Datasheet

LTC3814-5
24
38145fc
about 3mA and driver current can be calculated by:
I
GATE
= f(Q
G(TOP)
+ Q
G(BOT)
), where Q
G(TOP)
and Q
G(BOT)
are the gate charges of the top and bottom MOSFETs.
This loss is proportional to the supply voltage that
INTV
CC
is derived from, i.e., V
IN
, V
OUT
or an external
supply connected to INTV
CC
.
4. C
OUT
loss. The output capacitor has the diffi cult job
of fi l
tering the large RMS input current out of the synchro-
nous MOSFET. It must
have a very low ESR to minimize
the AC I
2
R loss .
Other losses, including C
IN
ESR loss, Schottky diode D1
conduction loss during dead time and inductor core loss
generally account for less than 2% additional loss. When
making adjustments to improve effi ciency, the input cur-
rent is the best indicator of changes in effi ciency. If you
make a change and the input current decreases, then the
effi ciency has increased. If there is no change in input
current, then there is no change in effi ciency.
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 ∆I
LOAD
(ESR), where ESR is the effective series
resistance of C
OUT
. ∆I
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
R2
=
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
so, 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 • 0.0075Ω •5A
1− 0.5
=128mV
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