LT1374IR Linear Technology, LT1374IR Datasheet - Page 23

• Manufacturer Part Number: LT1374IR
• Description: IC SW REG 4.5A ADJ STEP-DWN 7-DD
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT1374IR

• 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: D²Pak, TO-263 (7 leads + tab)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIONS
What About a Resistor in the Compensation Network?
It is common practice in switching regulator design to
add a “zero” to the error amplifier compensation to
increase loop phase margin. This zero is created in the
external network in the form of a resistor (R
with the compensation capacitor. Increasing the size of
this resistor generally creates better and better loop
stability, but there are two limitations on its value. First,
the combination of output capacitor ESR and a large
value for R
altogether, creating a gain margin problem. An approxi-
mate formula for R
G
G
ESR = Output capacitor ESR
2.42 = Reference voltage
With V
would yield zero gain margin, so this represents an upper
limit. There is a second limitation however which has
nothing to do with theoretical small signal dynamics.
This resistor sets high frequency gain of the error ampli-
fier, including the gain at the switching frequency. If
switching frequency gain is high enough, output ripple
voltage will appear at the V
to muck up proper operation of the regulator. In the
marginal case, subharmonic switching occurs, as evi-
denced by alternating pulse widths seen at the switch
node. In more severe cases, the regulator squeals or
hisses audibly even though the output voltage is still
roughly correct. None of this will show on a theoretical
Bode plot because Bode is an amplitude insensitive
analysis. Tests have shown that if ripple voltage on the V
is held to less than 100mV
behaved . The formula below will give an estimate of V
ripple voltage when R
that R
500kHz.
MP
MA
= Transconductance of power stage = 5.3A/V
= Error amplifier transconductance = 2(10
C
OUT
is large compared to the reactance of C
= 5V and ESR = 0.03Ω, a value of 6.5k for R
C
may cause loop gain to stop rolling off
U
C
where gain margin falls to zero is:
C
is added to the loop, assuming
INFORMATION
U
C
P-P
pin with enough amplitude
, the LT1374 will be well
W
C
) in series
U
–3
)
C
at
C
C
C
G
If a computer simulation of the LT1374 showed that a
series compensation resistor of 3k gave best overall loop
response, with adequate gain margin, the resulting V
ripple voltage with V
L = 10µH, would be:
This ripple voltage is high enough to possibly create
subharmonic switching. In most situations a compromise
value (< 2k in this case) for the resistor gives acceptable
phase margin and no subharmonic problems. In other
cases, the resistor may have to be larger to get acceptable
phase response, and some means must be used to control
ripple voltage at the V
is to add a capacitor (C
on the V
set at one-fifth of switching frequency so that it provides
significant attenuation of switching ripple, but does not
add unacceptable phase shift at loop unity-gain frequency.
With R
How Do I Test Loop Stability?
The “standard” compensation for LT1374 is a 1.5nF
capacitor for C
work for most applications, the “optimum” value for loop
compensation components depends, to various extent, on
parameters which are not well controlled. These include
inductor value (±30% due to production tolerance, load
MA
= Error amplifier transconductance (2000µMho)
C
C
= 3k,
pin. Pole frequency for this capacitor is typically
C
, with R
IN
C
F
) in parallel with the R
C
pin. The suggested way to do this
= 10V, V
= 0. While this compensation will
OUT
= 5V, ESR = 0.1Ω,
C
LT1374
/C
C
network
23
C
1374fd
pin