
SLUS272C FEBRUARY 2000 REVISED JUNE 2003
7
www.ti.com
DETAILED PIN DESCRIPTIONS (continued)
CURLIM
: CURLIM programs the primary current limit threshold and determines whether the device latches off
or retries after an overcurrent condition. When a shutdown signal is generated, a 200-
μ
A current source to
ground pulls down on CURLIM. If the voltage on the pin remains above 350 mV the device remains latched and
the power must be cycled to restart. If the voltage on the pin falls below 350 mV, the device attempts a restart.
The voltage threshold is typically set by a resistor divider from V
REF
to ground. To calculate the current limit
adjust voltage threshold the following equations can be used.
Current limit adjust latching mode voltage is calculated in equation (2)
V
V
REF
(R1
300 A
R1
R2
3)
1
350 mV
Current limit adjust non-latching mode voltage is calculated in equation (3)
V
V
REF
(R1
80 A
R1
R2
3)
1
350 mV
where
R1 is the resistance from the VREF to CURLIM
R2 is the resistance from CURLIM to GND
GND
: GND is the reference ground and power ground for all functions of this part. Bypass and timing capacitors
should be connected as close as possible to GND.
RT
: RT is the connection point for the oscillator timing resistor. It has a low impedance input and is nominally
at 1.25 V. The current through RT is mirrored to the timing capacitor pin, CT. This causes a linear charging of
CT from 0 V to 2.35 V. Note that the current mirror is limited to a maximum of 100
μ
A so R
T
must be greater
than 12.5 k
.
SYNC
: SYNC is a bi-directional pin, allowing or providing external synchronization with TTL compatible
thresholds. In a typical application RT is connected through a timing resistor to GND which allows the internal
oscillator to free run. In this mode SYNC outputs a TTL compatible pulse during the oscillator dead time (when
CT is being discharged). If RT is forced above 4.4 V, SYNC acts as an input with TTL compatible thresholds
and the internal oscillator is disabled. When SYNC is high, greater than 2 V the outputs are held active low.
When SYNC returns low, the outputs may be high until the ontime is terminated by the normal peak current
signal, a fault seen at SHUTDOWN or the next high assertion of SYNC. Multiple UCC3806s can be
synchronized by a single master UCC3806 or external clock.
VC
: VC is the input supply connection for the FET drive outputs and has an input range from 2.5 V to 15 V. VC
should be capacitively bypassed for proper operation.
VIN
: VIN is the input supply connection for this device. The UCC1806 has a maximum startup threshold of 8 V
and internally limited by means of a 15 V shunt regulator. The shunted supply current must be limited to 2.5 mA.
For proper operation, VIN must be bypassed to GND with at least a 0.01-
μ
F ceramic capacitor
VREF
: VREF is a 5.1 V
±
1% trimmed reference output with a 5 mA maximum available current. VREF must be
bypassed to GND with at least a 0.1-
μ
F ceramic capacitor for proper operation.
(2)
(3)