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LT1111 Micropower DC/DC Converter Adjustable and Fixed 5V, 12V
FEATURES
s s s s s s s s s s
DESCRIPTIO
Operates at Supply Voltages from 2V to 30V 72kHz Oscillator Works with Surface Mount Inductors Only Three External Components Required Step-Up or Step-Down Mode Low-Battery Detector Comparator On-Chip User Adjustable Current Limit Internal 1A Power Switch Fixed or Adjustable Output Voltage Versions Space Saving 8-Pin MiniDIP or SO-8 Package
The LT1111 is a versatile micropower DC/DC converter. The device requires only three external components to deliver a fixed output of 5V or 12V. Supply voltage ranges from 2V to 12V in step-up mode and to 30V in step-down mode. The LT1111 functions equally well in step-up, stepdown, or inverting applications. The LT1111 oscillator is set at 72kHz, optimizing the device to work with off-the-shelf surface mount inductors. The device can deliver 5V at 100mA from a 3V input in step-up mode or 5V at 200mA from a 12V input in stepdown mode. Switch current limit can be programmed with a single resistor. An auxiliary open-collector gain block can be configured as a low-battery detector, linear post regulator, undervoltage lock-out circuit, or error amplifier. For input sources of less than 2V use the LT1110.
APPLICATI
s s s s s s s s s s
S
3V to 5V, 5V to 12V Converters 9V to 5V, 12V to 5V Converters Remote Controls Peripherals and Add-On Cards Battery Backup Supplies Uninterruptible Supplies Laptop and Palmtop Computers Cellular Telephones Portable Instruments Flash Memory VPP Generators
TYPICAL APPLICATI
Typical Load Regulation
6 5
OUTPUT VOLTAGE (V)
All Surface Mount 3V to 5V Step-Up Converter
SUMIDA CD54-220M MBRS120T3 22H 3V INPUT
5V 100mA
4 3 2 1 0 0 25
VIN = 2V 2.2 2.4
I LIM
+
V IN SW1
10 F*
+
LT1111CS8-5 SENSE GND SW2
33 F
*OPTIONAL
LT1111 * TA01
U
2.6 2.8 3V 50 75 100 125 150 175 200
LT1111 * TA02
UO
UO
LOAD CURRENT (mA)
1
LT11 11 ABSOLUTE AXI U RATI GS
Operating Temperature Range LT1111C ............................................... 0C to 70C LT1111I ......................................... - 40C to 105C LT1111M ....................................... - 55C to 125C Storage Temperature Range ................ - 65C to 150C Lead Temperature (Soldering, 10 sec)................. 300C
Supply Voltage (VIN) ............................................... 36V SW1 Pin Voltage (VSW1) ......................................... 50V SW2 Pin Voltage (VSW2) ............................ - 0.5V to VIN Feedback Pin Voltage (LT1111) ............................. 5.5V Switch Current ....................................................... 1.5A Maximum Power Dissipation ............................ 500mW
PACKAGE/ORDER I FOR ATIO
TOP VIEW ILIM 1 VIN 2 SW1 3 SW2 4 J8 PACKAGE 8-LEAD CERAMIC DIP 8 7 6 5 FB (SENSE)* SET A0 GND
N8 PACKAGE 8-LEAD PLASTIC DIP
*FIXED VERSIONS
TJMAX = 150C, JA = 120C/W (J) TJMAX = 90C, JA = 130C/W (N)
ORDER PART NUMBER LT1111CN8 LT1111CN8-5 LT1111CN8-12 LT1111MJ8 LT1111MJ8-5 LT1111MJ8-12
Consult factory for Industrial grade parts
ELECTRICAL CHARACTERISTICS
SYMBOL IQ VIN PARAMETER Quiescent Current Input Voltage Comparator Trip Point Voltage VOUT Output Sense Voltage Comparator Hysteresis Output Hysteresis fOSC DC tON VSAT Oscillator Frequency Duty Cycle: Step-Up Mode Step-Down Mode Switch ON Time: Step-Up Mode Step-Down Mode SW Saturation Voltage, Step-Up Mode SW Saturation Voltage, Step-Down Mode IFB ISET VOL Feedback Pin Bias Current Set Pin Bias Current Gain Block Output Low
VIN = 3V, Military or Commercial Version
CONDITIONS Switch OFF Step-Up Mode Step-Down Mode LT1111 (Note 1) LT1111-5 (Note 2) LT1111-12 (Note 2) LT1111 LT1111-5 LT1111-12 Full Load ILIM Tied to VIN VOUT, = 5V, VIN = 12V VIN = 3.0V, ISW = 650mA VIN = 5.0V, ISW = 1A VIN = 12V, ISW = 650mA LT1111, VFB = 0V VSET = VREF ISINK = 300A, VSET = 1.00V
q q q q q q q q q q q
2
U
U
W
WW
U
W
TOP VIEW ILIM 1 VIN 2 SW1 3 SW2 4 8 7 6 5 FB (SENSE)* SET A0 GND
ORDER PART NUMBER LT1111CS8 LT1111CS8-5 LT1111CS8-12 S8 PART MARKING 1111 11115 11111
S8 PACKAGE 8-LEAD PLASTIC SO *FIXED VERSION
TJMAX = 90C, JA = 150C/W
MIN 2.0 1.20 4.75 11.40
TYP 300
MAX 400 12.6 30.0
UNITS A V V V V V mV mV mV kHz % % s s V V V nA nA V
1.25 5.00 12.00 8 32 75
1.30 5.25 12.60 12.5 50 120 88 59 50 9 7.8 0.65 1.0 1.5 120 300 0.4
54 43 24 5 3.3
72 50 34 7 5 0.5 0.8 1.1 70 70 0.15
LT1111
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER Reference Line Regulation AV ILIM Gain Block Gain Current Limit Current Limit Temperature Coefficient Switch OFF Leakage Current Maximum Excursion Below GND
VIN = 3V, Military or Commercial Version
CONDITIONS 5V VIN 30V 2V VIN 5V RL = 100k (Note 3) 220 from ILIM to VIN
q q q
MIN
TYP 0.02 0.20
MAX 0.075 0.400
UNITS %/V %/V V/V mA
%/C
1000
6000 400 - 0.3 1 - 400 10 - 350
Measured at SW1 Pin, VSW1 = 12V ISW1 10A, Switch OFF
A mV
VIN = 3V, - 55C TA 125C unless otherwise noted.
