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LT1108 Micropower DC/DC Converter Adjustable and Fixed 5V, 12V
FEATURES
s s s s s s s s s
DESCRIPTIO
Operates at Supply Voltages from 2V to 30V Consumes Only 110A Supply Current Works in Step-Up or Step-Down Mode Only Four External Components Required 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 S8 Package
The LT1108 is a versatile micropower DC/DC converter. The device requires only four 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 LT1108 functions equally well in step-up, stepdown, or inverting applications. The LT1108 is pin-for-pin compatible with the LT1173, but has a duty cycle of 70%, resulting in increased output current in many applications. The LT1108 can deliver 150mA at 5V from a 2 AA cell input and 5V at 300mA from 9V in step-down mode. Quiescent current is just 110A, making the LT1108 ideal for power conscious batteryoperated systems. Switch current limit can be programmed with a single resistor. An auxiliary gain block can be configured as a low battery detector, linear post regulator, undervoltage lockout circuit, or error amplifier.
APPLICATI
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S
Palmtop Computers 3V to 5V, 5V to 12V Converters 9V to 5V, 12V to 5V Converters LCD Bias Generators Peripherals and Add-On Cards Battery Backup Supplies Cellular Telephones Portable Instruments
TYPICAL APPLICATI
Palmtop Computer Logic Supply
L1* 100H 47 ILIM 2 x AA CELLS VIN SW1
EFFICIENCY (%)
84 1N5817 5V 150mA 82 80 78 76 74 72 70 VIN = 3V VIN = 2.5V VIN = 2V
+
100F
+
LT1108-5 SENSE GND SW2
AVX TPS 330F 6.3V
*L1 = GOWANDA GA20-103K COILTRONICS CTX100-4 SUMIDA CD105-101K
LT1108 * TA01
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Efficiency
1 10 100 LOAD CURRENT (mA)
LT1108 * TA02
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1
LT1108 ABSOLUTE AXI U RATI GS
Maximum Power Dissipation ............................ 500mW Maximum Switch Current ...................................... 1.5A Operating Temperature Range .................... 0C to 70C 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 (LT1108) ............................. 5.5V Sense Pin Voltage (LT1108, -5, -12) ...................... 36V
PACKAGE/ORDER I FOR ATIO
TOP VIEW ILIM 1 VIN 2 SW1 3 SW2 4 8 7 6 5 FB (SENSE*) SET A0 GND
ORDER PART NUMBER LT1108CN8 LT1108CN8-5 LT1108CN8-12
ILIM 1 VIN 2 SW1 3 SW2 4
N8 PACKAGE 8-LEAD PLASTIC DIP *FIXED VERSIONS
TJMAX = 90C, JA = 130C/W
ELECTRICAL CHARACTERISTICS
SYMBOL IQ PARAMETER Quiescent Current Quiescent Current, Boost Mode Configuration VIN Input Voltage Comparator Trip Point Voltage VOUT Output Sense Voltage Comparator Hysteresis Output Hysteresis fOSC tON Oscillator Frequency Duty Cycle Switch-ON Time Feedback Pin Bias Current Set Pin Bias Current VOL Gain Block Output Low Reference Line Regulation VSAT SWSAT Voltage, Step-Up Mode
TA = 25C, VIN = 3V, unless otherwise noted.
CONDITIONS Switch OFF No Load Step-Up Mode Step-Down Mode LT1108 (Note 1) LT1108-5 (Note 2) LT1108-12 (Note 2) LT1108 LT1108-5 LT1108-12 Full Load, Step-Up Mode ILIM Tied to VIN, Step-Up Mode LT1108, VFB = 0V VSET = VREF ISINK = 100A, VSET = 1V 2V VIN 5V 5V VIN 30V VIN = 3V, ISW = 650mA VIN = 5V, ISW = 1A LT1108-5 LT1108-12
q q q q q q q q q q q q q q q q q q
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WW
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TOP VIEW 8 7 6 5 FB (SENSE*) SET A0 GND
ORDER PART NUMBER LT1108CS8 LT1108CS8-5 LT1108CS8-12 S8 PART MARKING 1108 10805 10812
S8 PACKAGE 8-LEAD PLASTIC SOIC *FIXED VERSIONS
TJMAX = 90C, JA = 150C/W
MIN
TYP 110 135 250
MAX 150
UNITS A A A
2 1.2 4.75 11.4 1.245 5 12 5 20 50 14 63 28 19 70 36 10 20 0.15 0.20 0.02 0.5 0.8
12.6 30.0 1.3 5.25 12.6 10 40 100 25 78 48 50 100 0.4 0.400 0.075 0.65 1.00
V V V V V mV mV mV kHz % s nA nA V %/V %/V V V
LT1108
ELECTRICAL CHARACTERISTICS
SYMBOL VSAT AV PARAMETER SWSAT Voltage, Step-Down Mode Gain Block Gain Current Limit Current Limit Temperature Coefficient Switch OFF Leakage Current VSW2 Maximum Excursion Below GND
TA = 25C, VIN = 3V, unless otherwise noted.
