AN008 A Basis for LDO and It’s Thermal Design Introduction The AIC LDO family device, a 3-terminal regulator, can be easily used with all protection features that are expected in high performance voltage regulation application. These devices provide short-circuit protection, thermal shutdown protection and internal current limit protection against any overload condition that would create over heating junction temperature. (1)Current Limit Protection Like other power regulation IC, the AIC LDO family has safety protection area. The current limit protection works while outputting heavy-loading current and keeps the output current within a safe operating scope. The output voltage decreases to a lower voltage level at the same time. The AIC LDO family protection function is designed to set up output current limit when over-current happen and the downstream devices can be protected from being damaged. Upper: output voltage (1V/DIV) Lower: output current (2A/DIV) (2) Protection Diodes During normal operation, the AIC LDO device needs no protection diode. The internal diode between input and output pins can handle microsecond surge current. Even with large output capacitance, it is very difficult to get those values of surge current in normal operation. The damage will not occur, unless the high value output capacitors and the input pin are shorted to ground instantaneously. A crowbar circuit at the input of the LDO device can generate those kinds of current and a diode from output-to-input is then recommended. Normal power supply cycling or even plugging and unplugging in the system will not generate sufficient large current to damage device (see Figure 2). AIC1086 Topology D1 Function block diagram VIN + VOUT + CIN Figure 2 Diagram COUT AIC1086 Protection Diodes (3) Ripple Rejection Figure 1 AIC1086 Current Limit Test It is recommended to use the AIC LDO family May 2000 1 AN008 device in the application required improving ripple rejection. By connecting a bypass capacitor from the ADJ pin to the ground can reduce the output voltage ripple significantly (see Figure3). The bypass capacitor prevents output ripple from being amplified as the output voltage or loading current increases. The function is defined by: 1 ≤ R1 2p ∗ Fr ∗ C ADJ Here the Fr is the output ripple frequency and the CADJ is a bypass capacitor (For figure 4). The ripple rejection capability intensifies as output capacitor increases, the output ripple will then be reduced. For more information, please refer to AIC LDO family datasheet. AIC1086 Topology D1 Function block diagram VIN R1 + CIN Ripple Rejection (dB) (AIC1722D - 33) COUT R2 CADJ AIC1086 and Bypass Capacitor 60 C : COUT=1µF, IL=1mA D : COUT=1µF, IL=40mA VIN=5V DC + 1Vp-p 50 45 40 35 (b) Figure 6: When the adjustable type regulator is used, the load should be connected to the output terminal on the positive side and the ground terminal on the negative side. The output voltage is measured as the following equation: VL = VREF × R1 + R2 − Io (RS1 + RS2 R1 (c) Load regulation is the circuit’s ability to maintain the specified output voltage level under different load conditions, which is defined as: ∆VOUT ∆IO Here, Q1 is the series pass element, and β is the current gain of Q1. Gm is the transconductance of the error amplifier at its operating point. 30 25 20 15 0.1 VL = Vout − Io (RS1 + RS2) Figure 7 shows a PMOS voltage regulator. The ratio of output voltage variation to the given load current variation (∆VOUT/∆Iο) under constant input voltage Vi can be calculated as follow. B : COUT=10µF, IL=1Ma 55 Being a three-terminal device, the AIC LDO family is unable to provide true remote load sensing. The resistance of the wire connecting the regulator to the load will limit the load regulation. Please refer to the datasheet for the detail measurement. (a) Figure 5: When the fixed type regulator is used, the load should be connected to the output terminal on the positive side and the ground terminal on the negative side. The output voltage is measured as the following