UCC3954
PRELIMINARY INFORMATION
6/98
FEATURES
Converts Lithium-Ion Cell to
+3.3V at 700mA Load
Current
Load Disconnect in
Shutdown
High Efficiency Flyback
Operation
Internal 0.15
Switch
Low Battery LED Driver
Internal 2A Current Limit
Internal 200kHz Oscillator
8 Pin D, N, 14 Pin PW
Packages
Single Cell Lithium-Ion to +3.3V Converter
BLOCK DIAGRAM
UDG-96137-1
DESCRIPTION
The UCC3954, along with a few external components, develops a regulated +3.3V
from a single lithium-ion battery whose terminal voltage can vary between 2.5V and
4.2V. The UCC3954 employs a simple flyback (Buck-Boost) technique to convert the
battery energy to +3.3V. This is accomplished by referencing the lithium-ion cell's
positive terminal to system ground. The negative terminal of the battery is the return
point for the UCC3954. This approach enables the converter to maintain constant fre-
quency operation whether the cell voltage is above or below the output voltage. An
additional benefit of this technique is its inherent ability to disconnect the battery from
the load in shutdown mode.
The UCC3954 operates as a fixed 200kHz switching frequency voltage mode flyback
converter. The oscillator time base and ramp are internally generated by the
UCC3954 and require no external components. A 2A current limit for the internal
0.15
power switch provides protection in the case of an output short circuit. When
left open, an internal 100k
resistor pulls the SD pin to BAT, which puts the
UCC3954 in shutdown mode, and thereby reduces power consumption to sub-
A lev-
els. A low battery detect function will drive the LOWBAT pin low (minimum of 5mA
sink current) when the battery has been discharged to within 200mV of the prede-
fined lockout voltage. The LOWBAT pin is intended for use with an external LED to
provide visual warning that the battery is nearly exhausted. The lockout mode is acti-
vated when the battery is discharged to 2.55V. In lockout mode, the part consumes
15
A. Once the UCC3954 has entered lockout mode, the user must insert a fresh
battery whose open circuit voltage is greater than 3.1V. This prevents a system-level
oscillation of the lockout function due to the lithium-ion battery's large equivalent se-
ries resistance.
Additional features of the UCC3954 include a trimmed 1.1V reference and internal
feedback scaling resistors, a precision error amplifier, low quiescent current drain in
shutdown mode, and a softstart function. The UCC3954 is offered in the 8 pin D,
14 pin PW (surface mount), and N (through hole) packages.
2
UCC3954
ELECTRICAL CHARACTERISTICS:
Unless otherwise specified, T
A
= 20C to 70C for the UCC3954, SD = V
BAT+
=
3.5V (ref. to V
BAT-
), VOUT = 3.3 (ref. to V
BAT+
). T
A
= T
J
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
Input Supply
Supply Current (total) active
I
BAT+
+ I
VOUT
1
2
mA
Supply Current (BAT+) Shutdown
V
SDB
= 0V (reference to BAT)
0.2
5
A
Supply Current (BAT+) UVLO
30
40
A
BAT+ Turn On Threshold
With Respect to BAT+ Turnoff
250
300
375
mV
BAT+ Turn Off Threshold
2.35
2.55
2.75
V
Low BAT+ Indicate Threshold
With Respect to BAT+ Turnoff
50
100
325
mV
Error Amplifier
Output Voltage High
Maximum Duty Cycle, I
OH
= 1ma
2.0
2.4
V
Output Voltage Low
Minimum Duty Cycle, I
OL
= 1ma
0
0.14
0.5
V
VOUT Regulation Voltage
T
A
= 25C
3.22
3.3
3.38
V
3.20
3.3
3.39
V
ABSOLUTE MAXIMUM RATINGS
Input Supply Voltage (BAT+ to BAT) . . . . . . . . . . . . . . . . 4.5V
VOUT
Maximum Forced Voltage (ref. to BAT+) . . . . . . . . . . . . 5.5V
SWITCH
Maximum Forced Voltage (ref. to BAT) . . . . . . . . . . . 10.2V
Maximum Forced Current . . . . . . . . . . . . . . Internally Limited
SD
Maximum Forced Voltage (ref. to BAT+) . . . . . . . . . . . . 5.5V
Maximum Forced Current . . . . . . . . . . . . . . . . . . . . . . . 10mA
COMP
Maximum Forced Voltage (ref. to BAT) . . . . . . . . . . . . 4.5V
Maximum Forced Current. . . . . . . . . . . . . . . . . . Self Limiting
Storage Temperature . . . . . . . . . . . . . . . . . . . 65C to +150C
Junction Temperature . . . . . . . . . . . . . . . . . . . 55C to +150C
Lead Temperature (Soldering, 10 sec.) . . . . . . . . . . . . . +300C
Unless otherwise indicated, voltages are reference to BAT
and currents are positive into, negative out of the specified ter-
minal. Pulsed is defined as a less than 10% duty cycle with a
maximum duration of 500
s. Consult Packaging Section of Da-
tabook for thermal limitations and considerations of packages.
CONNECTION DIAGRAMS
DIL-8, SOIC-8 (Top View)
D Package, N Package
SOIC-14 (Top View)
PW Package
3
UCC3954
ELECTRICAL CHARACTERISTICS:
Unless otherwise specified, T
A
= 20C to 70C for the UCC3954, SD = V
BAT+
=
3.5V (ref. to V
BAT-
), VOUT = 3.3 (ref. to V
BAT+
). T
A
= T
J
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
Oscillator/PWM
Intital Accurancy
T
A
= 25C
180
200
220
kHz
175
200
225
kHz
PWM Modulator Gain
V
COMP
= 1.6V to 2V
40
50
60
%/V
PWM Maximum Duty Cycle
65
75
85
%
PWM Minimum Duty Cycle
3
5
%
Shutdown
Disable Threshold
Reference to BAT
0.8
1.5
2.5
V
Lowbat
On Resistance
V
LOWBAT
= 1V
40
100
220
Soft Start
Rise TIme
Note 2, R
LOAD
= 33
, C
COMP
= 39nF,
C
LOAD
= 330
F
10
msec
Output Switch
Saturation Voltage
I
SWITCH
= 200mA
30
70
mV
Overcurrent Threshold
Note 2
2.0
3.0
3.5
Amps
Note 1: V
BAT+
<2V to reset.
Note 2: Guaranteed by design. Not 100% tested in production.
PIN DESCRIPTIONS
BAT+: Logic supply voltage for the UCC3954. Connect to
the positive terminal of the lithium-ion battery and system
ground. Bypass with a low ESR, ESL capacitor if located
more than 1 inch from the battery positive terminal. This
is also the return for the +3.3V load
BAT: Return for the UCC3954. Switch current flows
through this pin to the negative terminal of the battery.
Proper board layout precautions should be taken to
minimize trace length in this path.
COMP: Output of the voltage error amplifier. Loop
compensation component C
COMP
is connected between
COMP and VFB.
LOWBAT: An open drain output that will pull low and sink
10mA (typ) to drive an external LED if the battery voltage
falls below the low BAT+ warning threshold. Note that this
output pulls low to BAT.
PVOUT: (PW Package only) This is the bootstrap input
for the internal FET drive. It should be tied to the 3.3V
output along with V
OUT
.
SD: Shutdown input for the UCC3954. An internal 100k
resistor pulls SD to BAT when the circuit is left open.
Pulling SD up to system ground (BAT+) or to V
OUT
, starts
the UCC3954. The UCC3954 enters a lockout mode
when a dead battery is detected (<2.55V). Until a fresh
battery is inserted (>3.1V), the part will remain in the low
current lockout state.
SGND: (PW Package only) This is a separate signal
ground pin which should be externally tied to BAT.
SWITCH: Drain terminal of the internal 0.15
power
switch. The current into this pin is internally limited.
VFB: This is the virtual ground of the error amplifier.
Nominally at the same voltage as BAT+, the pin is
provided for external compensation by means of a single
capacitor to form a simple dominant pole.
VOUT: Regulated 3.3V supply feedback to the UCC3954.
4
UCC3954
Figure 1. Simplified Circuit Diagram
UDG-97098
APPLICATION INFORMATION
Circuit Topology
The UCC3954 uses a fixed frequency (200KHz), voltage
mode PWM flyback topology. It can operate from a bat-
tery input voltage that is above or below the output volt-
age by referencing the battery's (+) terminal to the output
(system) ground and the battery's () terminal to the IC's
"ground" pin. It is typically operated in the continuous
conduction mode (CCM), except at light loads to reduce
losses due to high peak inductor current. The simplified
diagram in Figure 1 helps to visualize the circuit topology.
Figure 2 illustrates the current waveforms in the major
circuit elements.
Only a few external components are required to develop
a regulated 3.3V output from a single Lithium-Ion cell. A
low ESR (Equivalent Series Resistance) and ESL
(Equivalent Series Inductance) decoupling capacitor
should be placed as close as possible to BAT+ and
BAT. This is especially important when operating at low
battery voltages, where the peak current could cause ex-
cessive input ripple, causing the input voltage to drop be-
low the UCC3954's shutdown threshold. The other parts
required
are
a
compensation
capacitor,
inductor,
Schottky diode and output filter capacitor. The output fil-
ter capacitor should also be a good low ESR/ESL capaci-
tor.
Choosing an Inductor
The inductor value selected, for a given input voltage and
load current, will determine if the converter is operating
in the continuous or the discontinuous conduction mode.
In general, the efficiency will be higher in the continuous
mode (larger inductor value), due to the lower peak cur-
rents. This also reduces the demands on the output filter
capacitor and lowers output ripple voltage. However, a
larger inductor value will also be physically larger for the
same current rating, and reduces loop bandwidth, mak-
ing it more difficult to compensate. For the input voltage
range and fixed operating frequency of the UCC3954, an
inductor value of around 33
H is a good compromise.
See Table 1 for values and part numbers of inductors for
specific ranges of load current.
Remember that the inductor must be able to maintain
most of its inductance at the peak switching current.
Output Capacitor Selection
To minimize output voltage ripple, a good high frequency
capacitor(s) must be used. Low ESR tantalums or Sanyo
Figure 2. Current Waveforms
UDG-97099
5
UCC3954
OSCON's are recommended. Surface mounting will
eliminate the lead inductance. Suggested values and part
numbers for C
OUT
at different load currents are given in
Table 1.
Compensation Capacitor
For applications where the load is fairly constant, the
loop may be compensated with a single capacitor be-
tween COMP and VFB. The value shown in the Applica-
tion Circuit of Figure 3 provides good stability margin
over a wide range of load, using the values shown for L1
and C
OUT.
Lead-Lag Compensation for Dynamic Loads
When large dynamic load transients are expected, the
simple dominant pole compensation method may not
provide adequate dynamic load regulation. In this case,
lead-lag compensation is recommended, as shown in the
application circuit of Figure 4. The addition of R1 and C1
in the error amp feedback loop provides significantly
wider loop bandwidth, resulting in improved transient re-
sponse. The optimum values of these compensation
components will depend on a number of factors; includ-
ing input voltage, load current, inductor value and output
capacitance, as well as the ESR of the inductor and out-
put capacitor. The compensation values shown in Fig-
ure 4 will provide good loop stability and good transient
response over the full range of input voltage and output
load. They were chosen assuming a nominal inductor
value of 33
H.
Power Stage Component Selection
Recommended values and part numbers are given in Ta-
ble 1 for C
IN
, C
OUT
, L1 and D1 for two ranges of load cur-
rent. The ranges were selected based on the current
ratings for two common surface mount inductor sizes.
APPLICATION INFORMATION (cont.)
L1
33
H
+3.3V
D2
LOW BATT
V
+
LITHIUM-ION
CELL
2.5 4.3VDC
C1
0.039
F
D1
+
+
S1
OPEN =
SHUTDOWN
8
VOUT
7
BAT+
6
SWITCH
5
LOWBAT
1
COMP
2
VFB
3
BAT-
4
SD*
C
100
F
6.3V
UCC3954
C
100
F .
6.3V
V
Figure 3. Application Circuit Using Dominant Pole Compensation. Typical Values are Shown.
UDG-98009
Load Current
C
IN
C
OUT
L1
D1
I
OUT
< 200mA
47
F, 6.3V
AVX
TPSC476M006R0350
100
F, 6.3V
AVX
TPSC107M06R0150
33
H
Coilcraft
DO1608C-333
0.5A, 20V Schottky
Motorola
MBR0520LT1
I
OUT
> 200mA
100
F, 10V
AVX
TPSD107M010R0100
330
F, 6.3V
AVX
TPSE337M006R0100
33
H
Coilcraft
DO3316P-333
Coiltronics CTX33-4
1A, 30V Schottky
Motorola
MBRS130LT3
Table 1. Power Stage Component Selection Guide
6
UCC3954
Reducing Output Ripple for Noise Sensitive
Applications
In some applications it may be necessary to have very
low output voltage ripple. There are a number of ways to
achieve this goal. Since the ripple is dominated by the
ESR of the output filter capacitor, one way to reduce the
ripple is to put multiple low ESR capacitors in parallel.
However, this brute force method can be expensive and
take up excessive board real estate.
A more effective method of ripple reduction is shown in
Figure 4. By adding a small tantalum capacitor (C3) be-
tween the 3.3V output and the negative battery input
(BAT), both input and output voltage ripple are reduced.
