Datasheet

APPLICATION INFORMATION
overview
programming the output voltage on the UCC284
5
4
1
32
R1
R2
VOUT
VOUTS
GND
6 7
VIN VIN VIN VIN
CIN
VIN
8
CT
SD/CT
COUT
VOUT
+
+
(+)
(−)
(+)
(−)
C1
UCC384−ADJ
1
µ
F
4.7
µ
F
0.015
µ
F
VOUT + * 1.25
ǒ
1 )
R1
R2
Ǔ (1)
dropout performance
UCC284-EP
www.ti.com ....................................................................................................................................... SLVS862A – NOVEMBER 2008 – REVISED NOVEMBER 2008
The UCC284 – x family of negative low-dropout linear (LDO) regulators provides a regulated-output voltage for
applications with up to 0.5 A of load current. The regulators feature a low-dropout voltage and short-circuit
protection, making their use ideal for demanding applications requiring fault protection.
The UCC284 – 5 and UCC284 – 12 have output voltages that are fixed at – 5 V and – 12 V respectively. Connecting
VOUTS to VOUT gives the proper output voltage with respect to ground.
The UCC284 – ADJ can be programmed for any output voltage between – 1.25 V and – 15 V. This is easily
accomplished with the addition of an external resistor divider connected between GND and VOUT with VOUTS
connected to the center tap of the divider. For an output of – 1.25 V, no resistors are needed and VOUTS is
connected directly to VOUT. The regulator-input voltage cannot be more positive than the UVLO threshold, or
approximately – 3 V. Thus, low dropout cannot be achieved when programming the output voltage more positive
than approximately – 3.3 V. A typical application circuit is shown in Figure 1 .
Figure 1. Typical Application Circuit
For the UCC284-ADJ, the output voltage is programmed by the following equation:
When R1 or R2 are selected to be greater than about 100 k Ω , a small ceramic capacitor should be placed across
R1 to cancel the input pole created by R1 and the parasitic capacitance appearing on VOUTS. Values of
approximately 20 pF should be adequate.
The UCC284 is tailored for low-dropout applications where low-quiescent power is important. Fabricated with a
BCDMOS technology ideally suited for low input-to-output differential applications, the UCC284 passes 0.5 A
while requiring only 0.2 V of headroom. The dropout voltage is dependent on operating conditions such as load
current, input and load voltages, and temperature. The UCC284 achieves a low RDS(on) through the use of an
internal charge-pump that drives the MOSFET gate.
Figure 2 shows typical dropout voltages versus output voltage for the UCC284 – 5 V and – 12 V versions as well
as the UCC284-ADJ version programmed between – 3.3 V and – 15 V. Since the dropout voltage is also affected
by output current, Figure 3 shows typical dropout voltages versus load current for different values of VOUT.
Operating temperatures also affect the R
DS(on)
and the dropout voltage of the UCC284. Figure 4 shows typical
dropout voltages for the UCC284 over temperature under a full load of 0.5 A.
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Product Folder Link(s): UCC284-EP