SYMBOL IQ fOSC DC tON PARAMETER Quiescent Current Oscillator Frequency Duty Cycle: Step-Up Mode Step-Down Mode Switch ON Time: Step-Up Mode Step-Down Mode Reference Line Regulation VSAT SW Saturation Voltage, Step-Up Mode SW Saturation Voltage, Step-Down Mode Full Load ILIM Tied to VIN VOUT = 5V, VIN = 12V 2V VIN 5V, 25C TA 125C 2.4V VIN 5V, TA = - 55C 0C TA 125C, ISW = 500mA, TA = - 55C, ISW = 400mA VIN = 12V, ISW = 500mA 0C TA 125C TA = - 55C CONDITIONS Switch OFF
q q q q q q
MIN 45 40 20 5 3
LT1111M TYP 300 72 50 7 0.2 0.5
MAX 500 100 62 55 11 9 0.4 0.8 0.65 1.5 2.0
UNITS A kHz % % s s %/V %/V V V V
VIN = 3V, 0C TA 70C unless otherwise noted.
SYMBOL IQ fOSC DC tON PARAMETER Quiescent Current Oscillator Frequency Duty Cycle: Step-Up Mode Step-Down Mode Switch ON Time: Step-Up Mode Step-Down Mode Reference Line Regulation VSAT SW Saturation Voltage, Step-Up Mode SW Saturation Voltage, Step-Down Mode Full Load ILIM Tied to VIN VOUT = 5V, VIN = 12V 2V VIN 5V VIN = 3V, ISW = 650mA VIN = 12V, ISW = 650mA CONDITIONS Switch OFF
q q q q q q q q q
MIN 300 54 43 24 5.0 3.3
LT1111C TYP 72 50 34 7 5 0.2 0.5 1.1
MAX 450 95 59 50 9.0 7.8 0.7 0.65 1.50
UNITS A kH % % s s %/V V V
The q denotes specifications which apply over the full operating temperature range. Note 1: This specification guarantees that both the high and low trip points of the comparator fall within the 1.20V to 1.30V range.
Note 2: The output voltage waveform will exhibit a sawtooth shape due to the comparator hysteresis. The output voltage on the fixed output versions will always be within the specified range. Note 3: 100k resistor connected between a 5V source and the A0 pin.
3
LT11 11
TYPICAL PERFOR A CE CHARACTERISTICS
Oscillator Frequency
100
75 74 73
OSCILLATOR FREQUENCY (KHz)
90 80 70 60 50 40 -50
FREQUENCY (KHz)
ON TIME (s)
9 12 15 18 21 24 27 30 INPUT VOLTAGE (V)
LT1111 * TPC02
-25
25 75 0 50 TEMPERATURE (C)
Duty Cycle
60 58 1.0 0.9
SATURATION VOLTAGE (V)
SATURATION VOLTAGE (V)
56
DUTY CYCLE (%)
54 52 50 48 46 44 42 40 -50 -25 50 75 0 25 TEMPERATURE (C) 100 125
Switch ON Voltage Step-Down Mode
2.00 1.75 VIN = 12V ISW = 650mA
1.4 1.2 1.0
OSCILLATOR FREQUENCY (KHz)
ON VOLTAGE (V)
ON VOLTAGE (V)
1.50 1.25 1.00 0.75 0.50 -50 -25
50 75 0 25 TEMPERATURE (C)
4
UW
100
LT1111 * TPC01
LT1111 * TPC04
Oscillator Frequency
10 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0
68 67 72 71 70 69
Switch ON Time
5.5
0 3 6
125
5.0 -50
-25
50 75 0 25 TEMPERATURE (C)
100
125
LT111 * TPC03
Saturation Voltage Step-Up Mode
1.4
VIN = 3V ISW = 650mA
Saturation Voltage Step-Up Mode
VIN = 3V 1.2 1.0 0.8 0.6 0.4 0.2 0 0 0.2 0.4 0.6 0.8 1.0 1.2 SWITCH CURRENT (A) 1.4 1.6 VIN = 2V
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -50 - 25 50 75 0 25 TEMPERATURE (C) 100 125
VIN = 5V
LT1111 * TPC05
LT1111 * TPC06
Switch ON Voltage Step-Down Mode
100
VIN = 12V
Minimum/Maximum Frequency vs ON Time
90 0C TA 70C 80 70 60 50 -55C TA 125C 40 4 5 8 9 6 7 10 SWITCH ON TIME (s) 11 12
0.8 0.6 0.4 0.2 0
100
125
0
0.2
0.8 0.4 0.6 SWITCH CURRENT (A)
1.0
LT1111 * TPC07
LT1111 * TPC08
LT1111 * TPC09
LT1111
TYPICAL PERFOR A CE CHARACTERISTICS
Quiescent Current
400 380
QUIESCENT CURRENT (A)
QUIESCENT CURRENT (A)
360 340 320 300 280 260 240 220 200 0 3 6 9 12 15 18 21 24 27 30 INPUT VOLTAGE (V)
LT1111 * TPC10
SWITCH CURRENT (A)
Set Pin Bias Current
100 90 80
BIAS CURRENT (nA)
60 50 40 30 20 10 0 -50 -25 0 25 50 75 TEMPERATURE (C) 100 125
BIAS CURRENT (nA)
70
PI FU CTI
S
GND (Pin 5): Ground. A0 (Pin 6): Auxiliary Gain Block (GB) Output. Open collector, can sink 300A. SET (Pin 7): GB Input. GB is an op amp with positive input connected to SET pin and negative input connected to 1.25V reference. FB/SENSE (Pin 8): On the LT1111 (adjustable) this pin goes to the comparator input. On the LT1111-5 and LT1111-12, this pin goes to the internal application resistor that sets output voltage.
