MIC5156/5157/5158 Micrel MIC5156/5157/5158 Super LDO™ Regulator Controller General Description The MIC5156, MIC5157, and MIC5158 Super Low-Dropout (LDO) Regulator Controllers are single IC solutions for highcurrent low-dropout linear voltage regulation. Super LDO™ Regulators have the advantages of an external N-channel power MOSFET as the linear pass element. The MIC5156/7/8 family features a dropout voltage as low as the RDS(ON) of the external power MOSFET multiplied by the output current. The output current can be as high as the largest MOSFETs can provide. The MIC5156/7/8 family operates from 3V to 36V. The MIC5156 requires an external gate drive supply to provide the higher voltage needed to drive the gate of the external MOSFET. The MIC5157 and MIC5158 each have an internal charge pump tripler to produce the gate drive voltage. The tripler is capable of providing enough voltage to drive a logiclevel MOSFET to 3.3V output from a 3.5V supply and is clamped to 17.5V above the supply voltage. The tripler requires three external capacitors. The regulator output is constant-current limited when the controller detects 35mV across an optional external sense resistor. An active-low open-collector flag indicates a low voltage of 8% or more below nominal output. A shutdown (low) signal to the TTL-compatible enable control reduces controller supply current to less than 1µA while forcing the output voltage to ground. The MIC5156-3.3 and MIC5156-5.0 controllers have internally fixed output voltages. The MIC5156 [adjustable] output is configured using two external resistors. The MIC5157 is a fixed output controller which is externally configured to select either 3.3V, 5.0V, or 12V. The MIC5158 can be configured as a fixed 5V controller or programmed to any voltage from 1.3V to 36V using two external resistors. The MIC5156 is available in an 8-pin DIP or SOP. The MIC5157 and MIC5158 are available in a 14-pin DIP or SOP which operate from –40°C to +85°C. Features • • • • • • • • • • • • • 4.5mA typical operating current <1µA typical standby current Low external parts count Optional current limit (35mV typical threshold) 1% initial output voltage tolerance in most configurations 2% output voltage tolerance over temperature Fixed output voltages of 3.3V, 5.0V (MIC5156) Fixed output voltages of 3.3V, 5.0V, 12V (MIC5157) Programmable (1.3 to 36V) with 2 resistors (MIC5156/8) Internal charge pump voltage tripler (MIC5157/8) Enable pin to activate or shutdown the regulator Internal gate-to-source protective clamp All versions available in DIP and SOP Applications • • • • • Ultrahigh current ultralow dropout voltage regulator Constant high-current source Low parts count 5.0V to 3.3V computer supply Low noise/low-dropout SMPS post regulator High-current, current-limited switch Typical Applications +12V 1.0µF 5V VDD C1– C1+ 8 8 9 10 11 12 13 0.1µF 3mΩ VIN 5V 47µF RS RS = 0.035V / ILIMIT SMP60N03-10L 1 3.3V 2 EN 3 S EN 7 S G D VDD 6 4 MIC5157 MIC5156-3.3 5 5 FLAG FLAG 6 D GND 7 GND 1 G 2 C2– 3 C2+ 4 VCP 0.1µF Enable Shutdown VP 0.1µF CL* 47µF VOUT 3.3V, 10A VIN (3.61V min.) 3mΩ 47µF RS RS = 0.035V / ILIMIT * Improves transient response to load changes IRLZ44 (Logic Level MOSFET) 10A 5V to 3.3V Desktop Computer Regulator 14 Enable Shutdown CL* 47µF VOUT 3.3V, 10A * Improves transient response to load changes 10A Low-Dropout Voltage Regulator Super LDO is a trademark of Micrel, Inc. 3-112 March 1999 MIC5156/5157/5158 Micrel Ordering Information MIC5156 Part Number Temperature Range Voltage Package MIC5156-3.3BN –40°C to +85°C 3.3V 8-pin DIP MIC5156-5.0BN –40°C to +85°C 5.0V 8-pin DIP MIC5156BN –40°C to +85°C Adjustable 8-pin DIP MIC5156-3.3BM –40°C to +85°C 3.3V 8-pin SOP MIC5156-5.0BM –40°C to +85°C 5.0V 8-pin SOP MIC5156BM –40°C to +85°C