LTC3816_15 Datasheet, PDF (35/44 Pages) Linear Technology – Single-Phase Wide VIN Range DC/DC Controller for Intel IMVP-6/IMVP-6.5 CPUs

LTC3816
APPLICATIONS INFORMATION
Conveniently, the typical probe tip ground clip is spaced
just right to span the leads of a typical output capacitor. In
general, it is best to take this measurement with the 20MHz
bandwidth limit on the oscilloscope turned on to limit high
frequency noise. Note that microprocessor manufacturers
typically specify ripple ≤20MHz, as energy above 20MHz
is generally radiated and not conducted and will not affect
the load even if it appears at the 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 LTC3816 and the transient generator
must be minimized.
Figure 20 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
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 LTC3816
circuits. Solder the MOSFET and the resistor(s) as close
to the output of the LTC3816 circuit as possible and set
up the signal generator to pulse at a 100Hz rate with a 5%
duty cycle. This pulses the LTC3816 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.
A Design Example
As a design example, consider an IMVP-6.5 application
with inductor DCR current sense (see the last page
schematic) and the following requirements: assume V_IN
= 12V (nominal), V_IN = 24V (maximum), V_OUT = 0.75V,
V_OUT (minimum) = 0.725V, I_LOAD(MAX) = 27A, I_LOAD(MIN)
= 1.5A, AVP = –3mV/A, f_OSC = 400kHz, V_IMON = 1.0V.
For the input and output conditions given above, the
steady-state minimum on-time for this application at
V_IN = 24V is approximately:
t_ON(MIN)
=
V_OUT(MIN)
V_IN(MAX) • f_OSC
=
0.725V
24V • 400kHz
=
75.5ns
This is much longer than the LTC3816 minimum on-time.
To program the 400kHz operation, float the RFREQ pin.
The inductance value is chosen first based on a 20% ripple
current assumption. The highest value of ripple current
occurs at the maximum input voltage:
L
=
f_OUT
V_OUT
• ∆I_L(MAX)

 ^1–

V_OUT
V_IN(MAX)



=
0.75V
400kHz • 0.2 • 27A


1–
0.75V
24V


=
0.33µH
A commonly available 0.33µH inductor is chosen. This
results in 5.5A of ripple current. The peak inductor cur-
rent is the maximum DC load current plus one-half the
ripple current, or:
I_L(PEAK)
=
27A
+
1
2
•
5.5A
=
29.75A
LTC3816
PULSE
GENERATOR
50Ω
10k
V_OUT
R_LOAD
RENESAS RJK0305DPB
OR EQUIVALENT
0V TO 10V
100Hz, 1% TO 5%
DUTY CYCLE
LOCATE CLOSE TO THE OUTPUT
3816 F20
Figure 20. Transient Load Generator PC Board
3816f
35