ILIM (Pin 1): Connect this pin to VIN for normal use. Where lower current limit is desired, connect a resistor between ILIM and VIN. A 220 resistor will limit the switch current to approximately 400mA. VIN (Pin 2): Input Supply Voltage. SW1 (Pin 3): Collector of Power Transistor. For step-up mode connect to inductor/diode. For step-down mode connect to VIN. SW2 (Pin 4): Emitter of Power Transistor. For step-up mode connect to ground. For step-down mode connect to inductor/diode. This pin must never be allowed to go more than a Schottky diode drop below ground.
UW
Quiescent Current
500 450 400 350 300 250 200 150 100 -50 -25 0 25 50 75 TEMPERATURE (C) 100 125
1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
Maximum Switch Current vs RLIM
STEP-UP 2V VIN 5V
STEP-DOWN VIN = 12V
10
100 RLIM ()
1000
LT1111 * TPC12
LT1111 * TPC11
Feedback Bias Current
100 90 80 70 60 50 40 30 20 10 0 -50 -25 0 25 50 75 TEMPERATURE (C) 100 125
LT1111 * TPC13
LT1111 * TPC14
UO
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5
LT11 11
BLOCK DIAGRA S
LT1111
SET
+
A2 A0
V IN
V IN
-
GAIN BLOCK/ ERROR AMP I LIM SW1
1.25V REFERENCE
+
A1 OSCILLATOR
-
COMPARATOR GND FB
LT1111 OPERATI
The LT1111 is a gated oscillator switcher. This type architecture has very low supply current because the switch is cycled when the feedback pin voltage drops below the reference voltage. Circuit operation can best be understood by referring to the LT1111 block diagram. Comparator A1 compares the feedback (FB) pin voltage with the 1.25V reference signal. When FB drops below 1.25V, A1 switches on the 72kHz oscillator. The driver amplifier boosts the signal level to drive the output NPN power switch. The switch cycling action raises the output voltage and FB pin voltage. When the FB voltage is sufficient to trip A1, the oscillator is gated off. A small amount of hysteresis built into A1 ensures loop stability without external frequency compensation. When the comparator output is low, the oscillator and all high current circuitry is turned off, lowering device quiescent current to just 300A. The oscillator is set internally for 7s ON time and 7s OFF time, optimizing the device for circuits where VOUT and VIN differ by roughly a factor of 2. Examples include a 3V to 5V step-up converter or a 9V to 5V step-down converter.
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LT1111-5/LT1111-12
SET
+
A2 A0
-
GAIN BLOCK/ ERROR AMP I LIM SW1
1.25V REFERENCE
+
A1 OSCILLATOR DRIVER COMPARATOR SW2 SENSE LT1111-5: R1 = 73.5k LT1111-12: R1 = 25.5k
-
DRIVER SW2
LT1111 * BD01
R1
R2 220k
GND
LT1111 * BD02
Gain block A2 can serve as a low-battery detector. The negative input of A2 is the 1.25V reference. A resistor divider from VIN to GND, with the mid-point connected to the SET pin provides the trip voltage in a low-battery detector application. AO can sink 300A (use a 22k resistor pull-up to 5V). A resistor connected between the ILIM pin and VIN sets maximum switch current. When the switch current exceeds the set value, the switch cycle is prematurely terminated. If current limit is not used, ILIM should be tied directly to VIN. Propagation delay through the current limit circuitry is approximately 1s. In step-up mode the switch emitter (SW2) is connected to ground and the switch collector (SW1) drives the inductor; in step-down mode the collector is connected to VIN and the emitter drives the inductor. The LT1111-5 and LT1111-12 are functionally identical to the LT1111. The -5 and -12 versions have on-chip voltage setting resistors for fixed 5V or 12V outputs. Pin 8 on the fixed versions should be connected to the output. No external resistors are needed.