Adjustable 8-pin SOP Ordering Information MIC5157 Part Number Temperature Range Voltage Package MIC5157BN –40°C to +85°C Selectable 14-pin DIP MIC5157BM –40°C to +85°C Selectable 14-pin SOP Ordering Information MIC5158 Part Number Temperature Range Voltage Package MIC5158BN –40°C to +85°C 5.0V/Adj. 14-pin DIP MIC5158BM –40°C to +85°C 5.0V/Adj. 14-pin SOP Pin Configuration MIC5156-x.x EN 1 MIC5156 FLAG 2 7 D (Drain) FLAG 2 7 D (Drain) GND 3 6 G (Gate) GND 3 6 G (Gate) MIC5157 5V 1 3.3V 2 5 VD D VP 4 5 VD D VP 4 March 1999 8 EA EN 1 8 S (Source) MIC5158 EA 1 14 EN 14 EN 13 S (Source) 5V FB 2 13 S (Source) FLAG 3 12 D (Drain) FLAG 3 12 D (Drain) GND 4 11 G (Gate) GND 4 11 G (Gate) VCP 5 10 VD D VCP 5 10 VD D C2– 6 9 C1– C2– 6 9 C1– C2+ 7 8 C1+ C2+ 7 8 C1+ 3-113 3 MIC5156/5157/5158 Micrel Pin Description MIC5156 Pin Number Pin Name Pin Function 1 EN 2 FLAG Output Flag (Output): Open collector output is active (low) when VOUT is more than 8% below nominal output. Circuit has 3% hysteresis. 3 GND Circuit ground. 4 VP N-channel Gate Drive Supply Voltage: User supplied voltage for driving the gate of the external MOSFET. 5 VDD Supply Voltage (Input): Supply voltage connection. Connect sense resistor (RS) to VDD if current limiting used. Connect supply bypass capacitor to ground near device. 6 G Gate (Output): Drives the gate of the external MOSFET. 7 D Drain and Current Limit (Input): Connect to external MOSFET drain and external sense resistor (current limit), or connect to VDD and external MOSFET drain (no current limit). 8 (3.3V, 5V) S Source (Input): Top of internal resistive divider chain. Connect directly to the load for best load regulation. 8 (adjustable) EA Enable (Input): TTL high enables regulator; TTL low shuts down regulator. Error Amplifier (Input): Connect to external resistive divider. Pin Description MIC5157, MIC5158 Pin Number Pin Name Pin Function 1 (MIC5157) 5V 5V Configuration (Input): Connect to S (source) pin for 5V output. 1 (MIC5158) EA Error Amplifier (Input): Connect to external resistive divider to obtain adjustable output. 2 (MIC5157) 3.3V 2 (MIC5158) 5V FB 5V Feedback (Input): Connect to EA for fixed 5V output. 3 FLAG Output Voltage Flag (Output): Open collector is active (low) when VOUT is 8% or more below its nominal value. 4 GND Circuit ground. 5 VCP Voltage Tripler Output [Filter Capacitor]. Connect a 1 to 10µF capacitor to ground. 6 C2– Charge Pump Capacitor 2: Second stage of internal voltage tripler. Connect a 0.1µF capacitor from C2+ to C2–. 7 C2+ Charge Pump Capacitor 2: See C2– pin 6. 8 C1+ Charge Pump Capacitor 1: First stage of internal voltage tripler. Connect a 0.1µF capacitor from C1+ to C1–. 9 C1– Charge Pump Capacitor 1: See C1+ pin 8. 10 VDD Supply Voltage (Input): Supply voltage connection. Connect sense resistor (RS) to VDD if current limiting used. Connect supply bypass capacitor to ground near device. 11 G Gate (Output): Connect to External MOSFET gate. 12 D Drain and Current Limit (Input): Connect to external MOSFET drain and external sense resistor (current limit), or connect to VDD and external MOSFET drain (no current limit). 13 (MIC5157) S Source and 3.3V/5V Configuration: Top of internal resistor chain. Connect to source of external MOSFET for 3.3V, 5V, and 12V operation. Also see 3.3V and 5V pin descriptions. 13 (MIC5158) S Source (Input): Top of internal resistor chain. Connect to top of external resistive divider and source of external MOSFET. 14 EN 3.3V Configuration (Input): Connect to S (source) pin for 3.3V output. Enable (Input): TTL high enables regulator; TTL low shuts down regulator. 