Datasheet

11
LT1681
1681f
APPLICATIO S I FOR ATIO
WUUU
Events that trigger a GFC are:
a) Exceeding the current limit of the 5V
REF
pin
b) Detecting an undervoltage condition on V
CC
c) Detecting an undervoltage condition on 5V
REF
d) Pulling the SHDN pin below the shutdown threshold
e) Exceeding the I
MAX
pin threshold
f) Exceeding the 1.25V fault detector threshold on either
the OVLO or THERM pins
The OVLO and THERM pins are used to directly trigger a
GFC. If either of these pins are not used, they can be
disabled by connecting the pin to SGND. The intention of
the OLVO pin is to allow monitoring of the input supply to
protect from an overvoltage condition. Monitoring of
system temperature (THERM) is possible through use of
a resistor divider using a thermistor as a resistor divider
component. The 5V
REF
pin can provide the precision
supply required for these applications. When these fault
detection circuits are disabled during shutdown or V
CC
pin
UVLO conditions, a reduction in OVLO and THERM pin
input impedance to ground will occur. To prevent exces-
sive pin input currents, low impedance pull-up devices
must not be used on these pins.
Undervoltage Lockout
The LT1681 maintains a low current operational mode
when an undervoltage condition is detected on the V
CC
supply pin, or when V
CC
is below the undervoltage lockout
(UVLO) threshold. During a UVLO condition on the V
CC
pin, the LT1681 disables all internal functions with the
exception of the shutdown and UVLO circuitry. The exter-
nal 5V
REF
supply is also disabled during this condition.
Disabling of all switching control circuity reduces the
LT1681 supply current to <1mA, simplifying integration
of trickle charging in systems that employ output feedback
supply generation.
The function of the high side switch output (TG) is also
gated by UVLO circuitry monitoring the bootstrap supply
(V
BST
-BSTREF). Switching of the TG pin is disabled until
the voltage across the bootstrap supply is greater than
7.4V. This helps prevent the possibility of forcing the high
side switch into a linear operational region, potentially
causing excessive power dissipation due to inadequate
gate drive during start-up.
Error Amplifier Configurations
The converter output voltage information is fed back to the
LT1681 onto the V
FB
pin where it is transformed into an
output current control voltage by the error amplifier. The
error amplifier is generally configured as an integrator and
is used to create the dominant pole for the main converter
feedback loop. The LT1681 error amplifier is a true high
gain voltage amplifier. The amplifier noninverting input is
internally referenced to 1.25V; the inverting input is the
V
FB
pin and the output is the V
C
pin. Because both low
frequency gain and integrator frequency characteristics
can be controlled with external components, this amplifier
allows far greater flexibility and precision compared with
use of a transconductance error amplifier.
In a nonisolated converter configuration where a resistor
divider is used to program the desired output voltage, the
error amplifier can be configured as a simple active
integrator, forming the system dominant pole (see Fig-
ure␣ 1). Placing a capacitor C
ERR
from the V
FB
pin to the V
C
pin will set the single-pole crossover frequency at
(2πR
FB
C
ERR
)
–1
. Additional poles and zeros can be added
by increasing the complexity of the RC network.
V
FB
R
FB
C
ERR
V
OUT
V
C
1.25V
1681 F01
LT1681
9
–
+
10
Figure 1. Nonisolated Error Amp Configuration
Another common error amplifier configuration is for
optocoupler use in fully isolated converters with second-
ary-side control (see Figure 2). In such a system, the
dominant pole for the feedback loop is created at the sec-
ondary-side controller, so the error amplifier needs only to