LT1111
APPLICATI
S I FOR ATIO
Inductor Selection -- General A DC/DC converter operates by storing energy as magnetic flux in an inductor core, and then switching this energy into the load. Since it is flux, not charge, that is stored, the output voltage can be higher, lower, or opposite in polarity to the input voltage by choosing an appropriate switching topology. To operate as an efficient energy transfer element, the inductor must fulfill three requirements. First, the inductance must be low enough for the inductor to store adequate energy under the worst case condition of minimum input voltage and switch-on time. The inductance must also be high enough so maximum current ratings of the LT1111 and inductor are not exceeded at the other worst case condition of maximum input voltage and ON time. Additionally, the inductor core must be able to store the required flux; i.e., it must not saturate. At power levels generally encountered with LT1111 based designs, small surface mount ferrite core units with saturation current ratings in the 300mA to 1A range and DCR less than 0.4 (depending on application) are adequate. Lastly, the inductor must have sufficiently low DC resistance so excessive power is not lost as heat in the windings. An additional consideration is ElectroMagnetic Interference (EMI). Toroid and pot core type inductors are recommended in applications where EMI must be kept to a minimum; for example, where there are sensitive analog circuitry or transducers nearby. Rod core types are a less expensive choice where EMI is not a problem. Minimum and maximum input voltage, output voltage and output current must be established before an inductor can be selected. Inductor Selection -- Step-Up Converter In a step-up, or boost converter (Figure 4), power generated by the inductor makes up the difference between input and output. Power required from the inductor is determined by:
PL = VOUT + V D - VIN MIN IOUT
(
)(
)
where VD is the diode drop (0.5V for a 1N5818 Schottky). Energy required by the inductor per cycle must be equal or greater than:
U
PL / f OSC (02)
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in order for the converter to regulate the output. When the switch is closed, current in the inductor builds according to:
-R t V IL ( t) = IN 1- e L R
(03)
where R is the sum of the switch equivalent resistance (0.8 typical at 25C) and the inductor DC resistance. When the drop across the switch is small compared to VIN, the simple lossless equation:
V I L t = IN t L
()
(04)
can be used. These equations assume that at t = 0, inductor current is zero. This situation is called "discontinuous mode operation" in switching regulator parlance. Setting "t" to the switch-on time from the LT1111 specification table (typically 7s) will yield IPEAK for a specific "L" and VIN. Once IPEAK is known, energy in the inductor at the end of the switch-on time can be calculated as:
EL =
12 LI 2 PEAK
(05)
EL must be greater than PL/fOSC for the converter to deliver the required power. For best efficiency IPEAK should be kept to 1A or less. Higher switch currents will cause excessive drop across the switch resulting in reduced efficiency. In general, switch current should be held to as low a value as possible in order to keep switch, diode and inductor losses at a minimum. As an example, suppose 12V at 60mA is to be generated from a 4.5V to 8V input. Recalling equation (01),
PL = 12 V + 0.5 V - 4.5 V 60mA = 480mW
(01)
(
)(
)
(06)
Energy required from the inductor is
PL f OSC
=
480mW = 6.7J 72kHz
(07)
7
LT11 11
APPLICATI
S I FOR ATIO
Picking an inductor value of 47H with 0.2 DCR results in a peak switch current of:
I PEAK =
-1.0 x 7s 4.5 V 1 - e 47H = 623mA . 1.0
Substituting IPEAK into Equation 04 results in:
1 E L = 47H 0.623 A 2 = 9.1J 2
(
)(
)
Since 9.1J > 6.7J, the 47H inductor will work. This trial-and-error approach can be used to select the optimum inductor. Keep in mind the switch current maximum rating of 1.5A. If the calculated peak current exceeds this, consider using the LT1110. The 70% duty cycle of the LT1110 allows more energy per cycle to be stored in the inductor, resulting in more output power. A resistor can be added in series with the ILIM pin to invoke switch current limit. The resistor should be picked so the calculated IPEAK at minimum VIN is equal to the Maximum Switch Current (from Typical Performance Characteristic curves). Then, as VIN increases, switch current is held constant, resulting in increasing efficiency. Inductor Selection -- Step-Down Converter The step-down case (Figure 5) differs from the step-up in that the inductor current flows through the load during both the charge and discharge periods of the inductor. Current through the switch should be limited to ~650mA in this mode. Higher current can be obtained by using an external switch (see Figure 6). The ILIM pin is the key to successful operation over varying inputs. After establishing output voltage, output current and input voltage range, peak switch current can be calculated by the formula: IPEAK = 2 I OUT DC V OUT + V D V - V SW + V D IN (10)
where DC = duty cycle (0.50) VSW = switch drop in step-down mode VD = diode drop (0.5V for a 1N5818)
8
U
