LT3645
14
3645f
APPLICATIONS INFORMATION
pull large currents from the output through the SW pin
and the V
IN
pin. Figure 5 shows a circuit that will run only
when the input voltage is present and that protects against
a shorted or reversed input.
Frequency Compensation (Buck)
The LT3645 uses current mode control to regulate the
loop. This simpli es loop compensation. In particular, the
LT3645 does not require the ESR of the output capacitor
for stability, allowing the use of ceramic capacitors to
achieve low output ripple and small circuit size. A low
ESR output capacitor will typically provide for a greater
margin of circuit stability than an otherwise equivalent
capacitor with higher ESR, although the higher ESR will
tend to provide a faster loop response. Figure 6 shows an
equivalent circuit for the LT3645 control loop.
Figure 5. Diode D4 Prevents a Shorted Input from Discharging a
Backup Battery Tied to the Output; It Also Protects the Circuit from
a Reversed Input. The LT3645 Runs Only When the Input Is Present
Figure 6. Model for Loop Response
The error ampli er (g
m
) is a transconductance type with
nite output impedance. The power section, consisting
of the modulator, power switch, and inductor, is modeled
as a transconductance ampli er (G) generating an output
current proportional to the voltage at the V
C
node. Note
that the output capacitor integrates this current, and that
the capacitor on the V
C
node (C
C
) integrates the error
ampli er output current, resulting in two poles in the
loop. R
C
provides a zero. With the recommended output
capacitor, the loop crossover occurs above the R
C
C
C
zero.
This simple model works well as long as the value of the
inductor is not too high and the loop crossover frequency
is much lower than the switching frequency. With a larger
ceramic capacitor that will have lower ESR, crossover may
be lower and a phase lead capacitor connected across
R1 in the feedback divider may improve the transient
response. Large electrolytic capacitors may have an ESR
3645 F05
V
IN
D4
GND
SW
DA
V
CC2
NPG
FB
OUT2
V
IN
EN/UVLO
EN2
FB2
BACKUP
BOOST
LT3645
G
R
C
R1
R2
g
m
= 100糀/V
G = 1A/V
R
C
= 150k
C
C
= 60pF
OUT
C
OUT
CERAMIC
C
C
1M
0.8V
g
m
C
PL
ELECTROLYTIC
ESR
+
3645 F06
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