MIC2184 Micrel MIC2184 Low Voltage Buck PWM Control IC Final Information General Description Features Micrel’s MIC2184 is a high efficiency PWM buck control IC. With its wide input voltage range of 2.9V to 14V, the MIC2184 can be used to efficiently step voltages down in 1- or 2-cell Li Ion battery powered applications, as well as in fixed 3.3V, 5V, or 12V systems. Efficiencies over 90% are achievable over a wide range of load conditions with the MIC2184’s PWM control scheme. The operating frequency can be divided by two by raising the FREQ/2 pin to VDD. This allows the user to optimize efficiency versus board space. It also allows the MIC2184 to be externally synchronized to frequencies below its nominal 400KHz. The MIC2184 features an oscillator output, FreqOut, which can be used to implement a simple charge pump in low voltage applications. The output of the charge pump can be fed into the gate drive power circuitry via the VINP pin, allowing enhanced gate drive, hence higher efficiencies, despite low input voltages. MIC2184 features a 1µA shutdown mode, and a programmable undervoltage lockout, making it well suited for portable applications. The MIC2184 is available in 16 pin SOP and QSOP package options with a junction temperture range range from –40°C to +125°C. • • • • • • • • • • • • • • • • Input voltage range: 2.9V to 14V >90% efficiency Oscillator frequency of 400kHz Frequency divide-by-two pin Frequency sync to 600kHz FrequOut oscillator output allows simple charge pump implementation in low voltage systems 1.245V reference output Front edge blanking 5Ω output driver Soft start PWM current mode control 1µA shutdown current Cycle-by-cycle current limiting Frequency foldback short circuit protection Adjustable under-voltage lockout 16 pin narrow-body SOIC and QSOP package options Applications • • • • • • 3.3V to 2.5V/1.8V/1.5V conversion DC power distribution systems Wireless Modems ADSL line cards 1-and 2-cell Li Ion battery operated equipment Satellite Phones Typical Application VIN = 3.3V 16 15mΩ VINP 1 7 2 16 10 4 3 13 VINA CSH EN/UVLO CSL FreqOut FREQ/2 OUTP MIC2184 EFFICIENCY 9 100 8 95 90 14 Q3 Si9803DY(x2) VOUT 2.5V @5A MIC2184 4.7µH VDD COMP SS PGND VREF SYNC 11 FB 12 220µF(x2) 6.3V 6 SGND EFFICIENCY (%) 120µF (x2) 6.3V 85 80 75 70 65 60 55 50 0 VIN = 3.3V VOUT = 2.5V fS = 200kHz 1 2 3 4 OUTPUT CURRENT (A) 5 5 Adjustable Output Buck Converter Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com December 10, 2001 1 MIC2184 MIC2184 Micrel Ordering Information Part Number Output Voltage Frequency Junction Temp. Range Package MIC2184BM Adjustable 400KHz –40°C to +125°C 16-lead SOP MIC2184BQS Adjustable 400KHz –40°C to +125°C 16-lead QSOP Pin Configuration VINA 1 16 VINP FreqOut 2 15 FREQ/2 SS 3 14 OUTP COMP 4 13 VREF SGND 5 12 PGND FB 6 11 SYNC EN/UVLO 7 10 VDD CSL 8 9 CSH 16 Lead SOIC (M) MIC2184 2 December 10, 2001 MIC2184 Micrel Pin Description Pin Number Pin Name 1 VINA 2 FreqOut 3 SS 4 COMP Compensation (Output): Internal error amplifier output. Connect to a capacitor or series RC network to compensate the regulator’s control loop. 5 SGND Small signal ground: must be routed separately from other grounds to the (-) terminal of COUT. 6 FB 7 EN/UVLO 8 CSL The (-) input to the current limit comparator. A built in offset of 100mV between CSH and CSL in conjunction with the current sense resistor sets the current limit threshold level. This is also the (-) input to the current amplifier. 9 CSH The (+) input to the current limit comparator. A built in offset of 100mV between CSH and CSL in conjunction with the current sense resistor sets the current limit threshold level. This is also the (+) input to the current amplifier. 10 VDD 3V internal linear-regulator output. VDD is also the supply voltage bus for the chip. Bypass to SGND with 1µF. 11 SYNC Frequency Synchronization (Input): Connect an external clock signal to synchronize the oscillator. Leading edge of signal above 1.5V starts the switching cycle. Connect to SGND if not used. 12 PGND MOSFET driver power ground, connects to source of synchronous MOSFET and the (-) terminal of CIN. 13 VREF The 1.245V voltage reference is available on this pin. A 0.1µF capacitor should be connected from this pin to ground. 