MP6004 datasheet(14/23 Pages) MPS

14 / 23 page

MP6004—PRIMARY-SIDE REGULATED FLYBACK/BUCK 80V DCDC CONVERTER

MP6004 Rev. 1.0

www.MonolithicPower.com

4/14/2015

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

OPERATION

Start-Up and Power Supply

MP6004 features an 80 V start-up circuit. When

VIN is higher than 4.3 V, the capacitor at VCC is

charged through the internal LDO. Normally VCC

is regulated at 5.4 V (if VIN is high enough),

and the VCC UVLO is 4.7 V, typically. With the

exception of VCC UVLO, the MP6004 has an

additional 11.6 V VIN UVLO. When VIN is higher

than the 11.6 V UVLO, VCC is charged higher

than the 4.7 V UVLO, and EN pin is high,

MP6004 starts switching.

VCC can be powered from the transformer

auxiliary winding to save IC power loss. Refer

to the “Vcc Power Supply Setting” section on

page 18 for more details.

Flyback and Buck Mode

MP6004 supports both flyback and buck

topology applications. Connect MODE to GND

to set the MP6004 in flyback mode, and float

MODE to set the MP6004 in buck mode. MODE

is pulled up internally to VCC through a 1.5 µA

current source. Do not connect MODE to VIN in

buck mode, and do not place a resistor

between MODE and GND in flyback mode.

Switching Work Principle

After start-up, MP6004 works in discontinuous

conduction mode (DCM). The second switching

cycle will not start until the inductor current

drops to 0 A. In each cycle, the internal

MOSFET is turned on, and the current-sense

circuit senses the current IP(t) internally.

Use Equation (1) to calculate the rate at which

the current rises linearly in flyback mode:

p(t)

(1)

When IP(t) rises up to IPK, the internal MOSFET

turns off (see Figure 2). The energy stored in

the primary-side inductance transfers to the

secondary-side through the transformer.

Figure 2—Primary-side current waveform

The primary-side inductance (LM) stores energy

in each cycle as a function of Equation (2):

MPK

EL I

(2)

Calculate the power transferred from the input

to the output with Equation (3):

MPK S

PL I F

(3)

Where FS is the switching frequency. When IPK

is constant, the output power depends on FS

and LM.

Use Equation (4) to calculate the rate at which

the current rises linearly in buck mode:

p(t)

OUT

−

(4)

The internal MOSFET turns off when IP(t) rises

to IPK (see Figure 3). The output current is

calculated with Equation (5):

OUT

IDI

(5)

Where, D is the inductor current conducting

duty cycle.

Figure 3—Inductor current waveform

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