equation: VOUT + Figure 3 (CADJ) (4) Load Regulation 1 10 100 Frequency (KHz) Figure 4 AIC 1722D-33 Frequency and Ripple Rejection Assume that there is a small output current change (∆Iο), The change of output current causes the output voltage to change was calculated as: ∆Vout = ∆IoREQ(REQ = (R1 + R2) RL ≈ RL ) 2 AN008 Where REQ is the equivalent output resistor .The change of sensed voltage multiplied by Gm of the error amplifier input difference and β of the PMOS current gain (Figure7) must be large enough to achieve the specified change of output current. Thus, ∆IO = βGM∆V + = βGM( Q1 (β) Iο→ VIN VOUT - R2 )∆VOUT R1 + R2 GM + ERROR AMP. R1 RL R2 V Reference GND Figure 7 PMOS Voltage Regulator Then, the load regulator is obtained from above equation. (5) Quiescent Current or Ground Current ∆VOUT 1 R1 + R2 = ∆Io βGm R2 Since load regulation is a steady-state parameter, all frequency components are neglected. The load regulation is limited by the open loop current gain of the system. As noted from the above equation, increasing dc open loop current gain improved load regulation. RS1 VIN VIN VOUT VOUT GND RL RS2 GND Figure 5 AIC LDO Fixed Regulator VIN VIN VOUT GND RS1 R1 R2 VOUT RL RS2 GND Figure 6 AIC LDO Adjustable Regulator Quiescent current or ground current is the difference between input and output current for AIC LDO family. Minimum quiescent current is necessary to maximize current efficiency. It is defined: I q = Ii − I o Quiescent current consists of bias current and drive current of the series pass element, which does not contribute to output power. The series pass element, function diagram, ambient temperature, and etc, determine the value of quiescent current. Linear dropout voltage usually employ bipolar or MOS transistors as series pass elements. (a) Figure 8 :The collector current of bipolar transistors is defined by: IC = β IB Where IC is the collector current of bipolar transistor, β is the common-emitter current gain of bipolar transistor and IB is the base current of bipolar transistor. The base current of bipolar transistor is proportional to the collector current. When the output current increases, the base current increases, too. Since the base current contributes to quiescent current, bipolar transistors have higher quiescent current than MOS transistors. At the same time, during the dropout region the quiescent current will increase, because of the additional parasitic current path between the emitter and the base of bipolar transistors, which is caused by a lower base voltage than that of the output voltage. 3 AN008 (b) Figure 9 the drain source current of MOS Figure 10 and figure 11 show the ground current with respect to input voltage and temperature. transistors is defined by: ID = K(VGS − VT )2 (1 + λVDS )(λVDS ≈ 0) Ground Current vs. Input Voltage ⇒ ID = K(VDS − VT)2 The drain current is a function of the gate to source voltage, not the gate current. β Ground Current (µA) 60 K is a MOS transistor conductivity parameter Vgs is the gate to source voltage Vt is the MOS threshold voltage 50 40 30 20 10 0 0 IB3 IC 2 4 6 Input Voltage (V) 8 10 12 IB2 IB1 VCC Figure 8 Figure 10 AIC1722 Input and Current Characteristics Ground Ground Current vs. Temperature I-V Characteristics of Bipolar 60 Transistors K ID VGS4 VGS3 VGS2 Ground Current (µA) 58 IL =300mA 56 IL =150mA 54 IL=0.1mA 52 VGS1 50 VDS -50 -25 0 25 50 75 100 125 Temperature (°C) Figure 9 I-V Characteristic of MOS Transistors Figure 11 AIC 1722 Temperature and Ground Current Characteristics For bipolar transistors, the quiescent current increases proportionally with the output current because the series pass element is a current-driven device. For MOS transistors, the quiescent current has a near constant value with respect to the load current since the device is voltage-driven. The only things that contribute to the quiescent current for MOS transistors are the biasing currents of band-gap, sampling resistor, and error amplifier. In most applications where power consumption is critical or