200kHz MIC4574/5/6 Family Design Guide Kevin Lynn Introduction

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200kHz MIC4574/5/6 Family Design Guide Kevin Lynn Introduction

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200kHz MIC4574/5/6 Family Design Guide Kevin Lynn
Introduction
Micrel's MIC457x family BiCMOS simple buck voltage regulators feature faster rise/fall time, faster response fault conditions, improved efficiency light loads. Description MIC457x switching regulator basically (pulse width modulation) controller with fixed gain error amplifier, 200kHz oscillator, internal compensation network. non-inverting side error amplifier tied 1.23V bandgap reference.
+40V
Shutdown Enable
Definitions Critical Inductance Condition critical inductance condition when current through inductor decays zero just prior next "on" time regulator switch. This occurs boundary between continuous discontinuous operation. Discontinuous Operation Discontinuous operation occurs when, condition input voltage output current, inductor current decays zero before next "on" time regulator switch. Continuous Operation Continuous operation occurs when, condition input voltage output current, inductor current does decay zero before next "on" time regulator switch. Continuous Conduction Operation
SHDN
Internal Regulator 200kHz Oscillator Thermal Shutdown Current Limit
Comparator
Driver Switch
VOUT COUT
Error Amp. MIC457x-x.x
1.23V Bandgap
external adjustable versions
Figure Block Diagram (Fixed Version)
Critical Inductance Compute value critical inductance required application worst case combination input voltage output load current. This will minimum value inductance that will guarantee continuous conduction operation over input voltage output load conditions. critical inductance condition, peak inductor current twice average current. average current current delivered load. peak current critical inductance condition
Buck Regulator Design Procedure
Select MIC4574 (0.5A), MIC4575 (1A), MIC4576 (3A) based required output current. higher current rated regulators chosen current applications, make sure current limit range appropriate that application. Output Voltage fixed output voltages, 3.3V 5.0V versions available. output voltage adjustable regulators configured using external resistive divider.
IPEAK
Where: duty cycle switch time/switch cycle time, switch cycle time, input (supply) voltage VOUT regulator output voltage inductance filter inductor switching frequency (Hz) input power will assumed equal output power.
1.23V
best performance, should between 10k. Inductor Selection Criteria following criteria used inductor selection: Mode operation (continuous discontinuous). Peak inductor current Volt-seconds applied inductor
RLOAD
Where: estimated efficiency reasonable initial estimate (0.8) RLOAD load resistance
1997
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and,
CRITICAL
RLOAD
Where: COUT output capacitance (µF) inductance (µH) This guarantees that dominant pole pair filter does occur frequency that high regulator's internal loop compensation circuitry. This computation result capacitor value that small provide adequate peak-to-peak output ripple reduction. Peak-to-peak ripple voltage function capacitor value type. ESR/ESL (equivalent series resistance/equivalent series inductance) capacitor should used lower ripple voltage. (Standard capacitors paralleled reduce effective ESR/ESL value.) electrolytic capacitors available from Panasonic, Nichicon, United Chemicon. Maximum peak-to-peak ripple voltage (assuming filter capacitor) estimated follows:
Duty Cycle Compute duty cycle required maximum required input voltage minimum load current. your cannot guarantee minimum load current, additional resistive load required regulator output.
DMIN
VIN(max)
this value DMIN minimum value RLOAD equation determine value critical inductance. This minimum value inductance required. Changing minimum load and/or maximum input voltage requirement changes minimum required critical inductance. value inductance chosen allow regulator operate discontinuous mode under certain conditions. Discontinuous mode typically occurs maximum input minium load current. many cases this present problem, however, should verified that operation discontinuous mode still allows circuit satisfy load regulation requirement. Maximum Compute maximum volt-microseconds applied inductor:
VP-P
(VIN(max) VOUT
VIN2
VIN(max)
Input Capacitor Selection input bypass capacitor must least 47µF maintain stability. capacitors recommended. operating temperature range below -25°C, value this capacitor should increased. Adding ceramic solid tantalum capacitor near input will also increase regulator stability temperatures. capacitor's ripple current rating should more than ripple component inductor current:
Inductor Peak Current Compute peak current through inductor. This maximum load current peak ripple current though inductor.
IPEAK VIN(max) RLOAD IN(max)
IRIPPLE
VIN(max)
Inductor Selection Refer "Inductor Selection Cross Reference" table select appropriate inductor your application. selection should satisfy following: Inductance Calculated Critical Inductance Volt-second Capability Calculated applicable) Calculated IPEAK Current 0.85 Output Capacitor Selection stable operation, output capacitor must satisfy following:
Catch Diode Selection Although either Schottky fast recovery diode used, Schottky diode will provide best performance because lower voltage drop faster switching speed will result higher efficiency. Fast recovery diodes with abrupt turn-off characteristics cause problems and/or instabilities. reverse voltage rating catch diode should least 1.25 maximum input voltage. Standard 1N400x series diodes should used. reverse recovery time this type diode excessive which will cause additional noise heat dissipation diode regulator's internal power switch.
VIN(max) COUT 13300
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Typical Applications
Fixed 3.3V Buck Regulator Figure shows 3.3V buck regulator using inexpensive standard components. high efficiency (~80%) form factor afforded TO-263 surface mount package makes this ideal battery operated designs. lower ripple voltage desired, standard 220µF capacitor replaced with standard 330µF. lower ripple small size, Oscon 105A220M capacitor (220µF, ESR) used.
+4.5V +40V Unregulated Input
FEEDBACK INPUT
MIC45753.3
OUTPUT
60µH 1N5822
3.3V, Output 220µF
47µF
ON/OFF
Figure 3.3V Buck Regulator
Unregulated Input
FEEDBACK INPUT
MIC4575
OUTPUT
ON/OFF
Figure Undervoltage Lockout
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Inductor Selection Cross Reference
Renco Part1 Part RL5341-20-1 RL5341-48-1 RL5341-68-1 RL5341-100-1 RL5341-150-1 RL5341-220-1 RL5341-330-1 RL5341-470-1 RL5341-680-1 RL5341-1000-1 RL5342-20-1 RL5342-48-1 RL5342-68 RL5342-100-1 RL5342-150-1 RL5342-220-1 RL5342-330-1 RL5342-470-1 RL5342-680-1 RL5342-1000-1 RL5341-20-3 RL5341-48-3 RL5341-68-3 RL5341-100-3 RL5341-150-3 RL5341-220-3 RL5341-330-3 RL5341-470-3 RL5341-680-3 RL5341-1000-3 RL5342-20-3 RL5342-48-3 RL5342-68-3 RL5342-100-3 RL5342-150-3 RL5342-220-3 RL5342-330-3 RL5342-470-3 RL5342-680-3 RL5342-1000-3 1500 2000 1200 1272 2155 3221 4784 6000 1202 1946 2837 3900 (µH) 1000 1000 1000 1000 Description powdered iron
moly permalloy
powdered iron
moly permalloy
Renco Electronics Inc., Deer Park, York; tel: (516) 586-5566
4-162
1997

200kHz MIC4574/5/6 Family Design Guide Kevin Lynn Introduction

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