14 OUTP High current drive for high side P channel MOSFET. Voltage swing is from ground to VINP. On-resistance is typically 2.5Ω. 15 FREQ/2 When this is low, the oscillator frequency is 400KHz. When this pin is raised to VDD, the oscillator frequency is 200KHz. 16 VINP Power Input voltage to the circuit. The output gate drivers are powered from this supply. The current sense resistor RCS should be connected as close as possible to this pin. December 10, 2001 Pin Function Analog voltage input voltage to the circuit. This powers up the analog sections of the die and does not need to be the same voltage as Pin 16 (VINP). This provides a digital signal output signal at half the switching frequency. This signal swings from 0 to 3V, and can be used to drive an external capacitive doubler to provide a higher voltage to the VINP input. Soft start reduces the inrush current and delays and slows the output voltage rise time. A 5µA current source will charge the capacitor up to VDD. A 1µF capacitor will soft start the switching regulator in 1.5ms. Feedback Input - the circuit regulates this pin to 1.245V. Enable/UnderVoltage Lockout (input): A low level on this pin will power down the device, reducing the quiescent current to under 5uA. This pin has two separate thresholds, below 1.5V the output switching is disabled, and below 0.9V the part is forced into a complete micropower shutdown. The 1.5V threshold functions as an accurate undervoltage lockout (UVLO) with 140mV hysteresis. 3 MIC2184 MIC2184 Micrel Absolute Maximum Ratings (Note 1) Operating Ratings (Note 2) Supply Voltage (VINA, VINP) ......................................... 15V Digital Supply Voltage (VDD) ........................................... 7V Comp Pin Voltage (VCOMP) ............................ –0.3V to +3V Feedback Pin Voltage (VFB) .......................... –0.3V to +3V Enable Pin Voltage (VEN/UVLO) ..................... –0.3V to 15V Current Sense Voltage (VCSH–VCSL) ............... –0.3V to 1V Sync Pin Voltage (VSYNC) ................................ –0.3V to 7V Freq/2 Pin Voltage (VFREQ/2) ............................ -0.3V to 7V Power Dissipation (PD) 16 lead SOIC ................................. 400mW @ TA = 85°C 16 lead QSOP ............................... 245mW @ TA = 80°C Ambient Storage Temp ............................ –65°C to +150°C ESD Rating, ............................................................. Note 3 Supply Voltage (VINA, VINP) ........................ +2.9V to +14V Ambient Operating Temperature ......... –40°C ≤ TA ≤ +85°C Junction Temperature ....................... –40°C ≤ TJ ≤ +125°C Output Voltage Range ...................................... 1.3V to 12V PackageThermal Resistance θJA 16-lead SOP ............................................... 100°C/W θJA 16-lead QSOP ............................................. 163°C/W Electrical Characteristics VINA = VINP = VCSH = 5V, VOUT = 3.3V, VEN/UVLO = 5V, VFREQ/2 = 0V, TJ = 25ºC, unless otherwise specified. Bold values indicate –40ºC < TJ < +125ºC. Parameter Condition Min Typ Max Units (±1%) 1.233 1.245 1.257 V (±2%) 1.22 1.27 V Regulation Feedback Voltage Reference Feedback Bias Current 50 nA Output Voltage Line Regulation 5V ≤ VIN ≤ 12V 0.04 %/V Output Voltage Load Regulation 0mV < (VCSH – VCSL) < 75mV 0.9 % Output Voltage Total Regulation 5V ≤ VINA ≤ 12V, 0mV < (VCSH – VCSL) < 75mV (±3%) 1.208 1.282 V Input & VDD Supply VINA Input Current 0.7 mA mA VINP Input Current, Note 4 (Excluding external MOSFET gate current) 1.0 Shutdown Quiescent Current VEN/UVLO = 0V; (IVINA + IVINP) 0.5 5 µA Digital Supply Voltage (VDD) IL = 0 3.0 3.18 V Digital Supply load regulation IL = 0 to 1mA 0.03 V Undervoltage Lockout VDD upper threshold (turn on threshold) 2.75 V 100 mV 2.82 