LT3752 datasheet(25/52 Pages) AD

25 / 52 page

LT3752/LT3752-1

Rev. C

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APPLICATIONS INFORMATION

Integrating the housekeeping controller saves cost and

space and allows switching frequency to be inherently

synchronized to the main forward converter.

The housekeeping supply can be used to overdrive the

INTVCC pin to take power outside of the part, improve

efficiency, provide more drive current and optimize the

INTVCC level. It can also be used as a bootstrap bias to

the VIN pin as described in the section LT3752-1 Part

Start-Up. The housekeeping supply also allows bias to

any secondary side IC before the main forward converter

starts switching. This removes the need for external start-

up circuitry on the secondary side. Alternative methods

involve powering secondary side ICs directly from the

output voltage of the forward converter. This can cause

issues depending on the minimum and maximum allowed

input voltages for each IC.

Housekeeping: Operation

The LT3752/LT3752-1 housekeeping controller opera-

tion is best described by referring to the Block Diagram

and Figure 8. The housekeeping controller uses a ±0.7A

gate driver at HOUT to control an external N-channel

MOSFET.Whencurrentintheprimarywindingoftheflyback

transformer exceeds a level commanded by HCOMP and

sensed at the HISENSE pin, the duty cycle of the HOUT is

terminated. Stored energy in the transformer is delivered

to the output during the off time of HOUT. The housekeep-

ing output voltage is programmed using a resistor divider

to the HFB pin. A transconductance amplifier monitors

the error signal between HFB pin and a 1.0V reference to

control HCOMP level and hence peak switch current. A

simple RC network from HCOMP pin to ground provides

compensation. Overcurrent protection exists for the exter-

nal switch when 98mV is sensed at the HISENSE pin. This

causes a low power hiccup mode (repeated retry cycles’

of shutdown followed by soft-start) until the overcurrent

condition is removed.

Housekeeping: Soft-Start/Shutdown

Duringstart-upoftheLT3752/LT3752-1,thehousekeeping

controller has a built-in soft-start of approximately 2.2ms.

The time will vary depending on the HCOMP level needed

to achieve regulation. The housekeeping controller is shut

down and the internal soft-start capacitor is discharged

for any of the following conditions (typical values):

(1) UVLO_VSEC < 1.25V

(and SS1 < 0.15V)

:Soft-Stop Shutdown

(2) UVLO_VSEC < 0.4V

:Micropower Shutdown

(3) OVLO > 1.250V

:System Input OVLO

(4) HISENSE > 98mV

:Housekeeping Overcurrent

(5) INTVCC < X, > 16.5V :INTVCC UVLO, OVLO

(6) TJ > 170°C

:Thermal Shutdown

(7) VIN < Y

:VIN Pin UVLO

(X = 4.6V, Y = 5.5V for LT3752)

(X = 6.8V, Y = 7.6V for LT3752-1)

Housekeeping: Programming Output Voltage

The output voltage, VHK, of the housekeeping controller

is programmed using a resistor divider between VHK and

the HFB pin (Figure 8) using the equation:

VHK =1V • 1+









The HFB pin bias current is typically 85nA.

Housekeeping: Programming Cycle-by-Cycle Peak

Inductor Current and Slope Compensation

The housekeeping controller limits cycle-by-cycle peak

current in the external switch and primary winding of

the flyback transformer by sensing voltage at a resistor

(RHISENSE)connectedinthesourceoftheexternalN-chan-

nel MOSFET (Figure 8). This sense voltage is compared to

a sense threshold at the HISENSEpin,controlledbyHCOMP

with an upper limit of 79mV. Since there is only one sense

line from the positive terminal of the sense resistor, any

parasitic resistance in ground side will increase its effec-

tive value and reduce available peak switch current. For

operation in continuous mode and above 50% duty cycle,

required slope compensation can be programmed by

adding a resistor RHISLP in series with the HISENSE pin. A

ramped current always flows out of the HISENSE pin. The

current starts from 2µA at 0% duty cycle and ramps to

52µAat100%dutycycle.Minimizecapacitanceonthispin.

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