IOUT = output current VOUT = output voltage VIN = minimum input voltage VSW is actually a function of switch current which is in turn a function of VIN, L, time, and VOUT. To simplify, 1.5V can be used for VSW as a very conservative value. Once IPEAK is known, inductor value can be derived from:
(09)
(08)
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L=
VIN MIN - V SW - V OUT x t ON I PEAK
(11)
where tON = switch-on time (7s). Next, the current limit resistor RLIM is selected to give IPEAK from the RLIM Step-Down Mode curve. The addition of this resistor keeps maximum switch current constant as the input voltage is increased. As an example, suppose 5V at 300mA is to be generated from a 12V to 24V input. Recalling Equation (10),
IPEAK = 2 300mA 5 + 0.5 12 - 1.5 + 0.5 = 600mA 0.50
(
)
(12)
Next, inductor value is calculated using Equation (11):
L= 12 - 1.5 - 5 7s = 64H. 600mA (13)
Use the next lowest standard value (56H). Then pick RLIM from the curve. For IPEAK = 600mA, RLIM = 56. Inductor Selection -- Positive-to-Negative Converter Figure 7 shows hookup for positive-to-negative conversion. All of the output power must come from the inductor. In this case, PL = (VOUT+ VD)(IOUT) (14)
In this mode the switch is arranged in common collector or step-down mode. The switch drop can be modeled as a 0.75V source in series with a 0.65 resistor. When the
LT1111
APPLICATI S I FOR ATIO U
(15)
(16) (17) capacitors provide still better performance at more expense. We recommend OS-CON capacitors from Sanyo Corporation (San Diego, CA). These units are physically quite small and have extremely low ESR. To illustrate, Figures 1, 2, and 3 show the output voltage of an LT1111 based converter with three 100F capacitors. The peak switch current is 500mA in all cases. Figure 1 shows a Sprague 501D, 25V aluminum capacitor. VOUT jumps by over 120mV when the switch turns off, followed by a drop in voltage as the inductor dumps into the capacitor. This works out to be an ESR of over 0.24. Figure 2 shows the same circuit, but with a Sprague 150D, 20V tantalum capacitor replacing the aluminum unit. Output jump is now about 35mV, corresponding to an ESR of 0.07. Figure 3 shows the circuit with a 16V OS-CON unit. ESR is now only 0.02.
(18)
50mV/DIV 5s/DIV
LT1111 * F01
switch closes, current in the inductor builds according to
IL t =
()
VL R
-R t 1- e L
where R = 0.65 + DCRL VL = VIN - 0.75V As an example, suppose -5V at 50mA is to be generated from a 4.5V to 5.5V input. Recalling Equation (14), PL = (-5V+0.5V)(50mA) = 275mW Energy required from the inductor is: 275mW PL = = 3.8J. 72kHz fOSC
Picking an inductor value of 56H with 0.2 DCR results in a peak switch current of:
IPEAK =
(4.5V - 0.75V) 1 - e-0.85 x 7s = 445mA . 56H (0.65 + 0.2)
Substituting IPEAK into Equation (04) results in:
1 E L = 56H 0.445 A 2 = 5.54J. 2
(
)(
)
Since 5.54J > 3.82J, the 56H inductor will work. With this relatively small input range, RLIM is not usually necessary and the ILIM pin can be tied directly to VIN. As in the step-down case, peak switch current should be limited to ~650mA. Capacitor Selection Selecting the right output capacitor is almost as important as selecting the right inductor. A poor choice for a filter capacitor can result in poor efficiency and/or high output ripple. Ordinary aluminum electrolytics, while inexpensive and readily available, may have unacceptably poor Equivalent Series Resistance (ESR) and ESL (inductance). There are low ESR aluminum capacitors on the market specifically designed for switch mode DC/DC converters which work much better than general-purpose units. Tantalum
50mV/DIV
50mV/DIV
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Figure 1. Aluminum
(19)
5s/DIV
LT1111 * F02
Figure 2. Tantalum
5s/DIV
LT1111 * F01
Figure 3. OS-CON
9
LT11 11
APPLICATI
Diode Selection Speed, forward drop, and leakage current are the three main considerations in selecting a catch diode for LT1111 converters. General purpose rectifiers such as the 1N4001 are unsuitable for use in any switching regulator application. Although they are rated at 1A, the switching time of a 1N4001 is in the 10s to 50s range. At best, efficiency will be severely compromised when these diodes are used; at worst, the circuit may not work at all. Most LT1111 circuits will be well served by a 1N5818 Schottky diode, or its surface mount equivalent, the MBRS130T3. The combination of 500mV forward drop at 1A current, fast turn ON and turn OFF time, and 4A to 10A leakage current fit nicely with LT1111 requirements. At peak switch currents of 100mA or less, a 1N4148 signal diode may be used. This diode has leakage current in the 1nA to 5nA range at 25C and lower cost than a 1N5818. (You can also use them to get your circuit up and running, but beware of destroying the diode at 1A switch currents.) Step-Up (Boost Mode) Operation A step-up DC/DC converter delivers an output voltage higher than the input voltage. Step-up converters are not short-circuit protected since there is a DC path from input to output. The usual step-up configuration for the LT1111 is shown in Figure 4. The LT1111 first pulls SW1 low causing VIN - VCESAT to appear across L1. A current then builds up in L1.
L1 V IN R3* I LIM V IN SW1 FB R2 D1 V OUT
LT1111 * F05
S I FOR ATIO
+
C1
LT1111
GND
SW2
R1
*OPTIONAL
Figure 4. Step-Up Mode Hookup. Refer to Table 1 for Component Values.
