LT1765/LT1765-1.8/LT1765-2.5/
LT1765-3.3/LT1765-5
14
1765fd
APPLICATIONS INFORMATION APPLICATIONS INFORMATION
Figure 8 shows the overall loop response with a 330pF VC
capacitor and a typical 100μF tantalum output capacitor.
The response is set by the following terms:
Error amplifi er:
DC gain set by g
m
and R
L
= 850μ • 500k = 425.
Pole set by C
F
and R
L
= (2π • 500k • 330p)
–1
= 965Hz.
Unity-gain set by C
F
and g
m
= (2π • 330p • 850μ
–1
)
–1
=
410kHz.
Power stage:
DC gain set by g
m
and R
L
(assume 5Ω) = 5 • 5 = 25.
Pole set by C
OUT
and R
L
= (2π • 100μ • 10)
–1
= 159Hz.
Unity-gain set by C
OUT
and g
m
= (2π • 100μ • 5
–1
)
–1
=
8kHz.
Tantalum output capacitor:
Zero set by C
OUT
and C
ESR
= (2π • 100μ • 0.1)
–1
= 15.9kHz.
The zero produced by the ESR of the tantalum output capaci-
tor is very useful in maintaining stability. Ceramic output
capacitors do not have a zero due to very low ESR, but are
dominated by their ESL. They form a notch in the 1MHz to
10MHz range. Without this zero, the V
C
pole must be made
dominant. A typical value of 2.2nF will achieve this.
If better transient response is required, a zero can be
added to the loop using a resistor (R
C
) in series with the
compensation capacitor. As the value of R
C
is increased,
transient response will generally improve, but two effects
limit its value. First, the combination of output capacitor
ESR and a large R
C
may stop loop gain rolling off altogether.
Second, if the loop gain is not rolled suffi ciently at the
switching frequency, output ripple will perturb the V
C
pin
enough to cause unstable duty cycle switching similar
to subharmonic oscillation. This may not be apparent
at the output. Small signal analysis will not show this
since a continuous time system is assumed. If needed,
an additional capacitor (C
F
) can be added to the V
C
pin to
form a pole at typically one fi fth the switching frequency
(If R
C
= ~ 5k, C
F
= ~ 100pF)
When checking loop stability, the circuit should be operated
over the application’s full voltage, current and temperature
range. Any transient loads should be applied and the output
voltage monitored for a well-damped behavior.
CONVERTER WITH BACKUP OUTPUT REGULATOR
In systems with a primary and backup supply, for example,
a battery powered device with a wall adapter input, the
output of the LT1765 can be held up by the backup supply
with its input disconnected. In this condition, the SW pin
will source current into the V
IN
pin. If the SHDN pin is held
at ground, only the shutdown current of 6μA will be pulled
via the SW pin from the second supply. With the SHDN pin
fl oating, the LT1765 will consume its quiescent operating
current of 1mA. The V
IN
pin will also source current to
any other components connected to the input line. If this
load is greater than 10mA or the input could be shorted to
ground, a series Schottky diode must be added, as shown
in Figure 9. With these safeguards, the output can be held
at voltages up to the V
IN
absolute maximum rating.
BUCK CONVERTER WITH ADJUSTABLE SOFT-START
Large capacitive loads or high input voltages can cause
high input currents at start-up. Figure 10 shows a circuit
that limits the dv/dt of the output at start-up, controlling
the capacitor charge rate. The buck converter is a typical
confi guration with the addition of R3, R4, C
SS
and Q1. As
the output starts to rise, Q1 turns on, regulating switch
current via the V
C
pin to maintain a constant dv/dt at the
output. Output rise time is controlled by the current through
C
SS
defi ned by R4 and Q1’s V
BE
. Once the output is in
regulation, Q1 turns off and the circuit operates normally.
R3 is transient protection for the base of Q1.
Figure 8. Overall Loop Response
FREQUENCY (Hz)
GAIN (dB)
80
60
40
20
0
–20
–40
PHASE (DEG)
180
150
120
90
60
30
0
1765 F08
GAIN
PHASE
V
OUT
= 5V
C
OUT
= 100μF, 0.1Ω
C
C
= 330pF
R
C
/C
F
= 0
I
LOAD
= 1A
10 1k 10k 1M100 100k
