LTC3770EUH#TR Linear Technology, LTC3770EUH#TR Datasheet - Page 13

• Manufacturer Part Number: LTC3770EUH#TR
• Manufacturer: Linear Technology

Specifications of LTC3770EUH#TR

• Lead Free Status / RoHS Status: Not Compliant

APPLICATIONS INFORMATION
When there is no R
the on-time t
damage the converter. To prevent this, the LTC3770 will
detect this fault condition and provide a minimum I
current of 5μA to 10μA.
Changes in the load current magnitude will cause frequency
shift. Parasitic resistance in the MOSFET switches and
inductor reduce the effective voltage across the inductance,
resulting in increased duty cycle as the load current
increases. By lengthening the on-time slightly as current
increases, constant frequency operation can be maintained.
This is accomplished with a resistive divider from the I
to the V
on the parasitic resistances in the specifi c application. A
good starting point is to feed about 25% of the voltage
change at the I
3a. Place capacitance on the V
variations at the switching frequency. The resistor load on
I
load regulation, which can be avoided by using the PNP
emitter follower of Figure 3b.
Minimum Off-Time and Dropout Operation
The minimum off-time t
time that the LTC3770 is capable of turning on the bottom
MOSFET, tripping the current comparator and turning the
Figure 3. Correcting Frequency Shift with Load Current Changes
TH
reduces the DC gain of the error amp and degrades
ON
INTV
pin and V
V
V
ON
OUT
OUT
CC
TH
is theoretically infi nite, which in turn could
2N5087
R
R
R
VON1
10k
pin to the V
VON1
30k
VON2
100k
3k
ON
Q1
OUT
resistor connected to the I
R
10k
OFF(MIN)
. The values required will depend
VON2
C
R
C
C
(3a)
(3b)
ON
ON
C
0.01μF
C
0.01μF
C
is the smallest amount of
VON
VON
C
R
pin as shown in Figure
C
pin to fi lter out the I
V
I
V
I
TH
TH
LTC3770
LTC3770
ON
ON
3770 F03
ON
TH
pin,
pin
ON
TH
MOSFET back off. This time is generally about 250ns for
large peak-to-peak inductor ripple current. In applications
where the peak-to-peak inductor ripple current is small,
the minimum off-time can approach 400ns. The minimum
off-time limit imposes a maximum duty cycle of t
+ t
to a dropping input voltage for example, then the output
will drop out of regulation. The minimum input voltage
to avoid dropout is:
A plot of Maximum Duty Cycle vs Frequency is shown in
Figure 4.
Inductor Selection
Given the desired input and output voltages, the inductor
value and operating frequency determine the ripple
current:
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Highest effi ciency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a tradeoff between
component size, effi ciency and operating frequency.
OFF(MIN)
ΔI
V
Figure 4. Maximum Switching Frequency vs Duty Cycle
IN MIN
L
(
=
⎛
⎜
⎜
⎝
V
)
). If the maximum duty cycle is reached, due
f L
=
OUT
2.0
1.5
1.0
0.5
V
0
OUT
0
⎞
⎟
⎟
⎠
⎛
⎜
⎝
1
−
t
ON
V
0.25
V
DUTY CYCLE (V
OUT
+
IN
t
t
ON
OFF MIN
⎞
⎟
⎠
0.50
(
OUT
)
DROPOUT
REGION
/V
IN
0.75
)
LTC3770
3770 F04
1.0
13
ON
/(t
3770fc
ON