UVLO Hysteresis Reference Output(VREF) Reference Voltage (±1.5%) 1.226 (±2.5%) 1.213 1.245 1.264 V 1.276 V Reference Voltage Line Regulation 5V < VINA < 12V 2 mV Reference Voltage Load Regulation 0 < IREF < 100µA 1 mV Enable/UVLO Enable Input Threshold UVLO Threshold (Turn-on threshold) UVLO Hysteresis Enable Input Current MIC2184 0.6 0.9 1.2 V 1.4 1.5 1.6 V 140 VEN/UVLO = 5V 0.2 4 mV 5 µA December 10, 2001 MIC2184 Parameter Micrel Condiion Min Typ Max Units Soft Start Soft Start Current 5 µA 100 mV 20 V/V 3.0 V/V Current Limit Current Limit Threshold Voltage Voltage on CSH-CSL to trip current limit Error Amplifier Error Amplifier Gain Current Amplifier Current Amplifier Gain Oscillator Section Oscillator Frequency (fO) 360 Maximum Duty Cycle VFB = 1.0V Minimum On Time VFB = 1.5V Freq/2 Frequency (fO) VFREQ/2 = 5V Frequency Foldback Threshold Measured on FB 440 kHz 100 % 185 170 Frequency Foldback Frequency SYNC Threshold Level 0.6 SYNC Input Current SYNC Minimum Pulse Width SYNC Capture Range 400 200 ns 230 kHz 0.3 V 90 kHz 1.4 2.2 V 0.1 5 µA 200 Note 5 ns fO +15 % 600 kHz FreqOut Output FreqOut Frequency Note 6 fO / 2 kHz FreqOut Current Drive Sink 8 mA Source –6 mA Rise/Fall Time CL = 3300pF 50 ns Output Driver Impedance Source; VINP = 12V 1.8 4 Ω Sink; VINP = 12V 1.6 3.5 Ω Source; VINP = 5V 2.6 Ω Sink; VINP = 5V 2.4 Ω Gate Drivers Note 1: Absolute maximum ratings indicate limits beyond which damage to the component may occur. Electrical specifications do not apply when operating the device outside of its operating ratings. The maximum allowable power dissipation is a function of the maximum junction temperature, TJ(Max), the junction-to-ambient thermal resistance, θJA, and the ambient temperature, TA. Note 2. The device is not guaranteed to function outside its operating rating. Note 3. Devices are ESD sensitive. Handling precautions recommended. Note 4: See application information for I(VINP) vs. VINP. Note 5: See application information for limitations on maximum operating frequency. Note 6: The frequency on FreqOut is half the frequency of the oscillator, or half the frequency of the external Sync signal. December 10, 2001 5 MIC2184 MIC2184 Micrel Typical Characteristics Quiescent Current vs. Input Voltage 2.5 2 200kHz 1.5 1 0.5 0 0 3.005 ISTANDBY 5 10 INPUT VOLTAGE (V) VDD vs. Load 0.8 VINA = VINP = 5V 2.985 2.85 3.00 2.98 VIN = 5V 2.92 -40 -20 0 20 40 60 80 100120140 TEMPERATURE (°C) 1.2449 1.2447 1.2445 1.2443 1.2441 0 1.243 1.242 1.241 VIN = 3.3V 1.24 -40 -20 0 20 40 60 80 100120140 TEMPERATURE (°C) FREQUENCY VARIATION (%) 1.244 1 0.5 0 -0.5 -1 400kHz -1.5 Soft Start Current vs. Temperature 2 4 6 8 10 12 INPUT VOLTAGE (V) 14 94.0 4.5 92.0 4.4 -40 -20 0 20 40 60 80 100120140 TEMPERATURE (°C) 90.0 0 MIC2184 -13 -15 -50 -30 -10 10 30 50 70 90 110 TEMPERATURE (°C) Overcurrent Threshold vs. Temperature 101 96.0 400kHz -9 -11 100.0 98.0 200kHz -5 -7 102 THRESHOLD (mV) 4.6 -1 -3 Overcurrent Threshold vs. Input Voltage THRESHOLD (mV) CURRENT (µA) 4.7 3 1 102.0 4.9 4.8 4 6 8 10 12 14 INPUT VOLTAGE (V) 5 200kHz 1.5 -2 0 2 Frequency Variation vs. Temperature 2 1.245 15 1.2451 Frequency Variation vs. Input Voltage 1.246 5 10 INPUT VOLTAGE (V) Error Amp Reference Voltage vs. Input Voltage 3.02 Error Amp Reference Voltage vs. Temperature 2.80 0 VDD vs. Temperature 2.94 0.2 0.4 0.6 0.8 1 1.2 VDD LOAD CURRENT (mA) 2.95 2.90 IQ = IVINA + IVINP 0.2 V A = V P = 3.3V IN IN 2.96 2.975 VINA = VINP 3.00 0.4 3.04 2.980 REFERENCE VOLTAGE (V) ISTANDBY 0.6 3.08 VDD (V) VDD (V) 200kHz 3.06 2.990 2.970 0 1.0 VDD vs. Input Voltage 3.05 0 -40 -20 0 20 40 60 80 100120140 TEMPERATURE (°C) 15 3.000 2.995 400kHz VDD (V) 3 1.2 REFERENCE VOLTAGE (V) 3.5 IQ = IVINA + IVINP SOFT START CURRENT (µA) 4 3.10 1.4 400kHz QUIESCENT CURRENT (mA) QUIESCENT CURRENT (mA) 4.5 Quiescent Current vs. Temperature VIN = 3.3V 100 99 98 97 96 2 4 6 8 10 12 14 INPUT VOLTAGE (V) 6 95 -40 -20 0 20 40 60 80 100120140 TEMPERATURE (°C) December 10, 2001 MIC2184 Micrel OUTP Drive Impedance