ltc3770 Linear Technology Corporation, ltc3770 Datasheet - Page 13

• Manufacturer Part Number: ltc3770
• Description: Fast No Rsense Step-down Synchronous Controller With Margining, Tracking And Pll
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
When there is no R
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 fre-
quency shift. Parasitic resistance in the MOSFET switches
and inductor reduce the effective voltage across the induc-
tance, resulting in increased duty cycle as the load current
increases. By lengthening the on-time slightly as current
increases, constant frequency operation can be main-
tained. This is accomplished with a resistive divider from
the I
depend on the parasitic resistances in the specific applica-
tion. A good starting point is to feed about 25% of the
voltage change at the I
Figure 3a. Place capacitance on the V
I
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
variations at the switching frequency. The resistor load
TH
TH
reduces the DC gain of the error amp and degrades
pin to the V
ON
INTV
V
V
OUT
OUT
is theoretically infinite, which in turn could
CC
2N5087
R
R
R
ON
10k
30k
VON2
100k
VON1
VON1
3k
ON
U
Q1
pin and V
resistor connected to the I
R
10k
TH
OFF(MIN)
VON2
C
R
C
C
U
pin to the V
(3a)
(3b)
OUT
C
0.01µF
C
0.01µF
C
is the smallest amount of
VON
VON
C
R
. The values required will
C
W
ON
V
I
V
I
ON
TH
TH
LTC3770
LTC3770
ON
ON
pin to filter out the
pin as shown in
3770 F03
ON
U
pin, the
ON
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
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 efficiency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a tradeoff between
component size, efficiency and operating frequency.
OFF(MIN)
V
∆ =
IN MIN
Figure 4. Maximum Switching Frequency vs Duty Cycle
I
L
(
⎛
⎜
⎝
)
). If the maximum duty cycle is reached, due to
2.0
1.5
1.0
0.5
V
=
0
OUT
f L
0
V
OUT
⎞
⎟
⎠
⎛
⎜
⎝
1
0.25
t
DUTY CYCLE (V
ON
−
V
+
V
OUT
IN
t
t
ON
OFF MIN
0.50
⎞
⎟
⎠
(
OUT
DROPOUT
REGION
/V
IN
0.75
)
)
3770 F04
LTC3770
1.0
13
ON
/(t
3770fb
ON