LTC3703_15 Datasheet, PDF (27/34 Pages) Linear Technology – 100V Synchronous Switching Regulator Controller

LTC3703
Applications Information
With most output capacitors, several devices paralleled
to get the ESR down will have so much capacitance that
this drop term is negligible. Ceramic capacitors are an
exception; a small ceramic capacitor can have suitably low
ESR with relatively small values of capacitance, making
this second drop term more significant.
Optimizing Loop Compensation
Loop compensation has a fundamental impact on transient
recovery time, the time it takes the LTC3703 to recover
after the output voltage has dropped due to a load step.
Optimizing loop compensation entails maintaining the high-
est possible loop bandwidth while ensuring loop stability.
The feedback component selection section describes in
detail the techniques used to design an optimized Type 3
feedback loop, appropriate for most LTC3703 systems.
Measurement Techniques
Measuring transient response presents a challenge in
two respects: obtaining an accurate measurement and
generating a suitable transient to test the circuit. Output
measurements should be taken with a scope probe directly
across the output capacitor. Proper high frequency probing
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 and the transient generator
must be minimized.
Figure 19 shows an example of a simple transient gen-
erator. 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
LTC3703
V_OUT
R_LOAD
PULSE
GENERATOR
IRFZ44 OR
EQUIVALENT
50Ω
0V TO 10V
100Hz, 5%
DUTY CYCLE
LOCATE CLOSE TO THE OUTPUT
3703 F19
Figure 19. Transient Load Generator
is to take ten 1/4W film resistors and wire them in parallel
to get the desired value. This gives a noninductive resis-
tive 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 500µs transients
10ms 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 = 36V to 72V (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
=
12V
(250kHz)(0.4)(10A)


1–
12
72


=
10µH
With 10µH 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_IN:
t_ON(MIN)
=
V_OUT
V_IN(MIN)(f)
=
12
72(250kHz)
=
667ns
which is above the LTC3703’s 200ns minimum on-time.
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