mode Synchronous Switching Regulator Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC3810
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
load step occurs, V
equal to ΔI
resistance of C
charge C
regulator to return V
this recovery time, V
or ringing that would indicate a stability problem.
Design Example
As a design example, take a supply with the following
specifi cations: V
12V ±5%, I
timing resistor with V
and choose the inductor for about 40% ripple current at
the maximum V
With a 10μH inductor, ripple current will vary from 3.2A
to 4A (32% 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 72V(max)
plus any ringing, choose an 80V MOSFET to provide a
margin of safety. The Si7852DP has:
This yields a nominal sense voltage of:
32
BV
R
δ = 0.007/°C,
C
V
θ
V
R
L =
JA
MILLER
GS(MILLER)
SNS(NOM)
DS(ON)
ON
DSS
= 20°C/W.
250kHz • 0.4 • 10A
=
OUT
= 80V
2.4V • 250kHz • 76pF
OUT(MAX)
LOAD
= 16.5mΩ(max)/13.5mΩ(nom),
= (18.5nC – 7nC)/40V = 288pF ,
generating a feedback error signal used by the
= 10A • 1.3 • 0.0135Ω = 176mV
12V
= 4.7V,
OUT
IN
(ESR), where ESR is the effective series
IN
:
. ΔI
12V
= 36V to 72V (48V nominal), V
OUT
= 10A, f = 250kHz. First, calculate the
OUT
OUT
ON
LOAD
immediately shifts by an amount
to its steady-state value. During
can be monitored for overshoot
= INTV
also begins to charge or dis-
1
12V
72V
CC
= 263k
:
= 10μH
OUT
=
To guarantee proper current limit at worst-case conditions,
increase nominal V
V
V
80°C above a 70°C ambient (ρ
and double-check the assumed T
Verify that the Si7852DP is also a good choice for the top
MOSFET by checking its power dissipation at current limit
and maximum input voltage, assuming a junction tempera-
ture of 50°C above a 70°C ambient (ρ
The junction temperature will be signifi cantly less at
nominal current, but this analysis shows that careful at-
tention to heat sinking on the board will be necessary in
this circuit.
Since V
generated from V
V
the ZXMN10A07F can be used for the pass device if fault
timeout is enabled. Choose R
timeout is enabled when power dissipation of M3 exceeds
0.4W (max for 70°C ambient). Calculate power dissipation
at V
RNG
SNS
OUT
T
T
I
P
I
P
P
+ 72V
•
= 0.64W + 1.75W = 2.39W
CC
LIMIT
J
J
IN(MIN)
BOT
MAIN
M3
= 70°C + 3.8W • 20°C/W = 146°C
= 70°C + 2.39W • 20°C/W = 118°C
to the EXTV
= 320mV, assume a junction temperature of about
to 2V). To check if the current limit is acceptable at
10V 4.7V
= 250kHz • 2 • 34nC + 3mA = 20mA
= (36V – 10V)(0.02A) = 0.52W
OUT
=
=
2
72V 12V
2 • 0.0165
•
= 36V:
1
12V
72V
11.7A
> 6.7V, the INTV
320mV
72V
2
• 11.7A
CC
OUT
+
SNS
• 2 • 288pF
pin. A small SOT23 MOSFET such as
4.7V
with the internal LDO by connecting
• 11.7A
1
by at least 50% to 320mV (by tying
2
+
(
2
1
1.7 • 0.0165
• 250kHz
• 4A = 11.7A
NDRV
2
CC
150°C
• 2 • 0.0165 = 3.8W
/DRV
J
to guarantee that fault
in the MOSFET:
= 2):
CC
120°C
voltage can be
)
= 1.7):
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