AN8018SA View Datasheet(PDF) - Panasonic Corporation

Part Name

Description

Manufacturer

AN8018SA

1.8-volt 2-channel step-up, step-down, or inverting DC-DC converter control IC

Panasonic Corporation 

AN8018SA

Voltage Regulators

s Application Notes (continued)

[6] Time constant setting method for timer latch short-circuit protection circuit

The constructional block diagram of protection latch circuit is shown in figure 6. The comparator for short-circuit

protection compares the error amplifier 1 output FB1 with the reference voltage of 0.9 V for channel 1 side, and the

error amplifier 2 output FB2 with the reference voltage of 0.22 V for channel 2 side at all the time.

When the load conditions of DC-DC converter output is stabilized, there is no fluctuation of error amplifier

output and the short-circuit protection comparator also keeps the balance. At this moment, the output transistor Q1 is

in the conductive state and the S.C.P. terminal is held to approximately 60 mV.

When the load conditions for channel 1 side suddenly change and high-level signal (0.9 V or more) is input from

the error amplifier 1 block to the short-circuit protection comparator, the short-circuit protection comparator outputs

the low-level signal to cut off the output transistor Q1. Also, when the load conditions for channel 2 side suddenly

change and low-level signal (0.22 V or less) is inputted from the error amplifier 2 block to the short-circuit protection

comparator, the short-circuit protection comparator outputs the low-level signal to cut off the output transistor Q1.

The capacitor CSCP connected to the S.C.P. terminal starts charging. When the external capacitor CSCP has been charged

to approximately 1.19 V with the constant current of approximately 1.1 µA, the latch circuit is set, the output terminal

is fixed to low level, and the dead-time is set to 100%. Once the latch circuit is set, the S.C.P. terminal is discharged

to approximately 40 mV, However, the latch circuit is not reset unless the power for the latch circuit is turned off or

restarted by the on/off control.

1.19 V = ICHG

×

tPE

CSCP

VSCP [V]

∴ tPE [s] = 1.08 × CSCP

1.19 Short-circuit detection time tPE

When the power supply is turned on, the output is

considered to be short-circuited state so that the S.C.P.

terminal voltage starts charging. It is necessary to set the

external capacitor so as to start up the DC-DC converter

output voltage before setting the latch circuit in the later

stage. Especially, pay attention to the delay of the start-up

time when applying the soft-start.

0.06

t [s]

Figure 5. S.C.P. terminal charging waveform

FB1 5

On/off

control

IN+1 3 Error amp.1

IN−1 4

FB2 13

0.9 V

IN+2 14 Error amp.2

1.19 V

0.22 V

S.C.P.

comp.

Internal

reference

U.V.L.O.

1.1 µA

Latch

R

SQ

High-level detection comp.

1.19 V

Output cut-off

Q1

Figure 6. Short-circuit protection circuit

16

Share Link: