LTC1778EGN Linear Technology, LTC1778EGN Datasheet - Page 12

• Manufacturer Part Number: LTC1778EGN
• Description: IC CONTROLLR STP-DWN SYNC 16SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1778EGN

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Current - Output: 2A
• Voltage - Input: 4 ~ 36 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Output: -
• Power - Output: -
• Frequency - Switching: -

APPLICATIO S I FOR ATIO
LTC1778/LTC1778-1
Because the voltage at the I
current into this pin is not exactly inversely proportional to
V
To correct for this error, an additional resistor R
connected from the I
further stabilize the frequency.
Changes in the load current magnitude will also 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
required will depend on the parasitic resistances in the
specific application. A good starting point is to feed about
25% of the voltage change at the I
shown in Figure 4a. Place capacitance on the V
filter out the I
resistor load on I
and degrades load regulation, which can be avoided by
using the PNP emitter follower of Figure 4b.
Minimum Off-time and Dropout Operation
The minimum off-time t
time that the LTC1778 is capable of turning on the bottom
MOSFET, tripping the current comparator and turning the
MOSFET back off. This time is generally about 250ns. The
minimum off-time limit imposes a maximum duty cycle of
t
12
ON
IN
R
/(t
, especially in applications with lower input voltages.
ON
ON
2
+ t
=
TH
OFF(MIN)
0 7
5
.
TH
V
pin to the V
V
variations at the switching frequency. The
V
R
OUT
TH
ON
). If the maximum duty cycle is reached,
U
reduces the DC gain of the error amp
ON
R
R
VON1
30k
VON2
100k
OFF(MIN)
pin to the 5V INTV
U
ON
C
R
C
ON
pin and V
C
Figure 4. Correcting Frequency Shift with Load Current Changes
(4a)
is the smallest amount of
pin is about 0.7V, the
TH
W
pin to the V
C
0.01µF
VON
OUT
V
I
CC
TH
LTC1778
ON
. The values
supply will
U
ON
ON
pin as
pin to
ON2
due 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 5.
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
V
∆ =
Figure 5. Maximum Switching Frequency vs Duty Cycle
IN MIN
I
L
INTV
(
V
OUT
CC
⎛
⎜
⎝
)
2.0
1.5
1.0
0.5
V
0
=
2N5087
OUT
f L
R
0
VON1
10k
3k
V
OUT
Q1
⎞
⎟
⎠
⎛
⎜
⎝
R
10k
1
0.25
VON2
t
ON
−
DUTY CYCLE (V
V
(4b)
+
V
OUT
IN
t
t
ON
OFF MIN
0.50
C
C
0.01µF
⎞
⎟
⎠
VON
C
R
(
C
OUT
DROPOUT
REGION
/V
)
V
I
IN
TH
0.75
LTC1778
ON
)
1778 F04
1778 F05
1.0
1778fb