LTC3703 Linear Technology, LTC3703 Datasheet - Page 27

• Manufacturer Part Number: LTC3703
• Description: 100V Synchronous Switching Regulator Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
watching the output. If this isn’t convenient, a current step
generator is needed. This generator needs to be able to
turn on and off in nanoseconds to simulate a typical
switching logic load, so stray inductance and long clip
leads between the LTC3703 and the transient generator
must be minimized.
Figure 19 shows an example of a simple transient genera-
tor. Be sure to use a noninductive resistor as the load
element—many power resistors use an inductive spiral
pattern and are not suitable for use here. A simple solution
is to take ten 1/4W film resistors and wire them in parallel
to get the desired value. This gives a noninductive resistive
load which can dissipate 2.5W continuously or 50W if
pulsed with a 5% duty cycle, enough for most LTC3703
circuits. Solder the MOSFET and the resistor(s) as close to
the output of the LTC3703 circuit as possible and set up
the signal generator to pulse at a 100Hz rate with a 5% duty
cycle. This pulses the LTC3703 with 500 s transients10ms
apart, adequate for viewing the entire transient recovery
time for both positive and negative transitions while keep-
ing the load resistor cool.
Design Example
As a design example, take a supply with the following
specifications: V
12V 5%, I
to give the 250kHz operating frequency:
Next, choose the inductor value for about 40% ripple
current at maximum V
R
L
SET
(
250
= 7100/(250-25) = 31.6k
OUT(MAX)
kHz
Figure 19. Transient Load Generator
DUTY CYCLE
LOCATE CLOSE TO THE OUTPUT
12
GENERATOR
100Hz, 5%
)( . )(
0V TO 10V
LTC3703
IN
PULSE
0 4 10
V
U
= 36V to 72V (48V nominal), V
= 10A, f=250kHz. First, calculate R
IN
:
A
U
)
50
1
–
12
72
R
IRFZ44 OR
EQUIVALENT
LOAD
W
V
OUT
10
3703 F19
H
U
OUT
SET
=
With 10 H inductor, ripple current will vary from 3.2A to
4A (32% to 40%) over the input supply range.
Next, verify that the minimum on-time is not violated. The
minimum on-time occurs at maximum V
which is above the LTC3703’s 200ns minimum on-time.
Next, choose the top and bottom MOSFET switch. Since
the drain of each MOSFET will see the full supply voltage
72V(max) plus any ringing, choose a 100V MOSFET to
provide a margin of safety. Si7456DP has a 100V BV
R
10nC)/50V = 180pF, V
The power dissipation can be estimated at maximum input
voltage, assuming a junction temperature of 100 C (30 C
above an ambient of 70 C):
And double check the assumed T
Since the synchronous MOSFET will be conducting over
twice as long each period (almost 100% of the period in
short circuit) as the top MOSFET, use two Si7456DP
MOSFETs on the bottom:
Next, set the current limit resistor. Since I
limit should be set such that the minimum current limit is
>10A. Minimum current limit occurs at maximum R
P
MAIN
DS(ON)
T
T
P
t
ON MIN
J
J
SYNC
= 70 C + (1.64W)(20 C/W) = 103 C
= 70 C + (1.74W)(20 C/W) = 105 C
(
= 25m (max), = 0.009/ C, C
( )
0 70
12
72
72
.
)
( )
2
0 025
10 1 0 009 100 25 0 025
W
72 12
.
V
2
10
IN MIN
72
2
2
V
(
OUT
0 94
( )(
.
2 180
)
( )
( )
1 74
GS(MILLER)
.
W
f
10 1 0 009 100 25
.
2
pF
(
W
72 250
1 64
.
)•
(
12
–
W
10 4 7
.
J
= 4.7V,
kHz
in the MOSFET:
– .
1
) ( .
(
)
LTC3703
MILLER
IN
MAX
667
:
–
JA
4 7
)
1
.
= 20 C/W.
= 10A, the
ns
= (19nC –
) •
27
(
DS(ON)
250
DSS
3703f
k
)
,
.