LT1376CS Linear Technology, LT1376CS Datasheet - Page 20

• Manufacturer Part Number: LT1376CS
• Description: IC SW REG 1.5A ADJ STP-DWN16SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT1376CS

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 2.42 ~ 21.5 V
• Current - Output: 1.5A
• Frequency - Switching: 500kHz
• Voltage - Input: 5 ~ 25 V
• Operating Temperature: 0°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

LT1375/LT1376
APPLICATIONS
introduce multiple poles into the feedback loop. The
inductor and output capacitor on a conventional step-
down converter actually form a resonant tank circuit that
can exhibit peaking and a rapid 180° phase shift at the
resonant frequency. By contrast, the LT1376 uses a “cur-
rent mode” architecture to help alleviate phase shift cre-
ated by the inductor. The basic connections are shown in
Figure 12. Figure 13 shows a Bode plot of the phase and
gain of the power section of the LT1376, measured from
the V
ductance of the LT1376 power section and the effective
complex impedance from output to ground. Gain rolls off
smoothly above the 100Hz pole frequency set by the
100µF output capacitor. Phase drop is limited to about
85°. Phase recovers and gain levels off at the zero fre-
quency (≈16kHz) set by capacitor ESR (0.1Ω).
20
GND
LT1375
LT1376
C
pin to the output. Gain is set by the 2A/V transcon-
CURRENT MODE
POWER STAGE
C
Figure 13. Response from V
F
g
m
–20
–40
40
20
= 2A/V
Figure 12. Model for Loop Response
0
V
10
R
C
C
C
C
100
U
AMPLIFIER
GAIN
PHASE
ERROR
–
+
FREQUENCY (Hz)
INFORMATION
1k
U
2.42V
V
SW
10k
FB
V
V
I
OUT
IN
OUT
C
= 10V
W
= 500mA
Pin to Output
= 5V
100k
1375/76 F13
1M
40
0
–40
–80
–120
R1
R2
U
+
ESR
C1
OUTPUT
1375/76 F12
Error amplifier transconductance phase and gain are shown
in Figure 14. The error amplifier can be modeled as a
transconductance of 2000µMho, with an output imped-
ance 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 15, full loop phase/gain characteristics are
shown with a compensation capacitor of 0.0033µF, giving
the error amplifier a pole at 240Hz, with phase rolling off
to 90° and staying there. The overall loop has a gain of
Figure 14. Error Amplifier Gain and Phase
3000
2500
2000
1500
1000
–20
Figure 15. Overall Loop Characteristics
500
80
60
40
20
0
100
10
V
V
C
C
V
R
IN
OUT
OUT
C
ERROR AMPLIFIER EQUIVALENT CIRCUIT
FB 2 • 10
LOAD
= 3.3nF, R
= 10V
(
= 5V, I
= 100µF, 10V, AVX TPS
100
1k
= 50Ω
–3
OUT
)
GAIN
C
FREQUENCY (Hz)
FREQUENCY (Hz)
= 0, L = 10µH
PHASE
GAIN
= 500mA
10k
1k
R
200k
OUT
100k
10k
PHASE
100k
1M
C
12pF
OUT
1375/76 F15
1375/76 F14
V
C
10M
1M
200
150
100
50
0
–50
200
150
100
50
0
–50
C
pin to
13756fd