vs. Input Voltage 4 IMPEDANCE (Ω) 3.5 3 2.5 2 SINK 1.5 SOURCE 1 0 5 10 INPUT VOLTAGE (V) December 10, 2001 15 7 MIC2184 MIC2184 Micrel Functional Diagram VIN CIN CDECOUP VINA 1 OVERCURRENT COMPARATOR VREF 1.245V 0.1V EN/UVLO VDD 9 CSH 8 CSL 16 VINP 14 OUTP 7 RSENSE BIAS 10 GAIN 3.7 VDD CURRENT SENSE AMP ON fs/4 CONTROL Q1 L1 SYNC 11 FREQ/2 15 FreqOut 2 VOUT D1 OSC 12 SLOPE COMPENSATION ÷2 COUT RESET PGND ∑ PWM COMPARATOR SS 3 COMP 4 gm = 0.0002 VREF gain = 20 ERROR AMP 6 FB 5 SGND 100k 0.3V fs/4 VREF 13 VREF FREQUENCY FOLDBACK Figure 1. MIC2184 Block Diagram P-Channel MOSFET, Q1. Current flows from the input to the output through the current sense resistor, MOSFET and inductor. The current amplitude increases, controlled by the inductor. The voltage developed across the current sense resistor, RSENSE, is amplified inside the MIC2184 and combined with an internal ramp for stability. This signal is compared to the output of the error amplifier. When the current signal equals the error voltage signal, the P-channel MOSFET is turned off. The inductor current flows through the diode, D1. At the beginning of the next switching cycle, the P-channel MOSFET is turned on which turns off the diode, D1. Functional Characteristics Controller Overview and Functional Description The MIC2184 is a BiCMOS, switched mode, synchronous, step down (buck) converter controller. It uses a P-Channel MOSFET, which allows the controller to operate at 100% duty cycle and eliminates the need for a high side drive bootstrap circuit. Current mode control is used to achieve superior transient line and load regulation. An internal corrective ramp provides slope compensation for stable operation above a 50% duty cycle. The controller is optimized for high efficiency, high performance DC-DC converter applications. Figure 1 is a block diagram of the MIC2184 configured as a synchronous buck converter. At the beginning of the switching cycle, the OUTP pin pulls low and turns on the high-side MIC2184 8 December 10, 2001 MIC2184 Micrel The MIC2184 controller is broken down into 7 functions. • Control loop • PWM operation • Current mode control • Current limit • Reference, enable and UVLO • FreqOut • MOSFET gate drive • Oscillator and Sync • Soft-start Control Loop Current Limit The output current is detected by the voltage drop across the external current sense resistor (RSENSE in Figure 1.). The current sense resistor must be sized using the minimum current limit threshold. The external components must be designed to withstand the maximum current limit. The current sense resistor value is calculated by the equation below: RSENSE = The maximum output current is: PWM Control Loop IOUT _ MAX = The MIC2184 uses current mode control to regulate the output voltage. This dual control loop method (illustrated in Figure 2) senses the output voltage (outer loop) and the inductor current (inner loop). It uses inductor current and output voltage to determine the duty cycle of the buck converter. Sampling the inductor current effectively removes the inductor from the control loop, which simplifies compensation. VOUT Voltage Divider IINDUCTOR Switch Driver VERROR VREF IINDUCTOR VERROR tON tPER D = tON/tPER Figure 2. Current Mode Control Example As shown in Figure 1, the inductor current is sensed by measuring the voltage across the resistor, RSENSE. A ramp is added to the amplified current sense signal to provide slope compensation, which is required to prevent unstable operation at duty cycles greater than 50%. A transconductance amplifier is used for the error amplifier, which compares an attenuated sample of the output voltage with a reference voltage. The output of the error amplifier is the compensation pin (Comp), which is compared to the current sense waveform in the PWM block. When the current signal becomes greater than the error signal, the comparator turns off the high side drive. The COMP pin provides access to the output of the error amplifier and allows the use of external