HV302 / HV312
Design Information - continued
The following waveforms demonstrate the sequencing of the
Start up Overload Protection
PWRGD flags. These results were obtained with RTB = 120kΩ, RTC
= 60kΩ and RTD = 3kΩ
Start up must be achieved within a nominal 100ms as indicated by
the PWRGD-A pin transition to the active state or the circuit will
reset and an Auto-Retry will initiate. If there is an output overload
or short circuit during start up, the circuit will be in current limit
mode for the 100ms time limit (in servo mode). In feedback
capacitor mode the circuit breaker will shutdown the MOSFET
before 100ms.
Circuit Breaker Delay
The circuit breaker will trip in less than 5µs when the voltage on
the SENSE pin reaches a nominal 100mV. A resistor in series with
the SENSE pin and a capacitor connected between the SENSE
and VEE pins may be added to delay the rate of voltage rise on the
SENSE pin, thus permitting a current overshoot and delaying
Circuit Breaker activation. This method is particularly useful when
operating in Feedback Capacitor Mode. However, in Servo Mode
operation it will result in a current limit leading edge overshoot.
Auto-Retry and Auto-Retry Disable
The Auto-Retry delay time is directly proportional to the
capacitance at the RAMP pin. Auto-Retry sequence is activated
whenever the 100ms timeout is reached during start up or the
Circuit Breaker is tripped.
The value of the resistors determines the capacitor charging and
discharging current of a triangle wave oscillator. The oscillator
output is fed to an 8-bit counter to generate the desired time delay.
The respective delay time is defined by the following equation:
Auto-Retry can be approximated as a 555-timer with 2.5µA charge
up and charge down currents through 8V, to a count of 256.
255 × 2× COSC × VPP
tTX
=
ICD
and
Vbg
4RTX
ICD
=
Where
tTX = Delay Time between respective PWRGD flags
COSC = 120pF (Internal oscillator capacitor)
Therefore,
VPP = 8.2V (Peak-to-Peak voltage swing of oscillator)
ICD = Charge and Discharge current of oscillator
Vbg = 1.2V (Internal Band Gap Reference)
2× 8× 256
2.5µA
tAuto−Retry
=
=
× CRAMP
RTX = Programming resistor at TB, TC or TD pin
For CRAMP = 10nF
tAuto−Retry
2× 8× 256
2.5µA
Combining the above two equations and solving for RTX yields:
bg × tTX
×10nF = 16.4s
B
1.2V × tTX
2040 × CPP × VPP 2040 ×120pF× 8.2V
RTX
=
=
Due to the 2.5µA maximum charge current a resistor which draws
more than 2.5µA below 8V will disable Auto-Retry. Try to keep this
resistor as big as possible, e.g. 2.5MΩ. For most MOSFETs with
maximum Vt of 4V, this will vary the 10µA RAMP current source by
4V
RTX = 0.6×106 × tTX
only
= 1.6µA
For a delay time of 200ms we get:
2.5MΩ
−3
RTX
=
(
0.6×106
)
×
(
200 ×10
)
= 120kΩ
PWRGD Flag Delay Programming
For a delay time of 5ms we get:
Shortly after current limiting ends, PWRGD-A becomes active
indicating successful completion of the Hotswap operation.
PWRGD-B will change to an active state a programmed delay time
after PWRGD-A went active, PWRGD-C will change to an active
state a programmed delay time after PWRGD-B went active and
PWRGD-D will change to an active state a programmed delay time
after PWRGD-C went active. Resistors connected from the
respective TB, TC and TD pins to VEE pin are used to program the
delay times between the PWRGD flags sequentially going active.
−3
RTX
=
(
0.6×106
)
×
(
5×10
)
= 3kΩ
9
Rev. D
04/17/02
Supertex, Inc. 1235 Bordeaux Drive, Sunnyvale, CA 94089 TEL: (408) 744-0100 Fax: (408) 222-4895 www.supertex.com
SUPERTEX [ Supertex, Inc ]
