LT1375HV Linear Technology, LT1375HV Datasheet - Page 20

• Manufacturer Part Number: LT1375HV
• Description: 1.5A/ 500kHz Step-Down Switching Regulators
• Manufacturer: Linear Technology

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
100 F 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 (0.1 ).
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 2000 Mho, with an output imped-
ance of 200k
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 0.0033 F, 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
in parallel with 12pF. In all practical
= 10V
= 5V, I
= 100 F, 10V, AVX TPS
100
1k
= 50
–3
OUT
GAIN
C
FREQUENCY (Hz)
FREQUENCY (Hz)
= 0, L = 10 H
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