components to stabilize the voltage loop. December 10, 2001 MAX _ CURRENT _ SENSE _ THRESHOLD RSENSE The current sense pins CSH (pin 9) and CSL (pin 8) are noise sensitive due to the low signal level and high input impedance. The PCB traces should be short and routed close to each other. A small (1nF) capacitor across the pins will attenuate high frequency switching noise. When the peak inductor current exceeds the current limit threshold, the overcurrent comparator turns off the high side MOSFET for the remainder of the switching cycle, effectively decreasing the duty cycle. The output voltage drops as additional load current is pulled from the converter. When the voltage at the feedback pin (FB) reaches approximately 0.3V, the circuit enters frequency foldback mode and the oscillator frequency will drop to 1/4 of the switching frequency. This limits the maximum output power delivered to the load under a short circuit condition. Reference, Enable and UVLO Circuits The output drivers are enabled when the following conditions are satisfied: • The VDD voltage (pin 10) is greater than its undervoltage threshold. • The voltage on the enable pin (pin 7) is greater than the enable UVLO threshold. The enable pin (pin 7) has two threshold levels, allowing the MIC2184 to shut down in a low current mode, or turn off output switching in standby mode. An enable pin voltage lower than the shutdown threshold turns off all the internal circuitry and places the MIC2184 in a micropower shutdown mode. If the enable pin voltage is between the shutdown and standby thresholds, the internal bias, VDD and reference voltages are turned on. The soft start pin is forced low by an internal discharge MOSFET. The output drivers are inhibited from switching. The OUTP pin is in a high state and the OUTN pin remains in a low state. Raising the enable voltage above the standby threshold allows the soft start capacitor to charge and enables the output drivers. The standby threshold is specified in the electrical characteristics. A resistor divider can be used with the enable pin to prevent the power supply from turning on until a specified input voltage is reached. The circuit in Figure 3 shows how to connect the resistors. VIN Switching Converter MIN _ CURRENT _ SENSE _ THRESHOLD IOUT _ MAX 9 MIC2184 MIC2184 Micrel supply. The VINP pin and CSH pin must be connected to the same potential. MOSFET Selection The P-channel MOSFET must have a VGS threshold voltage equal to or lower than the input voltage when used in a buck converter topology. There is a limit to the maximum gate charge the MIC2184 will drive. Higher gate charge MOSFET will slow down the turn-on and turn-off time of the MOSFET. Slower transition times will cause higher power dissipation in the MOSFET due to higher switching transition losses. The MOSFET gate charge is also limited by power dissipation in the MIC2184. The power dissipated by the gate drive circuitry is calculated below: MIC2184 VIN 1.5V Typical R1 R2 Bias Circuitry EN/UVLO (7) 140mV Hysteresis (typical) Figure 3. UVLO Circuitry The line voltage turn on trip point is: VINPUT _ ENABLE = VTHRESHOLD × R2 R1 + R2 PGATE_DRIVE = Q GATE × VINP × fS where: VTHRESHOLD is the voltage level of the internal comparator reference, typically 1.5V The input voltage hysteresis is equal to: VINPUT _ HYST = VHYST × where: Qgate is the total gate charge of both the N and Pchannel MOSFETs. fS is the switching frequency VINP is the gate drive voltage at the VINP pin The graph in Figure 4 shows the total gate charge that can be driven by the MIC2184 over the input voltage range, for different values of switching frequency. R1 + R2 R2 where: VHYST is the internal comparator hysteresis level, typically 140mV. VINPUT_HYST is the hysteresis at the input voltage The MIC2184 will be disabled when the input voltage drops back down to: VINPUT_OFF = VINPUT_ENABLE – VINPUT_HYST = Frequency vs. Max. Gate Charge TOTAL GATE CHARGE (C) 200x10-9 R2 R1 + R2 Either of 2 UVLO conditions