3-114 March 1999 MIC5156/5157/5158 Micrel Absolute Maximum Ratings (Note 1) Operating Ratings (Note 2) Supply Input (VDD) ...................................................... +38V Enable Input (VEN) ......................................... –0.3V to 36V Gate Output (VG) MIC5156 ......................................... +55V Charge Pump Node (VCP) MIC5157/8 ........................ +55V Source Connection (VS) .................................... 1.3 to +36V Flag (VFLAG) .................................................... –0.3 to +40V Storage Temperature (TS) ....................... –65°C to +150°C Lead Temperature (soldering 10 sec.) ...................... 300°C Ambient Temperature Range (TA) MIC515xBM/BN ..................................... –40°C to +85°C Junction Temperature (TJ) ...................................... +150°C Thermal Resistance (θJA) Package MIC5156 MIC5157/8 DIP ............................... 100°C/W ..................... 90°C/W SOP .............................. 160°C/W ................... 120°C/W Electrical Characteristics VDD = 5V, VEN = 5V; TA = 25°C; unless noted. Symbol Parameter Condition Min VDD Supply Voltage IDD(ON) IDD(OFF) Supply Current MIC5156 Operating, VEN = 5V Shutdown, VEN = 0V IDD(ON) IDD(OFF) Supply Current MIC5157/8 Operating, VEN = 5V Shutdown, VEN = 0V VIH VIL Enable Input Threshold High Low EN IB Enable Input Bias Current VEN = 2.4V VCP Max. Charge Pump Voltage VCP – VDD, VDD > 10V fCP Charge Pump Frequency VOUT MAX Maximum Gate Drive Voltage (MIC5157/8) VSOURCE = 0V VDD = 3.5V VDD = 5V VDD = 12V VOUT MIN Minimum Gate Drive Voltage VSOURCE > VOUT(NOM) VLIM Current Limit Threshold VDD – VD @ ILIM VS Source Voltage Typ Max Units 36 V 2.7 0.1 10 5 mA µA 4.5 0.1 10 5 mA µA 1.3 1.3 0.8 V V 20 25 µA 17.5 18.5 V 3 2.4 160 5 9 24 7.0 11.3 28 kHz 9 15 30 1.0 V V V V 28 35 42 mV Short G (gate) to (S) source, Note 4 MIC5156-3.3 MIC5156-5.0 MIC5157, 3.3V pin to S pin (3.3V config.) MIC5157, 5V pin to S pin (5V config.) MIC5157, VDD = 7V, (12V config.) MIC5158, 5V FB pin to EA pin (5V config.) 3.267 4.950 3.250 4.950 11.70 4.925 3.3 5.0 3.3 5.0 12 5.0 3.333 5.050 3.350 5.050 12.30 5.075 V V V V V V 1.222 1.235 1.248 V 2 7 mV 16.6 20 V VBG Bandgap Reference Voltage MIC5156 [adjustable] and MIC5158 VLR Output Voltage Line Regulation 5V < VDD < 15V, VOUT = 3.3V VGS MAX Gate to Source Clamp VFT Flag Comparator Threshold % of nominal VSOURCE 92 % VFH Flag Comparator Hysteresis % of nominal VSOURCE 3 % VSAT Flag Comparator Sat. Voltage IFLAG = 1mA 14 0.09 Note 1. Exceeding the absolute maximum rating may damage the device. Note 2. The device is not guaranteed to function outside its operating rating. Note 3. Devices are ESD sensitive. Handling precautions recommended. Note 4. Test configuration. External MOSFET not used. March 1999 3-115 0.2 V 3 MIC5156/5157/5158 Micrel Typical Characteristics 5.0V Regulator Output Voltage vs. Temperature 3.33 5.03 10 3.31 3.30 3.29 3.28 3.27 MIC5157/8 On-State Supply Current vs. Supply Voltage 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 0 5 10 15 20 25 SUPPLY VOLTAGE (V) 30 5.01 5.00 4.99 4.98 4.96 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) MIC5157/8 On-State Supply Current vs. Temperature 5.0 4.5 4.0 3.5 3.0 2.5 VDD = 5V 2.0 1.5 1.0 0.5 0.0 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) 10 150 125 100 75 50 25 0 5 10 15 20 25 SUPPLY VOLTAGE (V) 0 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) 30 Enable Threshold Voltage vs. Temperature Enable Input Bias Current vs. Enable Voltage 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) 120 ENABLE BIAS CURRENT (µA) ENABLE THRESHOLD VOLTAGE (V) 0 VDD = 5V IFLAG = 1mA 175 20 4 LOGIC INPUT 2 3.3V OUTPUT 100 80 60 40 20 0 0 2 4 6 8 10 12 14 16 ENABLE VOLTAGE (V) 3-116 0.0 0.2 0.4 TIME (ms) 0.6 Off-State Supply Current vs. Temperature 5.0 4.5 VDD = 5V 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) Flag Output Voltage vs. Flag Current 200 30 6 -2 -0.2 1.0 0.9 FLAG VOLTAGE (V) 60 40 8 Flag Output Voltage vs. Temperature FLAG VOLTAGE (mV) CHARGE PUMP VOLTAGE (V) Charge-Pump Output Voltage vs. Supply Voltage 50 MOSFET = IRF540 VIN = 5V, IL = 0.5A CC1 = CC2 = 0.1µF CCP = 1µF CL = 50µF 0 4.97 VDD = 5V 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 CURRENT LIMIT THRESHOLD (mV) 2.0 5.02 OFF-STATE SUPPLY CURRENT (µA) 3.32 VOLTAGE (V) OUTPUT VOLTAGE (V) 12 3.26 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) ON-STATE SUPPLY CURRENT (mA) MIC5157/8 Turn-On Response Time for 3.3V 5.04 ON-STATE SUPPLY CURRENT (mA) OUTPUT VOLTAGE (V) 3.3V Regulator Output Voltage vs. Temperature 3.34 0 2 4 6 8 10 FLAG SINK CURRENT (mA) Current Limit Threshold vs. Temperature 70 60 50 40 30 20 10 0 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) March 1999 MIC5156/5157/5158 Micrel Block Diagram MIC5156 +12V Input +5V Input 0.1µF VP VDD Internal Bias EN RS 3mΩ Enable Shutdown 12k to all internal blocks ILIMIT Comparator D (Drain) [ILIMIT] 35mV 1.235V Bandgap Reference +5V Input Q2 FLAG G (Gate) Error Amp 16.6V Q1 SMP60N03-10L S* (Source) Regulated +3.3V Output 75mV * fixed version only 3.3V = 17k, 5V = 32k 17k† † ‡ 10k GND CL Load 1 Load 2 Switched 5V Load VOUT Comparator 3 EA‡ adjustable version only Block Diagram with External Components Fixed 3.3V Power Supply with 5.0V Load Switch Block Diagram MIC5157 +5V Input 0.1µF C1+ Oscillator C1 0.1µF C1– C2+ C3 1µF C2 C2– VCP VDD VCP Clamp Charge Pump Tripler Internal Bias ILIMIT Comparator 35mV FLAG RS 3mΩ Enable Shutdown to all internal blocks 1.235V Bandgap Reference VOUT Comparator EN D (Drain) [ILIMIT] G (Gate) Error Amp 16.6V Q1 IRFZ44 S (Source)* Regulated +3.3V Output 75mV 58k 15k 5V CL 3.3V 10k Block Diagram with External Components Fixed 3.3V 10A Power Supply March 1999 3-117 Load 17k GND MIC5156/5157/5158 Micrel Block Diagram MIC5158 +5V Input 0.1µF C1+ Oscillator C1 0.1µF C1– C2+ C3 1µF C2 C2– VCP VDD VCP Clamp Charge Pump Tripler Internal Bias ILIMIT Comparator 35mV FLAG RS 3mΩ Enable Shutdown to all internal blocks 1.235V Bandgap Reference VOUT Comparator EN D (Drain) [ILIMIT] G (Gate) Error Amp 16.6V Q1 IRFZ44 S (Source) Regulated +3.6V Output 32k 10k 5V FB CL Load 75mV GND EA 19.1k 10.0k Block Diagram with External Components Adjustable Power Supply, 3.6V Configuration Functional Description A Super LDO Regulator is a complete regulator built around Micrel’s Super LDO Regulator Controller. Refer to Block Diagrams MIC5156, MIC5157, and MIC5158. Version Differences The MIC5156 requires an external voltage for MOSFET gate drive and is available in 3.3V fixed output, 5V fixed output, or adjustable output versions. With 8-pins, the MIC5156 is the smallest of the Super LDO Regulator Controllers. The MIC5157 and MIC5158 each have an internal charge pump which provides MOSFET gate drive voltage. The MIC5157 has a selectable fixed output of 3.3V, 5V, or 12V. The MIC5158 may be configured for a fixed 5V or adjustable output. Enable (EN) With at least 3.0V on VDD, applying a TTL low to EN places the controller in shutdown mode. A TTL high on EN enables the internal bias circuit which powers all internal circuitry. EN must be pulled high if unused. The voltage applied to EN may be as high as 36V. The controller draws less than 1µA in shutdown mode. Gate Enhancement The Super LDO Regulator Controller manages the gate-tosource enhancement voltage for an external N-channel MOSFET (regulator pass element) placed between the supply and the load. The gate-to-source voltage may vary from 1V to 16V depending upon the supply and load conditions. Because the source voltage (output) approaches the drain voltage (input) when the regulator is in dropout and the MOSFET is fully enhanced, an additional higher supply voltage is required to