MC44608P40 데이터 시트보기 (PDF) - Motorola => Freescale

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MC44608P40

Few External Components Reliable and Flexible GreenLine™ Very High Voltage PWM Controller

Motorola => Freescale 

OSC

4V

Vcont

2.4 V

Clock

Figure 5.

MC44608

Figure 6. Hiccup Mode

V CC

13 V

10 V

6.5 V

DMG

Iprim

The OSC and Clock signals are provided according to the

figure 5. The Clock signals correspond to the CT capacitor

discharge. The bottom curve represents the current flowing

in the sense resistor Rcs. It starts from zero and stops when

the sawtooth value is equal to the control voltage Vcont. In

this way the SMPS is regulated with a voltage mode control.

Overvoltage Protection

The MC44608 offers two OVP functions:

– a fixed function that detects when VCC is higher than

15.4V

– a programmable function that uses the demag pin. The

current flowing into the demag pin is mirrored and compared

to the reference current Iovp (120µA). Thus this OVP is

quicker as it is not impacted by the VCC inertia and is called

QOVP.

In both cases, once an OVP condition is detected, the

output is latched off until a new circuit START–UP.

Start–up Management

The Vi pin 8 is directly connected to the HV DC rail Vin.

This high voltage current source is internally connected to the

VCC pin and thus is used to charge the VCC capacitor. The

VCC capacitor charge period corresponds to the Start–up

phase. When the VCC voltage reaches 13V, the high voltage

9mA current source is disabled and the device starts

working. The device enters into the switching phase.

It is to be noticed that the maximum rating of the Vi pin 8 is

700V. ESD protection circuitry is not currently added to this

pin due to size limitations and technology constraints.

Protection is limited by the drain–substrate junction in

avalanche breakdown. To help increase the application

safety against high voltage spike on that pin it is possible to

W insert a small wattage 1k series resistor between the Vin

rail and pin 8.

The figure 6 shows the VCC voltage evolution in case of no

external current source providing current into the VCC pin

during the switching phase. This case can be encountered in

SMPS when the self supply through an auxiliary winding is

not present (strong overload on the SMPS output for

example). The figure 16 also depicts this working

configuration.

Start–up

Phase

Latched off

Phase

Switching

Phase

In case of the hiccup mode, the duty cycle of the switching

phase is in the range of 10%.

Mode Transition

The LW latch figure 7 is the memory of the working status

at the end of every switching sequence.

Two different cases must be considered for the logic at the

termination of the SWITCHING PHASE:

1. No Over Current was observed

2. An Over Current was observed

These 2 cases are corresponding to the signal labelled

NOC in case of ”No Over Current” and ”OC” in case of Over

Current. So the effective working status at the end of the ON

time memorized in LW corresponds to Q=1 for no over

current and Q=0 for over current.

This sequence is repeated during the Switching phase.

Several events can occur:

1. SMPS switch OFF

2. SMPS output overload

3. Transition from Normal to Pulsed Mode

4. Transition from Pulsed Mode to Normal Mode

Figure 7. Transition Logic

Latched Off

Phase

VPWM

OUT

LEB out

1V

& NOC S Q

OC LW

&

SQ

Mode

Stand–by

&

R Q & R1

R2

+

–CS

m Start–up Idemag Switching Start–up

Phase > 24 A Phase Phase

S1

Switch

• 1. SMPS SWITCH OFF

When the mains is switched OFF, so long as the bulk

electrolithic bulk capacitor provides energy to the SMPS, the

controller remains in the switching phase. Then the peak

current reaches its maximum peak value, the switching

frequency decreases and all the secondary voltages are

reduced. The VCC voltage is also reduced. When VCC is

equal to 10V, the SMPS stops working.

MOTOROLA ANALOG IC DEVICE DATA

7

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