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At the end of the switch ON time the current in L1 is1:
IPEAK = VIN L t ON (20)
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Immediately after switch turn-off, the SW1 voltage pin starts to rise because current cannot instantaneously stop flowing in L1. When the voltage reaches VOUT + VD, the inductor current flows through D1 into C1, increasing VOUT. This action is repeated as needed by the LT1111 to keep VFB at the internal reference voltage of 1.25V. R1 and R2 set the output voltage according to the formula R2 VOUT = 1 + 1.25 V R1
(
)
(21)
Step-Down (Buck Mode) Operation A step-down DC/DC converter converts a higher voltage to a lower voltage. The usual hookup for an LT1111 based step-down converter is shown in Figure 5.
VIN R3 100
+
C2
I LIM
V IN
SW1 FB
LT1111 L1 SW2 GND D1 1N5818 R2 VOUT
+
C1 R1
Figure 5. Step-Down Mode Hookup
When the switch turns on, SW2 pulls up to VIN - VSW. This puts a voltage across L1 equal to VIN - VSW - VOUT, causing a current to build up in L1. At the end of the switch ON time, the current in L1 is equal to:
LT1111 * F04
IPEAK =
VIN - VSW - VOUT L
t ON
(22)
Note 1: This simple expression neglects the effect of switch and coil resistance. This is taken into account in the "Inductor Selection" section.
LT1111
APPLICATI S I FOR ATIO U
VIN 30V MAX R1 0.3 Q1 MJE210 OR ZETEX ZTX749 R2 220 R3 330 D1 1N5821 L1 VOUT
When the switch turns off, the SW2 pin falls rapidly and actually goes below ground. D1 turns on when SW2 reaches 0.4V below ground. D1 MUST BE A SCHOTTKY DIODE. The voltage at SW2 must never be allowed to go below -0.5V. A silicon diode such as the 1N4933 will allow SW2 to go to -0.8V, causing potentially destructive power dissipation inside the LT1111. Output voltage is determined by: R2 VOUT = 1 + 1.25 V R1
(
)
R3 programs switch current limit. This is especially important in applications where the input varies over a wide range. Without R3, the switch stays on for a fixed time each cycle. Under certain conditions the current in L1 can build up to excessive levels, exceeding the switch rating and/or saturating the inductor. The 100 resistor programs the switch to turn off when the current reaches approximately 700mA. When using the LT1111 in step-down mode, output voltage should be limited to 6.2V or less. Higher output voltages can be accommodated by inserting a 1N5818 diode in series with the SW2 pin (anode connected to SW2). Higher Current Step-Down Operation Output current can be increased by using a discrete PNP pass transistor as shown in Figure 6. R1 serves as a current limit sense. When the voltage drop across R1 equals a VBE, the switch turns off. For temperature compensation a Schottky diode can be inserted in series with the ILIM pin. This also lowers the maximum drop across R1 to VBE - VD, increasing efficiency. As shown, switch current is limited to 2A. Inductor value can be calculated based on formulas in the "Inductor Selection -- StepDown Converter" section with the following conservative expression for VSW:
VSW = V R1 + V Q1SAT 1.0 V (24)
R2 provides a current path to turn off Q1. R3 provides base drive to Q1. R4 and R5 set output voltage. A PMOS FET can be used in place of Q1 when VIN is between 10V and 20V.
W
U
UO
+
C2
VIN LT1111
IL SW1
+
C1
R4 FB GND SW2 R5 VOUT = 1.25V
(23)
(1 + R4 ) R5
LT1111 * TA08
Figure 6. Q1 Permits Higher Current Switching. LT1111 Functions as Controller.
Inverting Configurations The LT1111 can be configured as a positive-to-negative converter (Figure 7), or a negative-to-positive converter (Figure 8). In Figure 7, the arrangement is very similar to a step-down, except that the high side of the feedback is referred to ground. This level shifts the output negative. As in the step-down mode, D1 must be a Schottky diode, and VOUTshould be less than 6.2V. More negative output voltages can be accommodated as in the prior section.
VIN R3 I LIM V IN SW1 FB C2 LT1111 L1 SW2 GND D1 1N5818 R1
+
+
C1 R2 -VOUT
LT1111 * F07
Figure 7. Positive-to-Negative Converter
In Figure 8, the input is negative while the output is positive. In this configuration, the magnitude of the input voltage can be higher or lower than the output voltage. A level shift, provided by the PNP transistor, supplies proper polarity feedback information to the regulator.
11
LT11 11
APPLICATI
S I FOR ATIO
L1 D1
+
C1 I LIM VIN SW1
+
C2 A0 GND -VIN
LT1111 FB SW2 R2
LT1111 * F08
VOUT = R1 1.25V + 0.6V R2
()
Figure 8. Negative-to-Positive Converter
Using the ILIM Pin The LT1111 switch can be programmed to turn off at a set switch current, a feature not found on competing devices. This enables the input to vary over a wide range without exceeding the maximum switch rating or saturating the inductor. Consider the case wh ere analysis shows the LT1111 must operate at an 800mA peak switch current with a 2V input. If VIN rises to 4V, the peak switch current will rise to 1.6A, exceeding the maximum switch current rating. With the proper resistor selected (see the "Maximum Switch Current vs ILIM" characteristic), the switch current will be limited to 800mA, even if the input voltage increases. Another situation where the ILIM feature is useful occurs when the device goes into continuous mode operation. This occurs in step-up mode when:
IL
VOUT + VDIODE 1 < VIN - VSW 1 - DC
When the input and output voltages satisfy this relationship, inductor current does not go to zero during the switch OFF time. When the switch turns on again, the current ramp starts from the non-zero current level in the inductor just prior to switch turn-on. As shown in Figure 9, the inductor current increases to a high level before the comparator turns off the oscillator. This high current can cause excessive output ripple and requires oversizing the output capacitor and inductor. With the ILIM feature, however, the switch current turns off at a programmed level as shown in Figure 10, keeping output ripple to a minimum.
