Datasheet

20
LTC3704
3704fb
APPLICATIO S I FOR ATIO
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regulator feedback loop acts on the resulting error amp
output signal to return V
O
to its steady-state value. During
this recovery time, V
O
can be monitored for overshoot or
ringing that would indicate a stability problem.
A second, more severe transient can occur when connect-
ing loads with large (> 1μF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C
O
, causing a nearly instantaneous drop in V
O
. No
regulator can deliver enough current to prevent this prob-
lem if the load switch resistance is low and it is driven
quickly. The only solution is to limit the rise time of the
switch drive in order to limit the inrush current di/dt to the
load.
Design Example: A 5V to 15V Input, –5V at 2A Output
Positive-to-Negative Converter
The design example presented here will be for the circuit
shown in Figure 1. The input voltage range is 5V to 15V,
and the output is -5V. The maximum load current is 2A at
an input voltage of 5V (3A peak), and 3A at an input voltage
of 15V (5A peak).
1. The maximum duty cycle of the main switch is:
D
V
VV
MAX
OUT
OUT IN MIN
=
−
==
()
–
–
%
5
10
50
2. Pulse-Skip operation is chosen, so the MODE/SYNC pin
is connected to the INTV
CC
pin.
3. The operating frequency is chosen to be 300kHz to
reduce the size of the inductors. From Figure 5, the resistor
from the FREQ pin to ground is 80.6k.
4. A total inductor ripple current of 40% of the maximum
is chosen, so the inductor ripple current is:
Δ = −
Δ ==
II
D
D
IA
LOMAX
MAX
MAX
L
1
1
1
04 20
05
105
08
χ ••
–
.•.•
.
–.
.
()
For a standard 1:1 coupled inductor, the inductance is
therefore:
LL
V
If
D
k
H
IN MIN
L
MAX
12
2
5
208300
05 52
1
==
Δ
==μ
()
••
•
•.•
•. .
The minimum saturation current for this inductor is:
II
D
A
LSAT MIN O MAX
MAX
() ( )
–••
–
.•.•
–.
.
≥ +
⎛
⎝
⎜
⎞
⎠
⎟
==
1
2
1
1
12 20
1
105
48
χ
The inductor chosen is a BH Electronics part # 510-1009,
which has an open circuit parallel inductance of 4.56μH
and a maximum dc current rating of 6.5A.
5. For the power MOSFET,
RV
D
I
DS ON SENSE MAX
MAX
OMAX
() ( )
()
•
–
••
≤
+
⎛
⎝
⎜
⎞
⎠
⎟
1
1
2
χ
ρ
Τ
At the maximum duty cycle of 50%, the maximum SENSE
pin voltage is reduced to 130mV due to slope compensa-
tion, as shown in Figure 11. Assuming a maximum
junction temperature of 125°C for the power MOSFET,
ρ
Τ
= 1.5, and
Rm
DS ON()
.•
.–
–. • . •.
.≤ = Ω0 130
05 1
12 20 15
18 1
The MOSFET chosen was Siliconix/Vishay’s Si4884, which
has a maximum R
DS(ON)
= 16.5mΩ at V
GS
= 4.5V at 25°C.
The minimum BV
DSS
= 30V and the maximum gate charge
is Q
G
= 20nC.
6. The output diode must withstand a reverse voltage of
V
IN(MAX)
– V
O
= 20V and a continuous current of
I
O(MAX)
= 5.0A (peak output current at V
IN
= 15V). The peak
current in the diode is:
IIA
D PEAK O MAX() ()
•=+
⎛
⎝
⎜
⎞
⎠
⎟
=1
2
6
χ
The power dissipated in this diode at full load is: