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UCC3954
PRELIMINARY INFORMATION
Single Cell Lithium-Ion to +3.3V Converter
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
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 frequency 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 levels. 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 predefined 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 activated when the battery is discharged to 2.55V. In lockout mode, the part consumes 15A. 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 series 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.
BLOCK DIAGRAM
UDG-96137-1
6/98
UCC3954
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
CONNECTION DIAGRAMS
DIL-8, SOIC-8 (Top View) D Package, N Package
Unless otherwise indicated, voltages are reference to BAT- and currents are positive into, negative out of the specified terminal. Pulsed is defined as a less than 10% duty cycle with a maximum duration of 500s. Consult Packaging Section of Databook for thermal limitations and considerations of packages.
SOIC-14 (Top View) PW Package
ELECTRICAL CHARACTERISTICS: Unless otherwise specified, TA = -20C to 70C for the UCC3954, SD = VBAT+ = 3.5V (ref. to VBAT-), VOUT = 3.3 (ref. to VBAT+). TA = TJ
PARAMETER Input Supply Supply Current (total) - active Supply Current (BAT+)- Shutdown Supply Current (BAT+) -UVLO BAT+ Turn On Threshold BAT+ Turn Off Threshold Low BAT+ Indicate Threshold Error Amplifier Output Voltage High Output Voltage Low VOUT Regulation Voltage Maximum Duty Cycle, IOH = 1ma Minimum Duty Cycle, IOL = 1ma TA = 25C 2.0 0 3.22 3.20 2.4 0.14 3.3 3.3 0.5 3.38 3.39 V V V V With Respect to BAT+ Turnoff With Respect to BAT+ Turnoff 250 2.35 50 IBAT+ + IVOUT VSDB = 0V (reference to BAT-) 1 0.2 30 300 2.55 100 2 5 40 375 2.75 325 mA A A mV V mV TEST CONDITIONS MIN TYP MAX UNITS
2
UCC3954
ELECTRICAL CHARACTERISTICS: Unless otherwise specified, TA = -20C to 70C for the UCC3954, SD = VBAT+ = 3.5V (ref. to VBAT-), VOUT = 3.3 (ref. to VBAT+). TA = TJ
PARAMETER Oscillator/PWM Intital Accurancy PWM Modulator Gain PWM Maximum Duty Cycle PWM Minimum Duty Cycle Shutdown Disable Threshold Lowbat On Resistance Soft Start Rise TIme Output Switch Saturation Voltage Overcurrent Threshold ISWITCH = 200mA Note 2 2.0 30 3.0 70 3.5 mV Amps Note 2, RLOAD = 33, CCOMP = 39nF, CLOAD = 330F 10 msec VLOWBAT = 1V 40 100 220 Reference to BAT- 0.8 1.5 2.5 V TA = 25C VCOMP = 1.6V to 2V 180 175 40 65 200 200 50 75 3 220 225 60 85 5 kHz kHz %/V % % TEST CONDITIONS MIN TYP MAX UNITS
Note 1: VBAT+ <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 CCOMP 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 VOUT . 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 VOUT, 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.
3
UCC3954
UDG-97098
Figure 1. Simplified Circuit Diagram
APPLICATION INFORMATION
Circuit Topology The UCC3954 uses a fixed frequency (200KHz), voltage mode PWM flyback topology. It can operate from a battery input voltage that is above or below the output voltage 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 excessive input ripple, causing the input voltage to drop below the UCC3954's shutdown threshold. The other parts required are a compensation capacitor, inductor, Schottky diode and output filter capacitor. The output filter capacitor should also be a good low ESR/ESL capacitor. 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 currents. This also reduces the demands on the output filter 4 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, making it more difficult to compensate. For the input voltage range and fixed operating frequency of the UCC3954, an inductor value of around 33H 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
UDG-97099
Figure 2. Current Waveforms
UCC3954
V LITHIUM-ION CELL 2.5 - 4.3VDC
-
UCC3954
L1 33H 8 + C 100F 6.3V +3.3V
1 COMP
VOUT
2 VFB C1 0.039F 3 C 100F . 6.3V S1 + OPEN = SHUTDOWN 4 SD* BAT-
BAT+ 7 D1 SWITCH 6 V +
LOWBAT
5 D2 LOW BATT
UDG-98009
Figure 3. Application Circuit Using Dominant Pole Compensation. Typical Values are Shown.
APPLICATION INFORMATION (cont.)
OSCON's are recommended. Surface mounting will eliminate the lead inductance. Suggested values and part numbers for COUT 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 between COMP and VFB. The value shown in the Application Circuit of Figure 3 provides good stability margin over a wide range of load, using the values shown for L1 and COUT. 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 response. The optimum values of these compensation components will depend on a number of factors; including input voltage, load current, inductor value and output capacitance, as well as the ESR of the inductor and output capacitor. The compensation values shown in Figure 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 33H. Power Stage Component Selection Recommended values and part numbers are given in Table 1 for CIN, COUT, L1 and D1 for two ranges of load current. The ranges were selected based on the current ratings for two common surface mount inductor sizes.
Load Current IOUT < 200mA
CIN 47F, 6.3V AVX TPSC476M006R0350 100F, 10V AVX TPSD107M010R0100
COUT 100F, 6.3V AVX TPSC107M06R0150 330F, 6.3V AVX TPSE337M006R0100
L1 33H Coilcraft DO1608C-333 33H Coilcraft DO3316P-333 Coiltronics CTX33-4
D1 0.5A, 20V Schottky Motorola MBR0520LT1 1A, 30V Schottky Motorola MBRS130LT3
IOUT > 200mA
Table 1. Power Stage Component Selection Guide
5
UCC3954
UDG-98010
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
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) between 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 capacitors, 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 minimum voltage rating of 10V is required. For applications where extremely low output ripple is required, a small LC filter is recommended. This is shown in Figure 5. The addition of a small inductor and filter capacitor will reduce the ripple well below what could be achieved with capacitors alone. It is also very effective in 6
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 negligible effect on output regulation. Note that the corner frequency 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 feedback is taken from the output of the LC filter. By leaving the feedback (VOUT) connection point before the LC filter, the filter cap value can be increased to achieve even higher ripple attenuation without affecting stability margin.

L: Coilcraft DO 1608C-102 C: Sprague 594D156X0025C2T
Figure 5. LC Filter for Very Low Noise Applications
UCC3954

UDG-98098
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.
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 (10H) requires a minimum 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 15H, 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 internal MOSFET's gate drive, satisfies the minimum load requirement. However, the higher peak current resulting from inductor values below 15H requires a small minimum load to maintain output regulation. These lower value inductors are not optimal, and will not be as efficient due to the higher peak currents, but may be necessary 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 desired, 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 output 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 converter efficiency for different loads as a function of input voltage.
7
UCC3954
LOAD 90% 85% 80% 75% 70% 65% 60% 2.7
UDG-98011
0.3W
0.6W
1.0W
1.8W
2.3W
EFFICIENCY
3
3.3
VIN
3.6
3.9
4.2
Figure 7. Simple Resistive Level Shifter for the Low Battery Warn Output
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
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