where small bias current is requested in comparison with the output current, an LDO voltage regulator using MOS transistors is an essential choice. (6) Thermal Considerations The AIC LDO family has internal power and thermal-limiting circuitry, which is designed to protect the device against overload conditions. For continuous normal load conditions, however, maximum ratings of junction temperature must not be exceeded. It is important to pay more attention to all sources of thermal resistance from junction to ambient . This includes junction-to-case, case-to-heat sink interface, and heat sink resistance itself. 4 AN008 We take the following condition as an example of AIC 1086. VIN (max continuous)=5V, VOUT=3.3V, IOUT=1A , regulator, The dropout voltage and quiescent current must be reduced. In addition, the dropout voltage between input and output must be minimized since the power dissipation of LDO regulators affects to the efficiency significantly. Power dissipation = (Vi – Vo) Io TA=70ºC For example of AIC1722: θHEAT SINK=1ºC/W, θCASE-TO-HEATSINK=0.2ºC/W for package with thermal compound. Input voltage is 5V TO-220 Output voltage is 3.3V Output current is 300mA Power dissipation under these conditions can be calculated: PD=(VIN-VOUT)(IOUT)=1.7W Ground (max) current is 80µA 300mA × 3.3 × 100% (300mA + 88µ8 ) × 5 = 66% E= Junction temperature will be equal to: TJ=TA+PD(θHEAT SINK + θCASE-TO-HEAT SINK + θJC) For the operating junction temperature range: TJ =70ºC+1.7W(1ºC/W+0.2ºC/W+0.7ºC/W) (8) Layout Note According to the following parameter, we can achieve the maximum allowable Temperature Rise, (TR) TR= TJ (max)- TA (max) =73.23ºC 73.23ºC<125ºC=TJMAX (Operating Junction Temperature Range) For the storage temperature range: TJ =70ºC +1.7W (1ºC / W+0.2ºC / +3ºC /W) =77.14ºC where TJ (max) is the maximum allowable junction temperature (125ºC ), and TA (max) is the maximum ambient temperature suitable in the application. Use the calculated values for TR and PD, the maximum allowable value of the junction-to-ambient thermal resistance (θJA) can be calculated: θJA=TR/PD 77.14ºC<150ºC=TJMAX (Storage Temperature Range) In the above two cases, the junction temperature are lower than the maximum rating, and this ensure a reliable operation. (7) Efficiency The quiescent or ground current and input/output voltage are with respect to the efficiency of a LDO regulator input/output voltage with following equation: E= I o Vo × 100 % I o + I g Vi ( ) In order to achieve a higher efficiency for LDO If the maximum allowable value for θJA is achieved to be ≥ 133ºC /W for SOT-223 package or ≥ 74ºC /W for TO-220 package or ≥102ºC /W for TO-263 package, no heatsink is needed since the package will dissipate heat to satisfy these requirements. If the calculated value for θJA falls below these limits, extra heatsink for LDO device is required. TABLE 1. θJA Different Heatsink Area Table 1 shows the values of the θJA of SOT-223 and TO-263 for different heatsink area. The 5 AN008 copper patterns that we used to measure these θJA are shown as below. Copper Area Layout 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 2 2 Top Side (in )* Bottom Side (in )* 0.012 0.064 0.3 0.52 0.75 1 0 0 0 0 0 0.065 0.174 0.283 0.391 0.4 0 0 0 0 0 0 0.2 0.4 0.6 0.8 1 0.065 0.174 0.283 0.391 0.4 Thermal Resistance (θJA °C/W) (θJA °C/W) TO-263 SOT-223 102 133 83 122 61 82 53 73 51 67 46 63 83 117 69 94 62 87 54 81 55 78 88 123 71 92 60 82 55 75 53 70 TABLE 2. AIC LDO Series Temperature table (-)èSince IC’s temperature can rise up, these operation conditions are not recommended. Test IC TYPE AIC1722-33CZL(TO-92) without heat sink Power Long time test: 0.5W 0.7W(-) dissipation Ta: 28ºC Test time: 20min. Load current 298mA 417mA No load: Input voltage: 5VDC Output voltage 3.302V 3.307V Output Package 70ºC 81ºC voltage :3.322VDC Test IC TYPE AIC1722-33CZL(SOT-89)IC stick on PCB Power Long time test 0.5W 0.6W(-) dissipation Ta: 28ºC Test time: 20min. Load current 290mA 348mA No load: Input voltage: 5VDC Output voltage 