will pull the soft start capacitor low. • When the VDD voltage drops below its undervoltage lockout level. • When the enable pin drops below the its enable threshold The internal bias circuit generates an internal 1.245V bandgap reference voltage for the voltage error amplifier and a 3V VDD voltage for the internal control circuitry. The VREF pin (pin 13) should be decoupled with a 0.1µf capacitor placed close to the pin. The VDD pin must be decoupled with a 1µF ceramic capacitor. The capacitor must be placed close to the VDD pin. The other end of the capacitor must be connected directly to the ground plane. MOSFET Gate Drive The MIC2184 is designed to drive a high side P-channel MOSFET. The source pin of the P-channel MOSFET is connected to the input of the power supply. It is turned on when OUTP pulls the gate of the MOSFET low. The advantage of using a P-channel MOSFET is that it does not require a bootstrap circuit to boost the gate voltage higher than the input, as would be required for an N-channel MOSFET. The VINP pin (pin 16) supplies the drive voltage to the gate drive pin, OUTP. VINP pin is usually connected to the input (VTHRESHOLD – VHYST) × MIC2184 180x10-9 160x10-9 140x10-9 200kHz 120x10-9 100x10-9 300kHz 80x10-9 500kHz 60x10-9 40x10-9 400kHz 600kHz 20x10-9 0x1003 5 7 9 11 13 15 17 INPUT VOLTAGE (V) Figure 4. MIC2184 Frequency vs Max. Gate Charge Oscillator & Sync The internal oscillator is free running and requires no external components. The f/2 pin allows the user to select from two switching frequencies. A low level set the oscillator frequency to 400kHz and a high level set the oscillator frequency to 200kHz. The maximum duty cycle for both frequencies is 100%. This is another advantage of using a P-channel MOSFET for the high-side drive; it can continuously turned on. A frequency foldback mode is enabled if the voltage on the feedback pin (pin 6) is less than 0.3V. In frequency foldback, the oscillator frequency is reduced by approximately a factor of 4. Frequency foldback is used to limit the energy delivered to the output during a short circuit fault condition. The SYNC input (pin 11) lets the MIC2184 synchronize with an external clock signal. The rising edge of the sync signal generates a reset signal in the oscillator, which turns off the low side gate drive output. The high side drive then turns on, restarting the switching cycle. The sync signal is inhibited when the controller operates in frequency foldback. The sync signal frequency must be greater than the maximum speci10 December 10, 2001 MIC2184 Micrel fied free running frequency of the MIC2184. If the synchronizing frequency is lower, double pulsing of the gate drive outputs will occur. When not used, the sync pin must be connected to ground. The maximum recommended output switching frequency is 600kHz. Synchronizing to higher frequencies may be possible, however, higher power dissipation in the internal gate drive circuits will occur. The MOSFET gates require charge to turn on the device. The average current required by the MOSFET gate increases with switching frequency. Soft Start Soft start reduces the power supply input surge current at start up by controlling the output voltage risetime. The input surge appears while the output capacitance is charged up. A slower output risetime will draw a lower input surge current. Soft start may also be used for power supply sequencing. The soft start voltage is applied directly to the PWM comparator. A 5µA internal current source is used to charge up the soft start capacitor. The capacitor is discharged when either the enable pin voltage drops below the standby threshold or the VDD voltage drops below its UVLO level. The part switches at a low duty cycle when the soft start pin voltage is zero. As the soft start voltage rises from 0V to 0.7V, the duty cycle increases from the minimum duty cycle to the operating duty cycle. The oscillator runs at the foldback frequency (1/4 of the switching frequency) until the