LTC3703-5
27
37035fa
Measurement Techniques
Measuring transient response presents a challenge in two
respects: obtaining an accurate measurement and gener-
ating a suitable transient to test the circuit. Output mea-
surements should be taken with a scope probe directly
across the output capacitor. Proper high frequency prob-
ing techniques should be used. In particular, don’t use the
6" ground lead that comes with the probe! Use an adapter
that fits on the tip of the probe and has a short ground clip
to ensure that inductance in the ground path doesn’t cause
a bigger spike than the transient signal being measured.
Conveniently, the typical probe tip ground clip is spaced
just right to span the leads of a typical output capacitor.
Now that we know how to measure the signal, we need to
have something to measure. The ideal situation is to use
the actual load for the test and switch it on and off while
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 LTC3703-5 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 LTC3703-5
circuits. Solder the MOSFET and the resistor(s) as close to
the output of the LTC3703-5 circuit as possible and set up
the signal generator to pulse at a 100Hz rate with a 5% duty
cycle. This pulses the LTC3703-5 with 500
µ
s
transients10ms apart, adequate for viewing the entire
transient recovery time for both positive and negative
transitions while keeping the load resistor cool.
Design Example
As a design example, take a supply with the following
specifications: V
IN
= 20V to 60V (48V nominal), V
OUT
=
12V
±
5%, I
OUT(MAX)
= 10A, f=250kHz. First, calculate R
SET
to give the 250kHz operating frequency:
R
SET
= 7100/(250-25) = 31.6k
Next, choose the inductor value for about 40% ripple
current at maximum V
IN
:
L
V
kHz
A
H
=
⎛
⎝⎜
⎞
⎠⎟
= µ
12
250
0 4 10
1
12
60
10
(
)( . )(
)
–
With 10
µ
H inductor, ripple current will vary from 1.9A to
3.8A (19% to 38%) over the input supply range.
Next, verify that the minimum on-time is not violated. The
minimum on-time occurs at maximum V
IN
:
t
V
V
f
kHz
ns
ON MIN
OUT
IN MIN
(
)
(
)
( )
(
)
=
=
=
12
60 250
800
which is above the LTC3703-5’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
60V(max) plus any ringing, choose a 60V MOSFET.
Si7850DP has a 60V BV
DSS
, R
DS(ON)
= 22m
Ω
(max),
δ
=
0.007/
°
C, C
MILLER
= (9nC – 3nC)/30V = 200pF, V
GS(MILLER)
= 3.8V,
θ
JA
= 20
°
C/W. The power dissipation can be
APPLICATIO S I FOR ATIO
W
U
U
U
Figure 19. Transient Load Generator
LTC3703-5
V
OUT
IRFZ44 OR
EQUIVALENT
R
LOAD
50
Ω
0V TO 10V
100Hz, 5%
DUTY CYCLE
LOCATE CLOSE TO THE OUTPUT
37035 F19
PULSE
GENERATOR
