LT1374IS8-SYNC Linear Technology, LT1374IS8-SYNC Datasheet - Page 22

• Manufacturer Part Number: LT1374IS8-SYNC
• Description: IC SW REG 4.5A ADJ STP-DWN 8SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT1374IS8-SYNC

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 2.42 ~ 21.5 V
• Current - Output: 4.5A
• Frequency - Switching: 500kHz
• Voltage - Input: 5 ~ 25 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

LT1374
APPLICATIONS
Error amplifier transconductance phase and gain are
shown in Figure 11. The error amplifier can be modeled
as a transconductance of 2000µMho, with an output
impedance of 200kΩ in parallel with 12pF. In all practical
applications, the compensation network from V
ground has a much lower impedance than the output
impedance of the amplifier at frequencies above 500Hz.
This means that the error amplifier characteristics them-
selves do not contribute excess phase shift to the loop, and
the phase/gain characteristics of the error amplifier sec-
tion are completely controlled by the external compensa-
tion network.
In Figure 12, full loop phase/gain characteristics are
shown with a compensation capacitor of 1.5nF, giving the
error amplifier a pole at 530Hz, with phase rolling off to 90°
22
LT1374
GND
CURRENT MODE
POWER STAGE
C
Figure 10. Response from V
g
F
–20
–40
m
40
20
= 5.3A/V
0
Figure 9. Model for Loop Response
10
V
R
C
C
C
C
100
U
PHASE
GAIN
AMPLIFIER
FREQUENCY (Hz)
ERROR
–
+
INFORMATION
1k
U
2.42V
V
SW
FB
10k
C
W
V
V
I
100k
OUT
Pin to Output
IN
OUT
= 10V
= 2A
= 5V
1374 F10
1M
R1
R2
40
0
–40
–80
–120
+
U
ESR
C1
C
OUTPUT
pin to
1374 F09
and staying there. The overall loop has a gain of 74dB at
low frequency, rolling off to unity-gain at 100kHz. Phase
shows a two-pole characteristic until the ESR of the output
capacitor brings it back above 10kHz. Phase margin is
about 75° at unity-gain.
Analog experts will note that around 4.4kHz, phase dips
very close to the zero phase margin line. This is typical of
switching regulators, especially those that operate over a
wide range of loads. This region of low phase is not a
problem as long as it does not occur near unity-gain. In
practice, the variability of output capacitor ESR tends to
dominate all other effects with respect to loop response.
Variations in ESR will cause unity-gain to move around,
but at the same time phase moves with it so that adequate
phase margin is maintained over a very wide range of ESR
(≥ ±3:1).
Figure 11. Error Amplifier Gain and Phase
3000
2500
2000
1500
1000
500
–20
Figure 12. Overall Loop Characteristics
80
60
40
20
0
100
10
V
R
V
V
C
C
ERROR AMPLIFIER EQUIVALENT CIRCUIT
FB 2 × 10
IN
OUT
OUT
C
LOAD
= 1.5nF, R
(
= 10V
= 5V, I
= 100µF, 10V, AVX TPS
100
1k
= 50Ω
–3
)
OUT
C
FREQUENCY (Hz)
FREQUENCY (Hz)
PHASE
= 0, L = 10µH
GAIN
10k
= 2A
1k
GAIN
R
200k
OUT
100k
10k
100k
1M
C
12pF
PHASE
OUT
V
C
1374 F11
1374 F12
10M
1M
200
150
100
50
0
–50
200
150
100
50
0
–50
1374fd