MIC38C43 datasheet(9/24 Pages) MICROCHIP

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DS20006436B-page 9

MIC38C42/43/44/45

4.0

FUNCTIONAL DESCRIPTION

Familiarity with 384x converter designs is assumed.

4.1

MIC38C4x Advantages

4.1.1

START-UP CURRENT

Start-up current has been reduced to an ultra-low

50 μA

(typical)

permitting

higher-resistance,

lower-wattage, start-up resistors (powers controller

during power supply start-up). The reduced resistor

wattage reduces cost and printed circuit space.

4.1.2

OPERATING CURRENT

Quiescent operating current has been reduced to 4 mA

compared to 11 mA for a typical bipolar controller. The

controller runs cooler and the VDD hold-up capacitance

required during start-up may be reduced.

4.1.3

OUTPUT DRIVER

Complementary internal P-channel and N-channel

MOSFETs produce rail-to-rail output voltages for better

performance driving external power MOSFETs. The

driver transistor’s low on resistance and high peak

current capability can drive gate capacitances of

greater than 1000 pF. The value of output capacitance

which can be driven is determined only by the rise/fall

time requirements. Within the restrictions of output

capacity and controller power dissipation, maximum

switching frequency can approach 500 kHz.

4.2

Design Precautions

When operating near 20V, circuit transients can easily

exceed the 20V absolute maximum rating,

permanently damaging the controller’s CMOS

construction. To reduce transients, connect a 0.1 μF

low-ESR capacitor to next to the controller’s supply

VDD (or VD for ‘-1’ versions) and ground connections.

Film type capacitors, such as Wima MKS2, are

recommended.

When designing high-frequency converters, avoid

capacitive and inductive coupling of the switching

waveform into high impedance circuitry such as the

error amplifier, oscillator, and current sense amplifier.

Avoid long printed-circuit traces and component leads.

Locate oscillator and compensation circuitry near the

IC. Use high frequency decoupling capacitors on VREF,

and if necessary, on VDD. Return high di/dt currents

directly to their source and use large area ground

planes.

4.3

Buck Converter

Refer to Figure 4-1. When at least 26V is applied to the

input, C5 is charged through R2 until the voltage VDD is

greater than 14.5V (the undervoltage lockout value of

the MIC38C42). Output switching begins when Q1 is

turned on by the gate drive transformer T1, charging

the output filter capacitor C3 through L1. D5 supplies a

regulated +12V to VDD once the circuit is running.

Current sense transformer CT1 provides current

feedback to ISNS for current-mode operation and

cycle-by-cycle current limiting. This is more efficient

than a high-power sense resistor and provides the

required ground-referenced level shift.

When Q1 turns off, current flow continues from ground

through D1 and L1 until Q1 is turned on again.

The 100V Schottky diode D1 reduces the forward

voltage drop in the main current path, resulting in

higher efficiency than could be accomplished using an

ultra-fast-recovery diode. R1 and C2 suppress parasitic

oscillations from D1.

Using a high-value inductance for L1 and a low-ESR

capacitor for C3 permits small capacitance with

minimum output ripple. This inductance value also

improves circuit efficiency by reducing the flux swing in

L1.

Magnetic components are carefully chosen for minimal

loss at 500 kHz. CT1 and T1 are wound on Magnetics,

Inc. P-type material toroids. L1 is wound on a Siemens

N49 EFD core.

TABLE 4-1:

MAGNETIC COMPONENTS

Symbol

Custom Coils

(Note 1)

ETS

(Note 2)

CT1

4923

ETS 92420

4924

ETS 92419

4925

ETS 92421

Note 1: Custom Coils, Alcester, SD. Tel: (605)

934-2460.

2: Energy Transformation Systems, Inc. Tel:

(510) 656-2012.

TABLE 4-2:

COMPONENT TEST RESULTS

Test

Conditions

Results

Line

Regulation

VIN = 26V to 80V,

VOUT = 12V, IO = 2A

0.5%

Load

Regulation

VIN = 48V, VOUT = 12V,

IO = 0.2A to 2A

0.6%

Efficiency

VIN = 48V, VOUT = 12V,

IO = 2A

90%

Output Ripple VIN = 48V, VOUT = 12V,

IO = 2A (20 MHz BW)

100 mV

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