feedback voltage rises above 0.3V. The risetime of the output is dependent of the soft start capacitor output capacitance, input and output voltage and load current. Voltage Setting Components The MIC2184 requires two resistors to set the output voltage as shown in Figure 5. Efficiency Considerations Efficiency is the ratio of output power to input power. The difference is dissipated as heat in the buck converter. Under light output load, the significant contributors are: • The VINA supply current • The VINP supply current, which includes the current required to switch the external MOSFET • Core losses in the output inductor To maximize efficiency at light loads: • Use a low gate charge MOSFET or use the smallest MOSFET, which is still adequate for maximum output current. • Use a ferrite material for the inductor core, which has less core loss than an MPP or iron power core. Under heavy output loads the significant contributors to power loss are (in approximate order of magnitude): • Resistive on time losses in the MOSFET • Switching transition losses in the MOSFET • Inductor resistive losses • Current sense resistor losses • Input capacitor resistive losses (due to the capacitors ESR) To minimize power loss under heavy loads: • Use low on resistance MOSFETs. Use low threshold logic level MOSFETs when the input voltage is below 5V. Multiplying the gate charge by the on resistance gives a figure of merit, providing a good balance between low load and high load efficiency. • Slow transition times and oscillations on the voltage and current waveforms dissipate more power during the turn on and turn off of the MOSFET. A clean layout will minimize parasitic inductance and capaci tance in the gate drive and high current paths. This will allow the fastest transition times and waveforms without oscillations. Low gate charge MOSFETs will transition faster than those with higher gate charge requirements. • For the same size inductor, a lower value will have fewer turns and therefore, lower winding resistance. However, using too small of a value will require more output capacitors to filter the output ripple, which will force a smaller bandwidth, slower transient response and possible instability under certain conditions. • Lowering the current sense resistor value will de crease the power dissipated in the resistor. However, it will also increase the overcurrent limit and will require larger MOSFETs and inductor components. • Use low ESR input capacitors to minimize the power dissipated in the capacitors ESR. VOUT MIC2184 Voltage Amplifier R1 Pin 6 R2 VREF 1.245V Figure 5 The output voltage is determined by the equation below. R1 R2 Where: VREF for the MIC2184 is typically 1.245V. Lower values of R1 are preferred to prevent noise from appearing on the FB pin. A typically recommended value is 10kΩ. If R1 is too small in value it will decrease the efficiency of the power supply, especially at low output loads. Once R1 is selected, R2 can be calculated with the following formula. VOUT = VREF × 1 + R2= VREF × R1 VOUT – VREF December 10, 2001 11 MIC2184 MIC2184 Micrel Package Information PIN 1 0.157 (3.99) 0.150 (3.81) DIMENSIONS: INCHES (MM) 0.020 (0.51) REF 0.020 (0.51) 0.013 (0.33) 0.0098 (0.249) 0.0040 (0.102) 0.050 (1.27) BSC 0.0648 (1.646) 0.0434 (1.102) 0.394 (10.00) 0.386 (9.80) 45° 0°–8° 0.050 (1.27) 0.016 (0.40) SEATING PLANE 0.244 (6.20) 0.228 (5.79) 16-Pin SOP (M) PIN 1 DIMENSIONS: INCHES (MM) 0.157 (3.99) 0.150 (3.81) 0.009 (0.2286) REF 0.025 (0.635) BSC 0.0098 (0.249) 0.0040 (0.102) 0.012 (0.30) 0.008 (0.20) 0.0098 (0.249) 0.0075 (0.190) 0.196 (4.98) 0.189 (4.80) SEATING 0.0688 (1.748) PLANE 0.0532 (1.351) 45° 8° 0° 0.050 (1.27) 0.016 (0.40) 0.2284 (5.801) 0.2240 (5.690) 16-Pin QSOP (QS) MICREL INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131 TEL + 1 (408) 944-0800 FAX + 1 (408) 944-0970 WEB USA http://www.micrel.com This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc. © 2001 Micrel Incorporated MIC2184 12 December 10, 2001