produce the necessary gate-to-source enhancement. This higher gate drive voltage is provided by an external gate drive supply (MIC5156) or by an internal charge pump (MIC5157 and MIC5158). Gate Drive Supply Voltage (MIC5156 only) The gate drive supply voltage must not be more than 14V above the supply voltage (VP – VDD < 14V). The minimum necessary gate drive supply voltage is: VP = VOUT + VGS + 1 where: VP = gate drive supply voltage VOUT = regulator output voltage VGS = gate-to-source voltage for full MOSFET gate enhancement The error amplifier uses the gate drive supply voltage to drive the gate of the external MOSFET. The error amplifier output can swing to within 1V of VP. 3-118 March 1999 MIC5156/5157/5158 Micrel Charge Pump (MIC5157/5158 only) The charge pump tripler creates a dc voltage across reservoir capacitor C3. External capacitors C1 and C2 provide the necessary storage for the stages of the charge pump tripler. The tripler’s approximate dc output voltage is: VCP ≈ 3 (VDD – 1) where: VCP = charge pump output voltage VDD = supply voltage The VCP clamp circuit limits the charge pump voltage to 16V above VDD by gating the charge pump oscillator ON or OFF as required. The charge pump oscillator operates at 160kHz. The error amplifier uses the charge pump voltage to drive the gate of the external MOSFET. It provides a constant load of about 1mA to the charge pump. The error amplifier output can swing to within 1V of VCP. Although the MIC5157/8 is designed to provide gate drive using its internal charge pump, an external gate drive supply voltage can be applied to VCP . When using an external gate drive supply, VCP must not be forced more than 14V higher than VDD. When constant loads are driven, the ON/OFF switching of the charge pump may be evident on the output waveform. This is caused by the charge pump switching ON and rapidly increasing the supply voltage to the error amplifier. The period of this small charge pump excitation is determined by a number of factors: the input voltage, the 1mA op-amp load, any dc leakage associated with the MOSFET gate circuit, the size of the charge pump capacitors, the size of the charge pump reservoir capacitor, and the characteristics of the input voltage and load. The period is lengthened by increasing the charge pump reservoir capacitor (C3). The amplitude is reduced by weakening the charge pump—this is accomplished by reducing the size of the pump capacitors (C1 and C2). If this small burst is a problem in the application, use a 10µF reservoir capacitor at C3 and 0.01µF pump capacitors at C1 and C2. Note that the recovery time to repetitive load transients may be affected with small pump capacitors. Gate-to-Source Clamp A gate-to-source protective voltage clamp of 16.6V protects the MOSFET in the event that the output voltage is suddenly forced to zero volts. This prevents damage to the external MOSFET during shorted load conditions. Refer to “Charge Pump” for normal clamp circuit operation. The source connection required by the gate-to-source clamp is not available on the adjustable version of the MIC5156. Output Regulation At start-up, the error amplifier feedback voltage (EA), or internal feedback on fixed versions, is below nominal when compared to the internal 1.235V bandgap reference. This forces the error amplifier output high which turns on external MOSFET Q1. Once the output reaches regulation, the controller maintains constant output voltage under changing input and load conditions by adjusting the error amplifier output voltage (gate enhancement voltage) according to the feedback voltage. Out-of-Regulation Detection When the output voltage is 8% or more below nominal, the open-collector FLAG output (normally high) is forced low to signal a fault condition. The