LTC3805-5_15 Datasheet, PDF(14/20 Page) Linear Technology – Adjustable Frequency Current Mode Flyback/ Boost/SEPIC DC/DC Controller

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LTC3805-5_15 Datasheet, PDF (14/20 Pages) Linear Technology – Adjustable Frequency Current Mode Flyback/ Boost/SEPIC DC/DC Controller

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LTC3805-5

APPLICATIONS INFORMATION

Overcurrent Threshold Adjustment

Figure 5 shows the connection of the overcurrent pin, OC,

along with the ISENSE pin and the current sense resistor

RSENSE located in the source circuit of the power NMOS

which is driven by the GATE pin. The internal overcurrent

threshold on the OC pin is set at VOCT = 100 mV which is the

same as the peak current sense voltage VI(MAX) = 100 mV on

the ISENSE pin. The role of the slope compensation adjust-

ment resistor RSLOPE and the slope compensation current

ISLOPE is discussed in the prior section. In combination with

the overcurrent threshold adjust current IOC = 10ÂµA, an

external resistor ROC can be used to lower the overcurrent

trip threshold from 100mV. This section describes how

to pick ROC to achieve the desired performance. In the

discussion that follows be careful to distinguish between

âcurrent limitâ where the converter continues to run with

the ISENSE pin limiting current on a cycle-by-cycle basis

while the output voltage falls below the regulation point

and âovercurrent protectionâ where the OC pin senses an

overcurrent and shuts down the converter for a timeout

period before attempting an automatic restart.

One overcurrent protection strategy is for the converter

to never enter current limit but to maintain output volt-

age regulation up to the point of tripping the overcurrent

protection. Operation at minimum input voltage VIN(MIN)

hits current limiting for the smallest output current and

is the design point for this strategy.

First, for operation at VIN(MIN), calculate the duty cycle Duty

Cycle VIN(MIN) using the appropriate formula depending on

whether the converter is a boost, flyback or SEPIC. Then

GATE

LTC3805-5

ISENSE

RSLOPE

ISLOPE

ROC IOC = 10ÂµA

RSENSE

GND

38055 F05

Figure 5. Circuit to Decrease Overcurrent Threshold

use Duty Cycle VIN(MIN) to calculate DVSENSE(VIN(MIN))

using the formula in the prior section. For overcurrent

protection to trip at exactly the point where current limit-

ing would begin set:

ROC(CRIT )

DVSENSE ( VIN(MIN))

10ÂµA

To find the actual output current that trips overcurrent

protection, calculate the peak switch current IPK(VIN(MIN))

from:

IPK

(

VIN(MIN))

100mV

DVSENSE

RSENSE

(

VIN(MIN))

Then calculate the converter output current that corre-

sponds to IPK(VIN(MIN)). Again, the calculation depends

both on converter type and the details of converter design

including inductor current ripple. For minimum input volt-

age, ROC(CRIT) produces an overcurrent trip at an output

current just before loss of output voltage regulation and

the onset of current limiting. Note that the output current

that causes an overcurrent trip is higher for higher input

voltages but that an overcurrent trip will always occur

before loss of output voltage regulation. If desired to

meet a specific design target, an increase in ROC above

ROC(CRIT) can be used to reduce the trip threshold and

make the converter trip for a lower output current.

This calculation is based on steady-state operation. De-

pending on design, overcurrent protection can also be

triggered during a start-up transient, particularly if large

output filter capacitors are being charged as output voltage

rises. If that is a problem, output capacitor charging can

be slowed by using a larger value of SSFLT capacitor. It is

also possible to trip overcurrent protection during a load

step especially if the trip threshold is lowered by making

ROC > ROC(CRIT).

Another overcurrent protection strategy is keep the con-

verter running as current limiting reduces the duty cycle

and the output voltage sags. In this case, the goal is often

keep the converter in normal operation over as wide a range

as possible, including current limiting, and to trigger the

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