12
U
VOUT R1 2N3906 IL SWITCH ON OFF
LT1111 * F09
W
U
UO
Figure 9. No Current Limit Causes Large Inductor Current Build-Up
PROGRAMMED CURRENT LIMIT
SWITCH
ON OFF
LT1111 * F10
Figure 10. Current Limit Keeps Inductor Current Under Control
(25)
Figure 11 details current limit circuitry. Sense transistor Q1, whose base and emitter are paralleled with power switch Q2, is ratioed such that approximately 0.5% of Q2's collector current flows in Q1's collector. This current is passed through internal 80 resistor R1 and out through the ILIM pin. The value of the external resistor connected between ILIM and VIN sets the current limit. When sufficient switch current flows to develop a VBE across R1 + RLIM, Q3 turns on and injects current into the oscillator, turning off the switch. Delay through this circuitry is approximately 1s. The current trip point becomes less accurate for switch ON times less than 3s. Resistor values programming switch ON time for 1s or less will cause spurious response in the switch circuitry although the device will still maintain output regulation.
RLIM (EXTERNAL) VIN Q3 DRIVER OSCILLATOR Q1 ILIM R1 80 (INTERNAL) SW1 Q2 SW2
LT1111 * F11
Figure 11. LT1111 Current Limit Circuitry
LT1111
APPLICATI S I FOR ATIO U
when the trip point is reached. Values in the 1M to 10M range are optimal. However, the addition of R3 will change the trip point.
5V LT1111 R1 1.25V REF SET V IN 47k
Using the Gain Block The gain block (GB) on the LT1111 can be used as an error amplifier, low-battery detector or linear post regulator. The gain block itself is a very simple PNP input op amp with an open collector NPN output. The negative input of the gain block is tied internally to the 1.25V reference. The positive input comes out on the SET pin. Arrangement of the gain block as a low-battery detector is straightforward. Figure 12 shows hookup. R1 and R2 need only be low enough in value so that the bias current of the SET input does not cause large errors. 33k for R2 is adequate. R3 can be added to introduce a small amount of hysteresis. This will cause the gain block to "snap"
Table 1. Component Selection for Common Converters
INPUT VOLTAGE 2 to 3.1 2 to 3.1 2 to 3.1 2 to 3.1 5 5 6.5 to 11 12 to 20 20 to 30 5 12 S = Sumida C = Coiltronics OUTPUT VOLTAGE 5 5 12 12 12 12 5 5 5 -5 -5 OUTPUT CURRENT (MIN) 90mA 10mA 30mA 10mA 90mA 30mA 50mA 300mA 300mA 75mA 250mA CIRCUIT FIGURE 4 4 4 4 4 4 5 5 5 6 6
* Add 47 from ILIM to VIN ** Add 220 from ILIM to VIN
Table 2. Inductor Manufacturers
MANUFACTURER Coiltronics Incorporated 6000 Park of Commerce Blvd. Boca Raton, FL 33487 407-241-7876 Toko America Incorporated 1250 Feehanville Drive Mount Prospect, IL 60056 312-297-0070 Sumida Electric Co. USA 708-956-0666 PART NUMBERS CTX100-4 Series Surface Mount
W
U
UO
-
A0 TO PROCESSOR VLB - 1.25V 35.1A VLB = BATTERY TRIP POINT R2 = 33k R3 = 1.6M R1 =
LT1111 * F12
VBAT
+
GND R3
R2
Figure 12. Setting Low-Battery Detector Trip Point
INDUCTOR VALUE 15H 47H 15H 47H 33H 47H 15H 56H 120H 56H 120H
INDUCTOR PART NUMBER S CD75-750K S CD54-470K, C CTX50-1 S CD75-150K S CD54-470K, C CTX50-1 S CD75-330K S CD75-470K, C CTX50-1 S CD54-150K S CD105-560K, C CTX50-4 S CD105-121K, C CTX100-4 S CD75-560K, C CTX50-4 S CD105-121K, C CTX100-4
CAPACITOR VALUE 33F 10F 22F 10F 22F 15F 47F 47F 47F 47F 100F
NOTES *
** ** ** **
Table 3. Capacitor Manufacturers
