3/97
DESCRIPTION
Transformerless Off-Line
Operation
Low Voltage Operation to 0.8V
Ideal for Battery Trickle Charger
Applications
Current Mode Operation With
100mV Shunt
Voltage Mode Operation With
Fixed 1.25V Output or Resistor
Adjustable Output
Efficient BiCMOS Design
Inherent Short Circuit Protection
The UCC3890 controller is optimized for use as an off-line, low power, low
voltage, regulated current supply, ideally suited for battery trickle charger
applications. The unique circuit topology used in this device can be visual-
ized as two cascaded flyback converters; each operating in the discon-
tinuous mode, and both driven from a single external power switch. The
significant benefit of this approach is the ability to charge low voltage bat-
teries in off-line applications with no transformer, and low internal losses.
The control algorithm used by the UCC3890 forces a switch on time in-
versely proportional to the input line voltage, while the switch off time is
inversely proportional to the output voltage. This action is automatically
controlled by an internal feedback loop and reference. The cascaded con-
figuration allows a large voltage conversion ratio with reasonable switch
duty cycle.
While the UCC3890 is ideally suited for control of constant current battery
chargers, provision is also made to operate as a fixed 1.25V regulated
supply, or to use a resistor voltage divider to obtain output voltages higher
than 1.25V.
UCC1890
UCC2890
UCC3890
Off-Line Battery Charger Circuit
FEATURES
BLOCK DIAGRAM
UDG-96052
Note:
This device incorporates patented technology used under license from
Lambda Electronics, Inc.
UCC1890
UCC2890
UCC3890
I
DD
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5mA
Current into TON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5mA
Voltage on V
OUT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20V
Current into TOFF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250
A
Storage Temperature . . . . . . . . . . . . . . . . . . . 65C to +150C
Junction Temperature . . . . . . . . . . . . . . . . . . 55C to +150C
Lead Temperature (Soldering, 10 sec.) . . . . . . . . . . . . . +300C
CONNECTION DIAGRAMS
Currents are positive into, negative out of the specified terminal.
Consult Packaging Section of Databook for thermal limitations
and considerations of packages.
ABSOLUTE MAXIMUM RATINGS
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
General
VDD Zener Voltage
I
DD
= 4.75mA,I
TON
= 0mA
8.3
9.0
9.4
V
Minimum Operating Current I
TON
I
DD
= 1mA, F = 150kHz
1.65
2.0
mA
Undervoltage Lockout
Minimum Voltage to Start
FB = 0
7.8
8.6
9.2
V
Minimum Voltage after Start
FB = 0
5.75
6.3
6.65
V
Hysteresis
FB = 0
1.8
2.3
2.6
V
VDD V
START
FB = 0
0.2
0.4
0.7
V
Oscillator
Amplitude
I
TON
= 3mA; I
TOFF
= 50
A; V
FB
= 0V CT = 100pF
3.1
3.4
3.7
V
CT to DRIVE High Delay
Overdrive = 200mV
80
200
ns
CT to DRIVE Low Delay
Overdrive = 200mV
50
100
ns
Charge Coefficient I
CT
/I
TON
I
TON
= 3mA; V
CT
= 3.0V
0.135
0.15
0.165
A/
A
Discharge Coefficent I
CT
/I
TOFF
I
TOFF
= 50
A; V
CT
= 3.0V
0.95
1.00
1.05
A/
A
Driver
V
OL
I = 100mA (Note 1)
0.7
1.8
V
V
OH
I = 100mA referred to VDD (Note 1)
2.9
1.5
V
Rise Time
C
L
= 1nF
35
70
ns
Fall Time
C
L
= 1nF
30
60
ns
Line Voltage Detection
Minimum I
TON
for Fault
1.0
1.5
2.0
mA
I
TON
Detector Hysteresis
110
A
On Time During Fault
0.5
s
V
OUT
Error Amplifier
Reference Level
I
TOFF
= 50
A, I
CT
= 25
A, T
J
= 25C
1.20
1.25
1.30
V
I
TOFF
= 50
A, I
CT
= 25
A, Over Temperature
1.15
1.25
1.35
V
Voltage at TOFF
I
TOFF
= 50
A
0.3
0.4
0.5
V
Regulation gm
I
TOFF
= 50
A (Note 2)
2.0
4.0
7.7
mA/V
Current Sense Amplifier
Gain
V
CS
= 90 110mV
11.8
12.5
13.0
V/V
Input Offset Voltage
V
CS
= 90 110mV
5
0
5
mV
Input Voltage for CS Amplifier Enabled
I
TON
= 3mA, Referred to VDD
1.5
0.8
V
Input Voltage for CS Amplifier Disabled
I
TON
= 3mA, Referred to VDD
0.8
0.3
V
ELECTRICAL CHARACTERISTICS:
Unless otherwise stated, these specifications apply for T
A
= 55
C to 125
C for
UCC1890, 40C to 85C for the UCC2890, and 0C to 70C for the UCC3890. No load at DRIVE pin (C
LOAD
= 0), T
A
= T
J
.