FLAG output can be used to signal or control external circuitry. The FLAG output can also be used to shut down the regulator using the EN control. Current Limiting Super LDO Regulators perform constant-current limiting (not foldback). To implement current limiting, a sense resistor (RS) must be placed in the “power” path between VDD and D (drain). If the voltage drop across the sense resistor reaches 35mV, the current limit comparator reduces the error amplifier output. The error amplifier output is decreased only enough to reduce the output current, keeping the voltage across the sense resistor from exceeding 35mV. Application Information March 1999 3-119 VIN G MIC515x GND S Load MOSFET Selection Standard N-channel enhancement-mode MOSFETs are acceptable for most Super LDO regulator applications. Logic-level N-channel enhancement-mode MOSFETs may be necessary if the external gate drive voltage is too low (MIC5156), or the input voltage is too low, to provide adequate charge pump voltage (MIC5157/8) to enhance a standard MOSFET. Circuit Layout For the best voltage regulation, place the source, ground, and error amplifier connections as close as possible to the load. See figures (1a) and (1b). Figure 1a. Connections for Fixed Output 3 MIC5156/5157/5158 Micrel Adjustable Configurations Micrel’s MIC5156 [adjustable] and MIC5158 require an external resistive divider to set the output voltage from 1.235V to 36V. For best results, use a 10kΩ resistor for R2. See equation (1) and figure (2). VIN G S Load MIC5157 or MIC5158 EA GND 1) V R1 = 1× 104 OUT − 1 1.235 G VOUT S Figure 1b. Connections for Adjustable Output MIC5157/8 GND VIN R1 EA* R2 10k G * MIC5156 EA GND Load Figure 2. Typical Resistive Divider Input Filter Capacitor The Super LDO requires an input bypass capacitor for accommodating wide changes in load current and for decoupling the error amplifier and charge pump. A medium to large value low-ESR (equivalent series resistance) capacitor is best, mounted close to the device. * Optional 16V zener diode recommended in applications where VG is greater than 18V Figure 1c. MIC5156 Connections for Adjustable Output MOSFET Gate-to-Source Protection When using the adjustable version of the MIC5156, an external 16V zener diode placed from gate-to-source is recommended for MOSFET protection. All other versions of the Super LDO regulator controller use the internal gate-tosource clamp. Output Voltage Configuration Fixed Configurations The MIC5156-3.3 and MIC5156-5.0 are preset for 3.3V and 5.0V respectively. The MIC5157 operates at 3.3V when the 3.3V pin is connected to the S (source) pin; 5.0V when the 5.0V pin is connected to the S pin; or 12V if the 3.3V and 5.0V pins are open. The MIC5158 operates at a fixed 5V (without an external resistive divider) if the 5V FB pin is connected to EA. Output Filter Capacitor An output filter capacitor may be used to reduce ripple and improve load regulation. Stable operation does not require a large capacitor, but for transient load regulation the size of the output capacitor may become a consideration. Common aluminum electrolytic capacitors perform nicely; very lowESR capacitors are not necessary. Increased capacitance (rather than reduced ESR) is preferred. The capacitor value should be large enough to provide sufficient I = C × dV/dt current consistent with the required transient load regulation quality. For a given step increase in load current, the output voltage will drop by about dV = I × dt/C, where I represents the increase in load current over time t. This relationship assumes that all output current was being supplied via the MOSFET pass device prior to the load increase. Small (0.01µF to 10µF) film capacitors parallel to the load will further improve response to transient loads. Some linear regulators specify a minimum required output filter capacitance because the capacitor determines the dominant pole of the system, and thereby stabilizes the system. This is not the situation for the MIC5156/7/8; its dominant pole is determined within its error amplifier. 