Datasheet

LTC3813
28
3813fb
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
OUT
immediately shifts by an amount
equal to ΔI
LOAD
(ESR), where ESR is the effective series
resistance of C
OUT
. ΔI
LOAD
also begins to charge or dis-
charge 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 speci-
fi cations: V
IN
= 12V± 20%, V
OUT
= 24V ±5%, I
OUT(MAX)
=
5A, f = 250kHz. Since V
IN
can vary around the 12V nominal
value, connect a resistive divider from V
IN
to V
OFF
to keep
the frequency independent of V
IN
changes:
R1
R
2
=
12V
1.55V
 1= 6.74
Choose R1 = 133k and R2 = 20k. Now calculate timing
resistor R
OFF
:
R
OFF
=
1+ 133k / 20k
250kHz • 76pF
= 402.6k
The duty cycle is:
D= 1
12V
24V
= 0.5
and the maximum input current is:
I
IN(MAX)
=
5A
1 0.5
= 10A
Choose the inductor for about 40% ripple current at the
maximum V
IN
:
L =
12V
250kHz • 0.4 • 10A
1
12V
24V






= 6μH
The peak inductor current is:
I
L(PEAK)
=
5A
1 0.5
+
1
2
(4A)= 12A
Choose the CDEP147 5.9μH inductor with I
SAT
= 16.4A
at 100°C.
Next, choose the bottom MOSFET switch. Since the drain
of the MOSFET will see the full output voltage plus any
ringing, choose a 40V MOSFET to provide a margin of
safety. The Si7848DP has:
BV
DSS
= 40V
R
DS(ON)
= 9mΩ(max)/7.5mΩ(nom),
δ
= 0.006/°C,
C
MILLER
= (14nC – 6nC)/20V = 400pF,
V
GS(MILLER)
= 3.5V,
θ
JA
= 20°C/W.
This yields a nominal sense voltage of:
V
SNS(NOM)
=
1.7 • 0.0075 •5A
1 0.5
= 128mV
To guarantee proper current limit at worst-case conditions,
increase nominal V
SNS
by 50% to 190mV. To check if the
current limit is acceptable at V
SNS
= 190mV, assume a
junction temperature of about 30°C above a 70°C ambient
(ρ
100°C
= 1.4):
I
IN(MAX)

190mV
1.4 • 0.009

1
2
•4A= 13A
I
OUT(MAX)
= I
IN(MAX)
• (1-D
MAX
) = 6.5A
and double-check the assumed T
J
in the MOSFET:
P
TOP
=
1
1 0.5






6.5A
()
2
(1.4)(0.009)= 1.06W
T
J
= 70°C + 1.06W • 20°C/W = 91°C
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