MANUFACTURER Sanyo Video Components 1201 Sanyo Avenue San Diego, CA 92073 619-661-6322 Nichicon America Corporation 927 East State Parkway Schaumberg, IL 60173 708-843-7500 Sprague Electric Company Lower Main Street Sanford, ME 04073 207-324-4140 Matsuo 714-969-2491 PART NUMBERS OS-CON Series
Type 8RBS
PL Series
CD54 CDR74 CDR105 Surface Mount
150D Solid Tantalums 550D Tantalex
267 Series Surface Mount
13
LT11 11
TYPICAL APPLICATI UO
+
22F
S
3V to - 22V LCD Bias Generator
L1* 27H R1 100 I LIM V IN SW1 732k 1% 1N4148
2 x 1.5V CELLS
3V
LT1111 0.1F FB GND SW2
+
4.7F 39.2k 1%
MBRS130T3
MBRS130T3
+
22F * L1 = SUMIDA CD54-270K FOR 5V INPUT CHANGE R1 TO 47. CONVERTER WILL DELIVER -22V AT 40mA. 220k -22V OUTPUT 7mA AT 2V INPUT
LT1111 * TA03
9V to 5V Step-Down Converter
VIN 12V TO 28V
100 ILIM 9V BATTERY V IN SW1 LT1111-5 SENSE GND SW2 L1* 15H 5V OUTPUT 150mA AT 9V INPUT 50mA AT 6.5V INPUT
20V to 5V Step-Down Converter
100 ILIM V IN SW1 LT1111-5 SENSE GND SW2 L1* 68H
MBRS130T3
+
MBRS130T3
5V OUTPUT 300mA 47F
* L1 = SUMIDA CD54-150K
LT1111 * TA04
* L1 = SUMIDA CD74-680M
LT1111 * TA06
14
LT1111
TYPICAL APPLICATI UO
V IN GND
S
5V to -5V Converter
100 I LIM V IN SW1 LT1111-5 SENSE GND SW2 L1* 33H
VIN 5V INPUT
+
22F
MBRS130T3 33F
+
-5V OUTPUT 75mA
* L1 = SUMIDA CD54-330K
LT1111 * TA05
Voltage Controlled Positive-to-Negative Converter
VIN 5V TO 12V 0.22 ZETEX ZTX788A L1* 20H, 3A
+
BAT54 220 220 MBRD320 47F -VOUT = -5.13 x VC 2W MAXIMUM OUTPUT V IN 200k 39k VC (0V TO 5V)
ILIM SW1 LT1111 FB SW2
LT1006
* L1 = COILTRONICS CTX20-4 ZETEX INC. 516-543-7100
High Power, Low Quiescent Current Step-Down Converter
VIN 8V TO 18V 0.22 MTM20P08 L1* 10H, 3A 5V 500mA
BAT54
2k 2N3904
51
V IN
ILIM SW1 LT1111 FB 1N4148 121k
GND
SW2
40.2k * L1 = SUMIDA CDR105-100M
OPERATE STANDBY
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
- +
LT1111 * TA07
MBRD320
+
220F
LT1111 * TA20
15
LT11 11
PACKAGE DESCRIPTIO U
Dimensions in inches (millimeters) unless otherwise noted. J8 Package 8-Lead Ceramic DIP
CORNER LEADS OPTION (4 PLCS) 0.290 - 0.320 (7.366 - 8.128) 0.023 - 0.045 (0.584 - 1.143) HALF LEAD OPTION 0.045 - 0.068 (1.143 - 1.727) FULL LEAD OPTION 0.045 - 0.068 (1.143 - 1.727) 0.014 - 0.026 (0.360 - 0.660) 0.405 (10.287) MAX 8 7 6 5
0.200 (5.080) MAX 0.015 - 0.060 (0.381 - 1.524)
0.005 (0.127) MIN
0.008 - 0.018 (0.203 - 0.457) 0.385 0.025 (9.779 0.635)
0.025 (0.635) RAD TYP 1 2 3
0.220 - 0.310 (5.588 - 7.874)
0 - 15
4
0.125 3.175 0.100 0.010 MIN (2.540 0.254)
NOTE: LEAD DIMENSIONS APPLY TO SOLDER DIP OR TIN PLATE LEADS.
N8 Package 8-Lead Plastic DIP
0.400 (10.160) MAX 8 7 6 5
0.300 - 0.320 (7.620 - 8.128)
0.045 - 0.065 (1.143 - 1.651)
0.130 0.005 (3.302 0.127)
0.009 - 0.015 (0.229 - 0.381)
0.065 (1.651) TYP 0.125 (3.175) MIN 0.020 (0.508) MIN
0.250 0.010 (6.350 0.254)
(
+0.025 0.325 -0.015 8.255 +0.635 -0.381
)
0.045 0.015 (1.143 0.381) 0.100 0.010 (2.540 0.254)
1
2
3
4
0.018 0.003 (0.457 0.076)
S8 Package 8-Lead Plastic SOIC
0.189 - 0.197 (4.801 - 5.004) 0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254) 0- 8 TYP 0.228 - 0.244 (5.791 - 6.197) 0.150 - 0.157 (3.810 - 3.988) 0.053 - 0.069 (1.346 - 1.752) 0.004 - 0.010 (0.101 - 0.254) 8 7 6 5
0.016 - 0.050 0.406 - 1.270
0.014 - 0.019 (0.355 - 0.483)
0.050 (1.270) BSC
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006 INCH (0.15mm).
1
2
3
4
SO8 0294
16
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900 q FAX: (408) 434-0507 q TELEX: 499-3977
LT/GP 0594 5K REV C * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 1994

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