3-120 March 1999 MIC5156/5157/5158 Micrel Current Limiting Current sensing requires a low-value series resistance (Rs) between VDD and D (drain). Refer to the typical applications. The internal current-limiting circuit limits the voltage drop across the sense resistor to 35mV. Equation (2) provides the sense resistor value required for a given maximum current. Gate Supply Enable Shutdown VG EN VIN RS VDD D MIC5156-x.x G RS = 35mV ILIM S GND Load 2) where: RS = sense resistor value ILIM = maximum output current Most current-limited applications require low-value resistors. See Application Hints 21 and 25 for construction hints. Non-Current-Limited Applications For circuits not requiring current limiting, do not use a sense resistor between VDD and D (drain). See figure (3). The controller will not limit current when it does not detect a 35mV drop from VDD to D. VIN Figure 4a. High-Side Switch If a MIC5157 or MIC5158 is used and is shutdown for a given time, the charge pump reservoir VCP will bleed off. If recharging the reservoir causes an unacceptable delay in the load reaching its operating voltage, do not use the EN pin for on/ off control. Instead, use the MIC5158, hold EN high to keep the charge pump in continuous operation, and switch the MOSFET on or off by overriding the error amplifier input as shown in figure (4b). VIN VDD D MIC5156 G VDD EN G S MIC5158 Figure 3. No Current Limit 3.3V Microprocessor Applications For computer designs that use 3.3V microprocessors with 5V logic, the FLAG output can be used to suppress the 5V supply until the 3.3V output is in regulation. Refer to the external components shown with the MIC5156 Block Diagram. SMPS Post Regulator Application A Super LDO regulator can be used as a post regulator for a switch-mode power supply. The Super LDO regulator can provide a significant reduction in peak-to-peak ripple voltage. High-Current Switch Application All versions of the MIC5156/7/8 may be used for currentlimited, high-current, high-side switching with or without voltage regulation. See figure (4a). Simply leave the “S” terminal open. A 16V zener diode from the gate to the source of the MOSFET protects the MOSFET from overdrive during fault conditions. EA GND Output Off Output On Load S 1N4148 Figure 4b. Fast High-Side Switch Battery Charger Application The MIC5158 may be used in constant-current applications such as battery chargers. See figure (5). The regulator supplies a constant-current (35mV ÷ R3) until the battery approaches the float voltage: R1 VFL = 1. 235 1 + R2 where: VFL = float voltage At float voltage, the MOSFET is shut off. A trickle charge is supplied by R4. March 1999 3-121 3 MIC5156/5157/5158 Micrel VIN R3 VDD R4 D EN G MIC5158 S R1 EA GND R2 Figure 5. Battery Charger Concept Uninterruptible Power Supply The MIC5157 and two N-channel MOSFETs provide battery switching for uninterruptible power as shown in figure (6). Two MOSFETs are placed source-to-source to prevent current flow through their body diodes when switched off. The Super LDO regulator is continuously enabled to achieve fast battery switch-in. Careful attention must be paid to the ac-line monitoring circuitry to ensure that the output voltage does not fall below design limits while the battery is being switched in. VDD EN D Q1 G D G S MIC5158 S S MOSFET body diodes shown for clarity G GND Line Battery AC Line EA Q2 D 40V max. 1N4148 Off-line Power Supply Uninterruptable DC Figure 6. UPS Power Supply Concept 3-122 March 1999