This technique is a kind of ripple current cancellation
scheme, since the ripple voltage on these two nodes is
180 out of phase. Using this method, output ripple can
be reduced by up to 50%. As with the other filter capaci-
tors, it is imperative that stray inductance and resistance
in series with the capacitor be minimized for maximum
effectiveness. Note that this capacitor sees the sum of
the input and output voltages; therefore an absolute mini-
mum voltage rating of 10V is required.
For applications where extremely low output ripple is re-
quired, a small LC filter is recommended. This is shown
in Figure 5. The addition of a small inductor and filter ca-
pacitor will reduce the ripple well below what could be
achieved with capacitors alone. It is also very effective in
eliminating any high frequency noise spikes resulting
from the main output capacitor's ESL and the Schottky
diode's parasitic capacitance. The LC values shown will
provide significant ripple reduction while having a negligi-
ble effect on output regulation. Note that the corner fre-
quency of 41kHz was chosen to be well below the
200kHz switching frequency, but high enough to avoid
the loop crossover frequency, which is typically below
10kHz. This avoids loop stability issues in case the feed-
back is taken from the output of the LC filter. By leaving
the feedback (VOUT) connection point before the LC fil-
ter, the filter cap value can be increased to achieve even
higher ripple attenuation without affecting stability mar-
gin.
Figure 4. Application Circuit Showing Lead-Lag Compensation and Additional Cap to Reduce Output Ripple
Using Cancellation Technique.
See Table 1 for Suggested Component Values and Part Numbers
UDG-98010
Figure 5. LC Filter for Very Low Noise Applications
L: Coilcraft DO 1608C-102
C: Sprague 594D156X0025C2T
7
UCC3954
Very Low Profile Applications
The UCC3954 is available in a low profile (1.2mm) 14 pin
TSSOP package. The application circuit shown in Figure
6 is an example of a complete 200mA, 3.3V converter
which will fit within a 1.2mm max height envelope*. Note
that the low inductor value for L1 (10
H) requires a mini-
mum load of at least 1mA to guarantee output regulation.
Minimum Load
Note that the pulse width modulator within the UCC3954
cannot go to zero percent duty cycle. Therefore, it stores
a finite amount of energy in L1 every switching cycle.
Normally, this would prevent regulation under no-load
conditions. However, for inductor values greater than
15
H, no minimum load is required to maintain output
regulation. This is because the current drawn by the
VOUT pin, used for feedback and to bootstrap the inter-
nal MOSFET's gate drive, satisfies the minimum load re-
quirement. However, the higher peak current resulting
from inductor values below 15
H requires a small mini-
mum load to maintain output regulation. These lower
value inductors are not optimal, and will not be as effi-
cient due to the higher peak currents, but may be neces-
sary to reduce size in some applications, such as that of
Figure 6.
Low Battery Warn Output
The UCC3954 includes an open drain Low Battery Warn
output that turns on and pulls the LOWBAT pin down to
BAT when the battery input voltage drops to the
Low Bat threshold. This indicates that the battery voltage
is very low and approaching the UCC3954 turn off
threshold.
The LOWBAT output switch is designed to have a high
on-resistance, so an LED can be driven directly if de-
sired, with no current limiting resistor. The anode of the
LED can be connected to system ground (BAT+) or to
the +3.3V output (this will result in a higher LED current).
For systems where it is desired to read the LOWBAT out-
put as a digital signal referenced to the +3.3V ground, a
level shifter is needed. The circuit shown in Figure 7 is a
simple resistive level shifter, consisting of R1 and R2,
which provides a +3.3V compatible output. The output
will normally be pulled up to +3.3V until a low battery
condition exists, at which point it will be about 0.3V
above the 3.3V ground. Figure 8 shows the typical con-
verter efficiency for different loads as a function of input
voltage.
Figure 6. Application Circuit Using the 14 Pin TSSOP Package and Other Low Profile Components
to Achieve 1.2mm Overall Maximum Height.*
*The maximum height on D1 is 1.35mm.
UDG-98098
8
UCC3954
60%
65%
70%
75%
80%
85%
90%
2.7
3
3.3
3.6
3.9
4.2
V
IN
EFFICIENCY
0.3W
0.6W
1.0W
1.8W
2.3W
LOAD
Figure 8. Typical Efficiency as a Function of Input
Voltage and Load
UNITRODE CORPORATION
7 CONTINENTAL BLVD. MERRIMACK, NH 03054
TEL. (603) 424-2410 FAX (603) 424-3460
Figure 7. Simple Resistive Level Shifter
for the Low Battery Warn Output
UDG-98011
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