CONDITIONS VIN = 12V, ISW = 650mA
q
MIN
TYP 1.1
MAX 1.5 1.7
UNITS V V V/ V mA %/C
RL = 100k (Note 3) 220 from ILIM to VIN
q
400
1000 400 - 0.3 1 - 400 10 - 350
q
Measured at SW1 Pin ISW1 10A, Switch OFF
A mV
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 1.2V to 1.3V 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.
TYPICAL PERFOR A CE CHARACTERISTICS
Saturation Voltage Step-Up Mode (SW2 Pin Grounded)
1.2 1.0
SWITCH ON VOLTAGE (V)
SWITCH CURRENT (mA)
VIN = 3V 0.8
VCESAT (V)
VIN = 2V 0.6 VIN = 5V 0.4 0.2 0
0
0.2
0.4 0.6 0.8 SWITCH CURRENT (A)
Saturation Voltage Step-Up Mode (SW2 Pin Grounded)
1000 900 800
SWITCH CURRENT (mA)
VOUT = 5V
QUIESCENT CURRENT (A)
700 600 500 400 300 200 100 0 100
SUPPLY CURRENT (mA)
VIN = 24V L = 500H
VIN = 12V L = 250H
R LIM ()
LT1108 * TPC04
UW
1.0 1.2
LT1108 * TPC01
Switch ON Voltage Step-Down Mode (SW1 Pin Connected to VIN)
1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0 0.1 0.2 0.5 0.6 SWITCH CURRENT (A) 0.3 0.4 0.7 0.8
LT1108 * TPC02
Maximum Switch Current vs RLIM
1200 1100 1000 900 800 700 600 500 400 300 200 100 10 100 RLIM () 1000
LT1108 * TPC03
2V VIN 5V
Supply Current vs Switch Current
50
Quiescent Current
120 115
40
110 105 100 95 90 85
30 VIN = 5V
20
10
VIN = 2V
1000
0
0
200
600 800 400 SWITCH CURRENT (mA)
1000
80 -50
-25
0 50 25 TEMPERATURE (C)
75
100
LTC1108 * TPC05
LT1108 * TPC06
3
LT1108
TYPICAL PERFOR A CE CHARACTERISTICS
Oscillator Frequency
22 21 20 SWITCH-ON TIME (s)
FREQUENCY (kHz)
DUTY CYCLE (%)
19 18 17 16 15
14 13 -50 -25 0 50 25 TEMPERATURE (C) 75 100
Minimum/Maximum Frequency vs ON-Time
28 26 24
0.8 0.7 0.6
FREQUENCY (kHz)
22 VCESAT (V) 20 18 16 14 12 10 0 25 30 40 35 ON-TIME (s) 45 50
VSAT (V)
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S
GND (Pin 5): Ground. AO (Pin 6): Auxiliary gain block (GB) output. Open collector, can sink 100A. SET (Pin 7): GB input. GB is an op amp with positive input connected to SET pin and negative input connected to 1.245V reference. FB/SENSE (Pin 8): On the LT1108 (adjustable) this pin goes to the comparator input. On the LT1108-5 and LT1108-12, 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.
