LTC3786EUD#PBF Linear Technology, LTC3786EUD#PBF Datasheet - Page 21

• Manufacturer Part Number: LTC3786EUD#PBF
• Description: IC, DC-DC CONV, 550kHz, QFN16
• Manufacturer: Linear Technology

Specifications of LTC3786EUD#PBF

• Primary Input Voltage: 38V
• No. Of Outputs: 1
• Output Voltage: 60V
• No. Of Pins: 16
• Operating Temperature Range: -40°C To +125°C
• Peak Reflow Compatible (260 C): Yes
• Switching Frequency Max: 550kHz
• Msl: MSL 1 - Unlimited
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIONS INFORMATION
The ITH series R
loop compensation. The values can be modified slightly
to optimize transient response once the final PCB layout
is complete and the particular output capacitor type and
value have been determined. The output capacitors must
be selected because the various types and values determine
the loop gain and phase. An output current pulse of 20%
to 80% of full-load current having a rise time of 1μs to
10μs will produce output voltage and ITH pin waveforms
that will give a sense of the overall loop stability without
breaking the feedback loop.
Placing a power MOSFET and load resistor directly
across the output capacitor and driving the gate with an
appropriate pulse generator is a practical way to produce
a realistic load step condition. The initial output voltage
step resulting from the step change in output current may
not be within the bandwidth of the feedback loop, so this
signal cannot be used to determine phase margin. This
is why it is better to look at the ITH pin signal which is
in the feedback loop and is the filtered and compensated
control loop response.
The gain of the loop will be increased by increasing
R
decreasing C
C
thereby keeping the phase shift the same in the most
critical frequency range of the feedback loop. The output
voltage settling behavior is related to the stability of the
closed-loop system and will demonstrate the actual overall
supply performance.
A second, more severe transient is caused by switching
in loads with large (>1μF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C
alter its delivery of current quickly enough to prevent this
sudden step change in output voltage if the load switch
resistance is low and it is driven quickly. If the ratio of
C
should be controlled so that the load rise time is limited to
approximately 25 • C
C
LOAD
C
is decreased, the zero frequency will be kept the same,
and the bandwidth of the loop will be increased by
OUT
to C
, causing a rapid drop in V
OUT
C
. If R
is greater than 1:50, the switch rise time
C
-C
C
C
is increased by the same factor that
LOAD
filter sets the dominant pole-zero
. Thus, a 10μF capacitor would
OUT
. No regulator can
require a 250μs rise time, limiting the charging current
to about 200mA.
Design Example
As a design example, assume V
V
= 75mV and f = 350kHz.
The inductance value is chosen first based on a 30% ripple
current assumption. Tie the MODE/PLLIN pin to GND,
generating 350kHz operation. The minimum inductance
for 30% ripple current is:
The largest ripple happens when V
where the average maximum inductor is I
(V
current. The peak inductor current will be the maximum
DC value plus one-half the ripple current, or 9.25A.
The R
maximum current sense voltage specification with some
accommodation for tolerances:
Choosing 1% resistors: R
output voltage of 24.072V.
The power dissipation on the topside MOSFET in each chan-
nel can be easily estimated. Choosing a Vishay Si7848BDP
MOSFET results in: R
maximum input voltage with T(estimated) = 50°C:
IN
P
OUT
R
ΔI
MAIN
= 22V (max), V
SENSE
L
/V
SENSE
=
IN
=
f • L
) = 8A. A 6.8μH inductor will produce a 31% ripple
+ 1.7
• 1+ 0.005
V
(
≤
⎡ ⎣
IN
(
24V – 12V
resistor value can be calculated by using the
75mV
9.25A
⎛
⎜
⎝
1–
(
(
)
12V
(
24V
OUT
V
V
OUT
= 0.008Ω
IN
DS(ON)
)
2
)
)
= 24V, I
)
3
(
24V
⎞
⎟
⎠
50°C – 25°C
12V
4A
A
= 5k and R
= 0.012Ω, C
• 4A
OUT(MAX)
(
( )
150pF
2
IN
IN
)
)
⎤ ⎦ • 0.008Ω
(
B
= 4A, V
350kHz
= 1/2V
= 12V(nominal),
MILLER
LTC3786
= 95.3k yields an
MAX
= I
SENSE(MAX)
OUT
= 150pF . At
OUT(MAX)
)
= 0.7W
21
= 12V,
3786f
•