DIL-8, SOIC-8 (Top View)
J, N, or D Packages
Note 1: VDD forced to 100mV below VDD Zener Voltage
Note 2: gm is defined as
I
CT
V
FB
for the values of V
FB
where the error amp is in regulation. The two points used to calculate gm
are for I
CT
at 65% amd 35% of its maximum value.
2
UCC1890
UCC2890
UCC3890
PIN DESCRIPTIONS
CS: The high side of the current sense shunt is con-
nected to this pin. Short CS to VDD for voltage feedback
operation.
CT: Oscillator timing capacitor is connected to this pin.
DRIVE: Gate drive to external power switch.
FB: Output of current sense amplifier. This pin can be
used for direct output voltage feedback if the current
sense amp input pin CS is shorted to the VDD pin.
GND: Ground pin.
TOFF: Resistor R
OFF
connects from voltage output to
this pin to provide a maximum capacitor discharge cur-
rent proportional to output voltage.
TON: Resistor R
ON
connects from line input to this pin to
provide capacitor charge current proportional to line volt-
age. The current in R
ON
also provides power for the 9V
shunt regulator at VDD.
VDD: Output of 9V shunt regulator.
APPLICATION INFORMATION
OPERATION (VOLTAGE OUTPUT)
Figure 1 shows a typical voltage mode application.
When input voltage is first applied, all of the current
through R
DD
and 80% of the current through R
ON,
charge the external capacitor C3 connected to VDD. As
the voltage builds on VDD, undervoltage lockout holds
the circuit off and the output DRIVE low until VDD
reaches 8.4V. At this time, DRIVE goes high, turning on
the external power switch Q1, and 15% of the current
into TON is directed to the timing capacitor C
T
. The volt-
age at TON is fixed at approximately 11V, so C
T
charges to a fixed threshold with current
I
=
0.2
V
IN
11V
R
ON
Since the input line is much greater than 11V, the
charge current is approximately proportional to the input
line voltage. DRIVE is only high while C
T
is charging, so
the power switch on time is inversely proportional to line
voltage. This provides a constant line voltage-switch on
time product.
At the end of the switch on time, Q1 is turned off, and
the 15% of the R
ON
current which was charging C
T
is
diverted to ground. The power switch off time is control-
led by discharge of C
T
, which is determined by the outut
voltage as described here:
Figure 1. Typical Voltage Mode Application
UDG-96053
UDG-96054
3
UCC1890
UCC2890
UCC3890
1. When V
OUT
= 0, the off time is infinite. This feature
provides inherent short circuit protection. However,
to ensure output voltage startup when the output is
not a short, a high value resistor, R
S,
is placed in
parallel with C
T
to establish a minimum switching
frequency.
2. As V
OUT
rises above approximately 0.4V, I
DCHG
is
set by R
OFF
, and is defined by
I
DCHG
=
V
OUT
0.4V
R
OFF
As V
OUT
increases, I
DCHG
increases resulting in the
reduction of off time. The frequency of operation in-
creases and V
OUT
rises quickly to its regulated
value.
3. In this region, a transconductance amplifier reduces
I
DCHG
in order to maintain V
OUT
in regulation. The
input to the transconductance amplifier is the pin FB.
(In this mode the pin CS should be shorted to VDD.)
FB can either be connected directly to V
OUT
to regu-
late at nominal V
OUT
=
1.25V or to be connected to
V
OUT
through a resistor divider R
VS1
/R
VS2
to regu-
late at nominal
V
OUT
=
1.25V
(
R
VS1
+
R
VS2
)
R
VS2
4. If V
OUT
should rise above its regulation range, I
DCHG
falls to zero and the circuit returns to the minimum
frequency established by R
S
and C
T
.
The range of switching frequencies is established by
R
ON
, R
OFF
, R
S
, and C
T
as follows:
Frequency
=
1
TON
+
TOFF
TON
=
CT
3.4V
0.15
R
ON
V
IN
11V
TOFF
(
MAX
)
=
1.5
R
S
C
T
(
regions
1
and 4
)
TOFF
=
C
T
3.4V
R
OFF
V
OUT
-
0.4V
(
region
2
)
The above equations assume VDD = 9, the voltage
at TON = 11V, the voltage at TOFF = 0.4V.