4
UW
LT1108 * TPC07
Duty Cycle
80 75 70 65 60 55 50 -50
44 42 40 38 36 34 32
Switch-ON Time
-25
0 50 25 TEMPERATURE (C)
75
100
30 -50
-25
50 25 0 TEMPERATURE (C)
75
100
LT1108 * TPC08
LT1108 * TPC09
Switch Saturation Voltage Step-Up Mode
1.8
ISW = 650mA
Switch Saturation Voltage Step-Down Mode
1.7 1.6 1.5 ISW = 650mA
0.5 0.4 0.3 0.2 0.1 0 -50 -25 0 50 25 TEMPERATURE (C) 75 100
1.4 1.3 1.2 1.1 1.0 0.9 0.8 -50 -25 25 50 0 TEMPERATURE (C) 75 100
LT1108 * TPC10
LT1108 * TPC11
LT1108 * TPC12
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LT1108
1 OPERATI
The LT1108 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 LT1108 block diagram. Comparator A1 compares the feedback (FB) pin voltage with the 1.245V reference signal. When FB drops below 1.245V, A1 switches on the 19kHz 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 110A. The oscillator is set internally for 36s ON-time and 17s OFF-time, allowing continuous mode operation in many cases such as 2V to 5V converters. Continuous mode greatly increases available output power. Gain block A2 can serve as a low battery detector. The
BLOCK DIAGRA S
LT1108 LT1108-5/LT1108-12
SET A2 VIN GAIN BLOCK/ ERROR AMP 1.245V REFERENCE A1 OSCILLATOR DRIVER GND FB COMPARATOR
LT1108 * BD
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negative input of A2 is the 1.245V 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. A0 can sink 100A (use a 47k 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 currentlimit circuitry is approximately 2s. 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 LT1108-5 and LT1108-12 are functionally identical to the LT1108. 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.
SET A0 VIN ILIM SW1 1.245V REFERENCE A1 OSCILLATOR DRIVER COMPARATOR SW2 R1 GND R2 753k SENSE LT1108-5: R1 = 250k LT1108-12: R1 = 87.4k
LT1108-5 * BD
A2
A0
GAIN BLOCK/ ERROR AMP
ILIM
SW1
SW2
5
LT1108
APPLICATI
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 LT1108 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 LT1108 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 Electro-Magnetic 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. Step-Up Converter In a step-up, or boost converter (Figure 1), power generated by the inductor makes up the difference between input and output. Power required from the inductor is determined by
S I FOR ATIO
INDUCTOR SELECTION
PL = VOUT + V D - VIN MIN IOUT
(
)(
)
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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
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PL / f OSC
in order for the converter to regulate the output.
(02)
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 IL 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 LT1108 specification table (typically 36s) 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 30mA is to be generated from a 2V to 3V input. Recalling equation (01),
PL = 12V + 0.5V - 2V 30mA = 315mW
(01)
(
)(
)
(06)
LT1108
APPLICATI
PL f OSC =
S I FOR ATIO
Energy required from the inductor is
315mW = 16.6J 19kHz
Picking an inductor value of 100H with 0.2 DCR results in a peak switch current of
2V I PEAK = 1.0
-1.0 x 36s 1 - e 100H = 605mA
Substituting IPEAK into Equation 04 results in
EL = 1 100H 6.605 A 2 = 18.3J 2
(
)(
)
Since 18.3J > 16.6J, the 100H 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, an external power transistor can be used. 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. Step-Down Converter The step-down case (Figure 2) 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 3). 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)
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(07)
where DC = duty cycle (0.60) VSW = switch drop in step-down mode VD = diode drop (0.5V for a 1N5818) 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)
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(08)
L=
VIN MIN - V SW - V OUT I PEAK
x t ON
(11)
where tON = switch-ON time (36s). 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 = 500mA 0.60
(
)
(12)
Next, inductor value is calculated using Equation (11)
L= 12 - 1.5 - 5 36s = 396H 500mA (13)
Use the next lowest standard value (330H). Then pick RLIM from the curve. For IPEAK = 500mA, RLIM = 220. Positive-to-Negative Converter Figure 4 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)
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LT1108
APPLICATI
S I FOR ATIO
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 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 100mA is to be generated from a 4.5V to 5.5V input. Recalling Equation (14), PL = (-5V+ 0.5V)(100mA) = 550mW. Energy required from the inductor is 550mW PL = = 28.9J 19kHz fOSC (17) (16)
Picking an inductor value of 220H with 0.3 DCR results in a peak switch current of IPEAK =
(4.5V - 0.75V) 1 - e -0.95 x 36s 220H (0.65 + 0.3)
= 568mA Substituting IPEAK into Equation (04) results in
EL = 1 220H 0.568 A 2 = 35.5J 2
(
)(
)
Since 35.5J > 28.9J, the 220H inductor will work. Finally, RLIM should be selected by looking at the Switch Current vs RLIM curve. In this example, RLIM = 150. 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.
