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

MP6004 Rev. 1.0

www.MonolithicPower.com

15

4/14/2015

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Light-Load Control

In flyback mode (if the load decreases),

MP6004

stretches

down

the

frequency

automatically to reduce the power transferring

approximate 10 kHz minimum frequency is

applied to detect the output voltage even at a

very light load. During this condition, the

reduce the power transferring. The MP6004 still

transfers some energy to the output even if

there is no load on the output due to the 10 kHz

minimum frequency. This means that some

load is required to maintain the output voltage,

In buck mode, the MP6004 has no minimum

frequency limit, so it stretches down to a very

low frequency and regulates the output

automatically even there is no load on the

output.

Frequency Control

By monitoring the auxiliary winding voltage in

flyback mode or monitoring the SW voltage in

buck mode, the MP6004 detects and regulates

the inductor current in DCM. The frequency is

controlled by the peak current, the current ramp

slew rate, and the load current. The maximum

frequency occurs when the MP6004 runs in

critical

conduction

mode,

providing

the

maximum load power. The MP6004 switching

frequency should be lower than 200 kHz in the

design.

Voltage Control

In flyback application, the MP6004 detects the

auxiliary winding voltage from FB1 during the

secondary-side diode conduction period.

Assume the secondary winding is the master,

and the auxiliary winding is the slave. When the

secondary-side

diode

conducts,

the

FB1

voltage is calculated with Equation (6):

A2

FB1

OUT

D1F

S1

2

NR

V(V

V )

NR

R

=×

+

×

+

(6)

Where:

and the secondary-side winding, respectively.

R1 and R2 are the resistor dividers for sampling.

The output voltage differs from the secondary-

winding voltage due to the current-dependant

diode forward voltage drop. If the secondary-

winding voltage is always detected at a fixed

secondary current, the difference between the

output voltage and the secondary-winding

the auxiliary-winding voltage after the internal

power MOSFET turns off for 0.7 μs and finishes

the sampling after the secondary-side diode

conducts for 3 μs. This provides good

regulation when the load changes. However,

the secondary diode conducting period must be

longer than 3 μs in each cycle, and the FB1

signal must be smooth 0.7 μs after the switch

turns off.

With a buck solution, there is one FB2 pin

referenced to VIN. It can be used as the

reference voltage for the buck application. The

output voltage is referenced to VIN and does

not have the same GND as the input power.

Programming the Current Limit

The MP6004 current limit is set by an external

resistor (R3) from ILIM to ground. The value of

R3 can be estimated with Equation (7):

L

LIM

V0.18

100

I

R3

L

×

=+

(7)

voltage applied on the inductor when the

MOSFET turns on, R3 is the setting resistor in

kΩ, and L is the inductor in μH.

The current limit cannot be programmed higher

than 3 A.

Leading-Edge Blanking

Transformer parasitic capacitance induces a

current spike on the sense resistor when the

power switch turns on. The MP6004 includes a

450 ns leading-edge blanking period to avoid

falsely terminating the switching pulse. During

this

blanking

period,

the

current

sense

comparator is disabled, and the gate driver

cannot switch off.