OPERATION (CURRENT OUTPUT)
Figure 2 shows a typical current mode application. In
current mode, operation is the same as in voltage
mode, except that in region 3 the transconductance am-
plifier is controlled by the current sense amplifier which
senses the voltage across a shunt resistor R
SH
. The cir-
cuit then regulates the current in the shunt to the nomi-
nal value
I
SH
=
100mV
R
SH
The circuit shown in this schematic would be suitable
for an application which trickle charges a battery at a
low current, (e.g. C/10), and has a battery load which
draws a high current, (e.g. C), when turned on. In that
case, R
SH1
value is chosen so that
100mV
R
SH1
=
C
10
Figure 2. Typical Current Mode Application
UDG-96055
APPLICATION INFORMATION (cont.)
4
UCC1890
UCC2890
UCC3890
If R
SH2
is chosen so that
100mV
R
SH2
=
C
then the regulator output will assist the battery, minimiz-
ing or eliminating battery output current.
DESIGN EXAMPLE
A typical design has the following requirements:
V
IN
=
80 to 132 VAC or 100 to 180 VDC
V
OUT
=
1.25V
V
OUT
=
2.0V (assumes 1.25
V
OUT
with
750mV forward drop in D3)
I
LOAD
=
500mADC max
F
SWITCHING
=
100kHz
(eff.)
=
50% (excluding efficiency losses in
D3 which will be very large due to the
low output voltage. Losses in D3 are
accounted for by using V
OUT
in the
calculations).
Component values are indicated in Figure 3. The expla-
nation for the choices in component values follows.
First calculate the maximum duty cycle, d(max). To cal-
culate this assume that at maximum load/minimum line
conditions, the converter will be at the continuous con-
duction boundary and there will be no idle time after the
inductors are discharged. For all other load/line condi-
tions, the UCC3890 will stretch the off time, to create an
idle time after the inductors are discharged, in order to
maintain a constant output voltage. For a single flyback
stage at continuous conduction boundary
d
=
1
1
+
V
IN
V
OUT
For the cascaded flyback stages of the UCC3890 topol-
ogy, the corresponding equation is
d
(
max
)
=
1
1
+
V
IN
V
OUT
in this case
d
(
max
)
=
1
1
+
100V
2V
=
0.125
Next using the operating frequency and the maximum
duty cycle to calculate the maximum on time
TON
(
max
)
=
d
(
max
)
F
SWITCHING
in this case
TON
(
max
)
=
0.125
100kHz
=
1.25
s
correspondingly
TOFF
(
min
)
=
1
-
0.125
100kHz
=
8.75
s
Figure 3. Example Application
UDG-96056
APPLICATION INFORMATION (cont.)
5
The average input current at minimum line and maxi-
mum load will be
I
IN
=
I
OUT
V
OUT
V
IN
in this case
I
IN
=
500mA
0.5
2V
100V
=
20mA
Knowing that input current is drawn from the line only
during TON, calculate the peak current in L1 to be
I
L1
(
pk
)
=
2
I
IN
TON
+
TOFF
TON
in this case
I
L1
(
pk
)
=
2
20mA
1.25
s
+
8.75
s
1.25
s
=
320mA
Now calculate the value for L1
L
1
=
V
IN
TON
I
L1
(
pk
)
in this case
L
1
=
100V
1.25
s
320mA
=
390
H
The output voltage of the first flyback stage is
V
C1
=
V
IN
TON
TOFF
in this case
V
C1
=
100V
1.25
s
8.75
s
=
14.3V
Knowing that output current is provided to the load only
during TOFF, calculate the peak current in L2 to be
I
L2
(
pk
)
=
2
I
OUT
TON
+
TOFF
TOFF
in this case
I
L2
(
pk
)
=
2
0.5A
1.25
s
+
8.75
s
8.75
s
=
1.14A
Now calculate the value of L
2
L
2
=
V
OUT
TOFF
I
L2
(
pk
)
in this case
L
2
=
2V
8.75
s
1.14A
=
15
H
For all of the calculations so far only the maximum
load/minimum line condition have been considered. The
entire range of operation must be considered to choose
values for the rest of the components.