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The usual step-up configuration for the LT1108 is shown in Figure 1. The LT1108 first pulls SW1 low causing VIN - VCESAT to appear across L1. A current then builds up in L1. At the end of the switch-ON time the current in L1 is
IPEAK = VIN L t ON *
L1 V IN R3 I LIM V IN SW1 FB R2 D1 V OUT
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(15)
(20)
+
C1
LT1108
GND
SW2
R1
LT1108 * F01
Figure 1. Step-Up Mode Hookup
(18)
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 LT1108 to keep VFB at the internal reference voltage of 1.245V. R1 and R2 set the output voltage according to the formula R2 VOUT = 1 + 1.245 V R1
(
)
(21)
(19)
STEP-DOWN (BUCK MODE) OPERATION A step-down DC/DC converter converts a higher voltage to a lower voltage. The usual hookup for an LT1108 based step-down converter is shown in Figure 2. 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
*Expression 20 neglects the effect of switch and coil resistance. This is taken into account in the "Inductor Selection" section.
LT1108
APPLICATI
IPEAK =
S I FOR ATIO
t ON
VIN - VSW - VOUT L
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 LT1108. Output voltage is determined by R2 VOUT = 1 + 1.245 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 LT1108 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).
VIN R3 100
+
C2
ILIM
VIN
SW1 FB
LT1108 L1 SW2 GND D1 1N5818 VOUT
+
C1
Figure 2. Step-Down Mode Hookup
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HIGHER CURRENT STEP-DOWN OPERATION
(22)
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Output current can be increased by using a discrete PNP pass transistor as shown in Figure 3. R1 serves as a current limit sense. When the voltage drop across R1 equals 0.5VBE, the switch turns off. As shown, switch current is limited to 2A. Inductor value can be calculated based on formulas in the Inductor Selection Step-Down Converter section with the following conservative expression for VSW:
VSW = VR1 + VQ1SAT 1.0 V
(24)
(23)
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.
VIN 30V MAX R1 0.15 Q1 ZETEX ZTX749 R2 100 R6 100 VIN C2 LT1108 R4 FB GND SW2 R5 VOUT = 1.245V 1 + R5 IL SW1 R3 330 D1 1N5821 L1 VOUT
+
+
C1
(
R4
)
LT1108 * F03
Figure 3. Q1 Permits Higher Current Switching The LT1108 Functions as Controller
INVERTING CONFIGURATIONS The LT1108 can be configured as a positive-to-negative converter (Figure 4), or a negative-to-positive converter (Figure 5). In Figure 4, 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. In Figure 5, 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,
R2 R1
LT1108 * F02
9
LT1108
APPLICATI S I FOR ATIO
provided by the PNP transistor, supplies proper polarity feedback information to the regulator.
VIN R3 ILIM VIN SW1 FB C2 LT1108 L1 SW2 GND D1 1N5818
+
+
C1
Figure 4. Positive-to-Negative Converter
L1 D1 VOUT
+
C1 ILIM VIN SW1
2N3906
+
C2 LT1108 AO GND
FB SW2 R2
VOUT = R1 1.245V + 0.6V R2
LT1108 * F05
()
-VIN
Figure 5. Negative-to-Positive Converter
USING THE ILIM PIN The LT1108 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 where analysis shows the LT1108 must operate at an 800mA peak switch current with a 2.0V 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 RLIM" characteristic), the switch current will be limited to 800mA, even if the input voltage increases.
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Another situation where the ILIM feature is useful occurs when the device goes into continuous mode operation. This occurs in step-up mode when
VOUT + VDIODE 1 < . VIN - VSW 1 - DC (25)
R1 R2 -VOUT
LT1108 * F04
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R1
When the input and output voltages satisfy this relationship, inductor current does not go to zero during the switchOFF 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 6, 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 7, keeping output ripple to a minimum.
