LT1959CR#TRPBF Linear Technology, LT1959CR#TRPBF Datasheet - Page 20

• Manufacturer Part Number: LT1959CR#TRPBF
• Description: IC SW REG STEP-DWN 500KHZ 7-DD
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT1959CR#TRPBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.21 ~ 38 V
• Current - Output: 4.5A
• Frequency - Switching: 500kHz
• Voltage - Input: 4.3 ~ 15 V
• Operating Temperature: 0°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: D²Pak, TO-263 (7 leads + tab)
• Dc To Dc Converter Type: Step Down
• Pin Count: 7 +Tab
• Input Voltage: 16V
• Output Voltage: 1.21 to 2.42V
• Switching Freq: 540kHz
• Output Current: 8.5A
• Package Type: DDPAK
• Output Type: Adjustable
• Switching Regulator: Yes
• Line Regulation: 0.03%/V
• Mounting: Surface Mount
• Input Voltage (min): 4V
• Operating Temperature Classification: Commercial
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Lead Free Status / Rohs Status: Compliant

LT1959
APPLICATIONS
G
G
ESR = Output capacitor ESR
1.21 = 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 amplifier,
including the gain at the switching frequency. If switching
frequency gain is high enough, output ripple voltage will
appear at the V
proper operation of the regulator. In the marginal case,
subharmonic switching occurs, as evidenced by alternat-
ing 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
LT1959 will be well behaved . The formula below will give
an estimate of V
loop, assuming that R
of C
G
If a computer simulation of the LT1959 showed that a
series compensation resistor of 6k gave best overall loop
response, with adequate gain margin, the resulting V
ripple voltage with V
L = 10 H, 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
20
V
MP
MA
MA
C RIPPLE
V
C
C RIPPLE
= Transconductance of power stage = 5.3A/V
= Error amplifier transconductance = 2(10
= Error amplifier transconductance (2000 Mho)
at 500kHz.
OUT
= 5V and ESR = 0.03 , a value of 6.5k for R
6
C
k
C
pin with enough amplitude to muck up
R
10 10 10
ripple voltage when R
C
2 10
U
•
IN
C
C
G
is large compared to the reactance
is held to less than 100mV
MA
= 10V, V
INFORMATION
•
3
U
V
10 5 0 1 1 21
IN
6
V
IN
OUT
500 10
V
L f
W
OUT
= 5V, ESR = 0.1 ,
•
.
C
3
ESR
is added to the
.
U
–3
1 21
.
0 144
)
P-P
.
C
, the
pin
V
C
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 LT1959 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
current and ripple current variations), output capacitance
( 20% to 50% due to production tolerance, tempera-
ture, aging and changes at the load), output capacitor ESR
( 200% due to production tolerance, temperature and
aging), and finally, DC input voltage and output load
current . This makes it important for the designer to check
out the final design to ensure that it is “robust” and tolerant
of all these variations.
I check switching regulator loop stability by pulse loading
the regulator output while observing transient response at
the output, using the circuit shown in Figure 13. The
regulator loop is “hit” with a small transient AC load
current at a relatively low frequency, 50Hz to 1kHz. This
causes the output to jump a few millivolts, then settle back
to the original value, as shown in Figure 14. A well behaved
loop will settle back cleanly, whereas a loop with poor
phase or gain margin will “ring” as it settles. The number
of rings indicates the degree of stability, and the frequency
of the ringing shows the approximate unity-gain fre-
quency of the loop. Amplitude of the signal is not particu-
larly important, as long as the amplitude is not so high that
the loop behaves nonlinearly.
C
F
C
C
= 6k,
pin. Pole frequency for this capacitor is typically
2
5
C
f R
, with R
C
C
F
) in parallel with the R
C
pin. The suggested way to do this
2 500 10
= 0. While this compensation will
5
3
6
k
C
/C
275
C
network
pF