3.305V 3.299V Output Package 70ºC 80ºC voltage:3.278VDC 6 AN008 Test IC TYPE AIC1723-33CE(TO-252)IC stick on PCB Power Long time test 0.5W 0.9W dissipation Ta: 28ºC Test time: 20min. Load current 300mA 538mA No load : Input voltage: 5VDC Output voltage 3.321V 3.313V Output Package 40ºC 50ºC voltage:3.328VDC Test IC TYPE AIC1723-33CF(TO-251) without heat sink Power 0.9W 1W Dissipation Long time test Ta: 28ºC Load Current 524mA 582mA Test time: 20min. No load: Output Voltage 3.294V 3.295V Input voltage: 5VDC Output Package 63ºC 66ºC voltage:3.284VDC Junction 80ºC 87ºC Test IC TYPE AIC1084CT(TO-220) without heat sink Power 1W 3W(-) Long time test Dissipation Ta: 28ºC Load Current 600mA 1.802A Test time: 20min. No load: Output Voltage 3.331V 3.311V Input voltage: 5VDC Output Package 55ºC 99ºC voltage:3.335VDC Junction 66ºC 124ºC Test IC TYPE AIC1084CT(TO-220) with heat sink Power Long time test 1W 3W Dissipation Ta: 28ºC Test time: 20min. Load Current 600mA 1.802A No load: Output Voltage 3.333V 3.322V Input voltage: 5VDC Output Package 47ºC 61ºC voltage:3.335VDC Junction 54ºC 85ºC Test IC TYPE AIC1084CT(TO-220) IC stick on PCB Power Long time test 1W 3W Dissipation Ta : 28ºC Test time: 20min. Load Current 600mA 1.802A No load: Output Voltage 3.333V 3.324V Input voltage: 5VDC Output Package 41ºC 66ºC voltage:3.335VDC Junction 46ºC 75ºC 1W(-) 598mA 3.316V 57ºC 1.1W(-) 641mA 3.296V 73ºC 96ºC 6W(-) 3.604A 3.291V 127ºC 176ºC 6W(-) 3.604A 3.219V 88ºC 113ºC 6W(-) 3.604A 3.197V 93ºC 110ºC 7 AN008 Test IC TYPE AIC1084CM(TO-263) IC stick on PCB Power 1W 3W 6W(-) Long time test Dissipation Ta: 28ºC Load Current 594mA 1.784A 3.567A Test time: 20min. No load: Output Voltage 3.314V 3.296V 3.242V Input voltage : 5VDC Package Output 40ºC 74ºC 88ºC voltage:3.318VDC Junction 44ºC 90ºC 108ºC Test IC TYPE AIC1085CT(TO-220) without heat sink Power 1W 3W(-) Long time test Dissipation Ta: 28ºC Load Current 556mA 1.667A Test time: 20min. No load: Output Voltage 3.193V 3.173V Input voltage: 5VDC Output Package 56ºC 90ºC voltage:3.200VDC Junction 76ºC 146ºC Test IC TYPE AIC1085CT(TO-220) with heat sink Power 1W 3W Dissipation Long time test Ta : 28ºC Test time: 20min. No load: Input voltage: 5VDC Output voltage:3.200VDC 7W(-) 4.162A 3.077V 100ºC 120ºC 6W(-) 3.333A 3.285V 130ºC 193ºC 6W(-) Load Current 556mA 1.667A 3.333A Output Voltage 3.192V 3.179V 3.176V Package 40ºC 56ºC 95ºC Junction 50ºC 80ºC 138ºC Test IC TYPE AIC1085CT(TO-220) IC stick on PCB Power 1W 3W Long time test Dissipation Ta : 28 Load Current 556mA 1.667A Test time: 20min. No load: Output Voltage 3.199V 3.192V Input voltage: 5VDC Package Output 45ºC 65ºC voltage:3.200VDC Junction 54ºC 85ºC Test IC TYPE AIC1085CM(TO-263) IC stick on PCB Power Long time test 1W 3W Dissipation Ta : 28ºC Load Current 595mA 1.788A Test time: 20min. No load: Output Voltage 3.321V 3.310V Input voltage: 5VDC Output Package 40ºC 64ºC voltage:3.322VDC Junction 47ºC 88ºC 6W(-) 3.333A 3.174V 100ºC 132ºC 6W(-) 3.576A 3.192V 80ºC 100ºC 8 AN008 Test IC TYPE AIC1117CE(TO-252) IC stick on PCB Power Long time test 1W 1.5W Dissipation Ta : 28ºC Load Current 561mA 841mA Test time: 20min. No load: Output Voltage 3.204V 3.192V Input voltage : 5VDC Output Package 55ºC 68ºC voltage:3.217VDC Junction 60ºC 70ºC 2W(-) 1.122A 3.184V 80ºC 84ºC (9) Summary Install a 10µF (or greater) capacitor is required between the AIC LDO family device’s output and ground pins for the reason of stability. Without this capacitor, the part will oscillate. Even though most types of capacitor may work, the equivalent series resistance (ESR) should be held to 5Ω or less, if aluminum electrolytic type is used. Many Aluminum electrolytic capacitors have electrolytes that will freeze under -30°C, so solid tantalums are recommended for operation below -25°C. The value of this capacitor may be increased without limit. A 10µF (or greater) capacitor should be placed from the AIC LDO family input to ground if the lead inductance between the input and power source exceeds 500nH (approximately 10 inches of trace). 9