IL
SWITCH
ON OFF
LT1108 * F06
Figure 6. No Current Limit Causes Large Inductor Current Build-Up
IL
PROGRAMMED CURRENT LIMIT
SWITCH
ON OFF
LT1108 * F07
Figure 7. Current Limit Keeps Inductor Current Under Control
LT1108
APPLICATI
S I FOR ATIO
Figure 8 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 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 2s. The current trip point becomes less accurate for switch-ON times less than 5s. Resistor values programming switch-ON time for 2s or less will cause spurious response in the switch circuitry although the device will still maintain output regulation.
RLIM (EXTERNAL) VIN R1 80 (INTERNAL) Q3 DRIVER OSCILLATOR Q1 SW1 Q2 SW2
LT1108 * F08
ILIM
Figure 8. LT1108 Current Limit Circuitry Table 2. Capacitor Manufacturers
USING THE GAIN BLOCK The gain block (GB) on the LT1108 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 1.245V reference. The positive input comes out on the SET pin. Arrangement of the gain block as a low battery detector is straightforward. Figure 9 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" when the trip point is reached. Values in the 1M to 10M range are optimal. The addition however, of R3 will change the trip point.
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.
U
5V V IN LT1108 R1 VBAT 1.245V REF SET 47k
W
U
UO
-
AO TO PROCESSOR
+
R2
GND R3
VLB - 1.25V 35.1A VLB = BATTERY TRIP POINT R2 = 33k R3 = 1.6M R1 =
LT1108 * F09
Figure 9. Setting Low Battery Detector Trip Point Table 1. Inductor Manufacturers
MANUFACTURER Coiltronics International 984 S.W. 13th Court Pompano Beach, FL 33069 305-781-8900 Sumida Electric Co. USA 708-956-0666 PART NUMBERS OCTA-PACTM Series
CD54 CDR74 CDR105
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 AVX Corporation Myrtle Beach, SC 803-946-0690
PART NUMBERS OS-CON Series
PL Series
TPS Series
Table 3. Transistor Manufacturers
MANUFACTURER Zetex Inc. 87 Modular Avenue Commack, NY 11725 516-543-7100 PART NUMBERS ZTX 749 (NPN) ZTX 849 (NPN) ZTX 949 (PNP)
11
LT1108
TYPICAL APPLICATI
VIN 5V INPUT 220 ILIM VIN SW1 33pF LT1108-5 SENSE GND SW2 L1* 300H
LT1108-5 SENSE
LT1108 * TA04
5V to - 5V Converter
VIN 6.5V TO 20V
+
* L1 = COILTRONICS CTX300-4
LT1108 * TA03
PACKAGE DESCRIPTIO
0.300 - 0.320 (7.620 - 8.128)
0.009 - 0.015 (0.229 - 0.381)
0.065 (1.651) TYP 0.125 (3.175) MIN 0.020 (0.508) MIN
(
+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)
0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254) 0.016 - 0.050 0.406 - 1.270 0- 8 TYP
0.053 - 0.069 (1.346 - 1.752) 0.004 - 0.010 (0.101 - 0.254) 0.228 - 0.244 (5.791 - 6.197)
12
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900 q FAX: (408) 434-0507 q TELEX: 499-3977
U
UO
S
6.5V-20V to 5V Step-Down Converter
0.22 ZETEX ZTX-949 100 47F VIN ILIM SW1 100 1N5818 L1* 100H 5VOUT 200mA AT 6.5VIN 500mA AT 8VIN
+
+
330F
220
MBRS130T3
+
GND
SW2 * L1 = COILTRONICS CTX100-4
330F -5V OUTPUT 150mA
Dimensions in inches (millimeters) unless otherwise noted. N8 Package 8-Lead Plastic DIP
0.045 - 0.065 (1.143 - 1.651)
0.130 0.005 (3.302 0.127)
0.400 (10.160) MAX 8 7 6 5
0.250 0.010 (6.350 0.254)
1
2
3
4
N8 0393
0.018 0.003 (0.457 0.076)
S8 Package 8-Lead Plastic SOIC
0.189 - 0.197 (4.801 - 5.004) 8 7 6 5
0.014 - 0.019 (0.355 - 0.483)
0.050 (1.270) BSC
0.150 - 0.157 (3.810 - 3.988)
SO8 0393
1
2
3
4
LT/GP 0493 10K REV 0
(c) LINEAR TECHNOLOGY CORPORATION 1993

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