LTC1778EGN Linear Technology, LTC1778EGN Datasheet - Page 17

• 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
Other losses, including C
conduction loss during dead time and inductor core loss
generally account for less than 2% additional loss.
When making adjustments to improve efficiency, the
input current is the best indicator of changes in efficiency.
If you make a change and the input current decreases, then
the efficiency has increased. If there is no change in input
current, then there is no change in efficiency.
Checking Transient Response
The regulator loop response can be checked by looking at
the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, V
equal to ∆I
resistance of C
discharge C
the regulator to return V
During this recovery time, V
overshoot or ringing that would indicate a stability prob-
lem. The I
will provide adequate compensation for most applica-
tions. For a detailed explanation of switching control loop
theory see Application Note 76.
Design Example
As a design example, take a supply with the following
specifications: V
±5%, I
timing resistor with V
and choose the inductor for about 40% ripple current at
the maximum V
R
L
ON
=
(
OUT(MAX)
=
250
TH
(
LOAD
0 7
OUT
kHz
.
pin external components shown in Figure 9
V
2 5
generating a feedback error signal used by
IN
)(
)( )( )
OUT
IN
(ESR), where ESR is the effective series
.
= 10A, f = 250kHz. First, calculate the
:
0 4 10
250
V
= 7V to 28V (15V nominal), V
.
U
2 5
. ∆I
OUT
.
ON
kHz
V
LOAD
immediately shifts by an amount
OUT
= V
A
U
)(
OUT
10
OUT
⎛
⎜
⎝
ESR loss, Schottky diode D1
1
also begins to charge or
OUT
pF
to its steady-state value.
−
:
)
2 5
28
can be monitored for
=
.
W
V
V
1 42
.
⎞
⎟ =
⎠
M
2 3
Ω
.
µ
U
OUT
H
= 2.5V
Selecting a standard value of 1.8µH results in a maximum
ripple current of:
Next, choose the synchronous MOSFET switch. Choosing
a Si4874 (R
θ
Tying V
for a nominal value of 110mV with current limit occurring
at 146mV. To check if the current limit is acceptable,
assume a junction temperature of about 80°C above a
70°C ambient with ρ
and double check the assumed T
Because the top MOSFET is on for such a short time, an
Si4884 R
40°C/W will be sufficient. Checking its power dissipation
at current limit with ρ
The junction temperatures will be significantly less at
nominal current, but this analysis shows that careful
attention to heat sinking will be necessary in this circuit.
JA
V
T
T
I
P
P
∆ =
LIMIT
J
J
= 40°C/W) yields a nominal sense voltage of:
SNS(NOM)
BOT
TOP
I
L
= 70°C + (1.97W)(40°C/W) = 149°C
= 70°C + (0.7W)(40°C/W) = 98°C
RNG
=
( )( ) ( )(
=
=
≥
1 7 28
(
DS(ON)(MAX)
250
.
0 30
2 5
28
28
to 1.1V will set the current sense voltage range
( )
.
.
1 5 0 010
DS(ON)
= (10A)(1.3)(0.0083Ω) = 108mV
V
.
V
V
kHz
28
146
W
– .
2 5
( ) ( )
(
12
V
.
V
2 5
+
)
.
mV
V
(
A
2
LTC1778/LTC1778-1
1 8
= 0.0083Ω (NOM) 0.010Ω (MAX),
0 40
150°C
V
.
100°C
12
.
2
= 0.0165Ω, C
( ) ( )
µ
Ω
12
1 4 0 0165
A
H
W
.
)
A
)
= 1.5:
+
= 1.4:
100
⎛
⎜
⎝
=
(
2
1
2
1
0 7
–
.
1 5 0 010
( )
.
pF
5 1
.
2 5
28
J
.
W
.
)(
in the MOSFET:
A
(
V
V
250
RSS
Ω
.
⎞
⎟ =
⎠
=
)
kHz
12
+
= 100pF, θ
5 1
Ω
A
.
)
)
A
=
1 97
.
17
W
JA
1778fb
=