Under all normal operating conditions the current I
TON,
(which is the current in R
ON
), should be greater than
2mA and less than 7.5mA. In this case set R
ON
to give
I
TON
= 2.8mA at low line. The voltage at TON will be
about 11V so
R
ON
=
100V
-
11V
2.8mA
=
33k
With R
ON
= 33k, I
TON
at high line will be
I
TON
=
180V
-
11V
33k
=
5.1mA
At high line, the power dissipation in R
ON
will be
P
(
R
ON
)
=
(
180V
-
11V
)
5.1mA
=
860mW
R
ON
will need to be at least a 1W resistor. Alternately it
could be four 1/4W 8.2k
resistors in series.
Once R
ON
is set, C
T
can be chosen. The charge current
for C
T
is nominally 15% of I
TON
, and the nominal oscilla-
tor amplitude is 3.4V, so
TON
=
CT
3.4V
0.15
I
TON
solving for CT
CT
=
TON
0.15
I
TON
3.4V
I
TON
at low line is 2.8mA, and the target TON at low line
is 1.25
s, so in this case
CT
=
1.25
s
0.15
2.8mA
3.4V
=
150pF
The final component to be chosen is R
OFF
, which deter-
mines the minimum value of TOFF. When the output
voltage is below the regulation point, the discharge cur-
rent for CT is equal to I
TOFF
(the current in R
OFF
). Un-
der that condition
TOFF
=
CT
3.4V
I
TOFF
since the voltage at the TOFF pin = 0.4V
I
TOFF
=
V
OUT
-
0.4V
R
OFF
substituting and solving for R
OFF
R
OFF
=
T
OFF
(
V
OUT
-
0.4V
)
CT
3.4V
The largest discharge current, and hence the minimum
off time, will occur when the output is about 10mV be-
UCC1890
UCC2890
UCC3890
APPLICATION INFORMATION (cont.)
6
UCC1890
UCC2890
UCC3890
low the regulation point of 1.25V. The minimum value
for TOFF is 8.75
s. So in this case
R
OFF
=
8.75
s
(
1.24V
-
0.4V
)
150pF
3.4V
=
15k
OTHER APPLICATION CONSIDERATIONS
Output Capacitor: For best regulation of the output
voltage or current, the output capacitor should be a low
ESR type. This is especially true when operating in cur-
rent sense mode with a non-linear load such as a bat-
tery. If a low ESR capacitor cannot be used, excellent
regulation can also be achieved by placing a low pass
R/C filter between the current shunt and the CS input.
No Load Operation: The UCC3890 is inherently pro-
tected for short circuits, but not for open circuits. If the
load is removed, the output voltage will quickly rise up
to the regulation point. Once the output is above the
regulation voltage, the oscillator will drop to the mini-
mum frequency set by R
S
/C
T
. With no load on the out-
put, even at this low frequency the output voltage can
quickly rise to a dangerous level. To protect against this,
it is recommended that a zener or other voltage clamp
always be connected across the output. The clamp
should be chosen to be above the normal range of out-
put voltage, but low enough to protect the output ca-
pacitor. In current sense operation, removal of the load
will also break the regulation loop, in which case a sim-
ple clamp on the output may not be adequate. In current
sense mode it is recommended that a second zener be
connected from the output to the FB pin, the breakdown
voltage of this clamp chosen to be high enough so that
it will not conduct during normal operation, but will con-
duct at least 2V lower than the breakdown voltage of
the other clamp.
Gate Drive for the External FET: The UCC3890 is
guaranteed to be able to deliver at least 1mA of steady
state current to the gate of the external FET at I
TON
=
2mA. If I
TON
is higher than 2mA, 80% of the additional
current is available to drive the FET gate. If, as in the
design example above, a moderate sized FET such as
the IRF820 is used, the operating frequency is 100kHz,
and the minimum I
TON
at low line is 2.8mA, then the
available gate drive current may be adequate. The
IRF820 needs about 13nC to charge the gate on each
cycle. At 100kHz, this is equivalent to 1.3mA steady
state; below the minimum 1.64mA available. In some
combinations of a larger FET, and/or higher frequency
operation, the current available for driving the gate may
not be adequate. In that case extra current may be pro-
vided by connecting a resistor R
DD
from the line input to
the VDD pin. This resistor should be sized so that under
all conditions the current input to VDD is below the
7.5mA absolute maximum limit. R
DD
will likely need to
be a power resistor.
UNITRODE CORPORATION
7 CONTINENTAL BLVD.
MERRIMACK, NH 03054
TEL. (603) 424-2410
FAX (603) 424-3460
APPLICATION INFORMATION (cont.)
7
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