HV111
Thermal Shutdown
In addition to the above parameters, the HV111 will
shutdown if the temperature on the die reaches ~135°C and
it will not restart until the temperature drops to 100°C or less
(could be significantly less). The thermal sensor is key in
providing a bullet proof power management solution
because it ensures that the device will turn off long before
damage can occur. This is a significant advantage over
solutions that do not contain an integral MOSFET as then
the temperature cannot be easily sensed quickly and
accurately.
Thermal engineering using the HV111 is key to proper
system operation. The 1Ω MOSFET pass element may
reach a value as high as 1.5Ω at high temperatures. There
are numerous methods to reduce the thermal resistance of
the R
θ
JA
. The following table describes some options:
Method
FR4
FR4 Heat Sink
FR4 + H/S
IMS (40cm )
IMS* w/ H/S
* IMS is a metal substrate board
2
Auto-Retry
Any fault condition will cause an automatic 9sec retry to
occur. This retry will occur indefinitely (as long as the
thermal supervisor is satisfied). Figure 6 shows typical
waveforms for the auto-retry.
PWRGD
I
DS
200ma/div
V
PP
-V
NN
50V/div
R
θ
JA
70-80° C/W
40° C/W
13° C/W
9° C/W
4.5° C/W
Description
Straight Convection
10cm PCB H/S
External Sink + Holes
Floating in Air
External Heatsink
2
Timers
The timer subsystems are critical to successful operation of
the HV111. Timers are as follows:
To determine your required thermal impedance, R
θ
JA
, is quite
simple. In a parallel to Ohms law, Power x R
θ
JA
=
∆T.
Junction temperature, which is limited to 120
°
C minimum, is
Tmaxambient +
∆T.
For example, if the highest operating
ambient temperature were 55
°
C as with many networking
applications, and the current were 1.6A, then the required
thermal resistance would be calculated as follows:
Determine maximum ambient = 55
°
C.
Determine max junction temp. = 120
°
C .
Determine max operating current = 1.6A.
Therefore
∆T
= 120-55
°
C = 65
°
C.
2
2
Max. Power = 1.23A *1.5Ω = 3.84W.
1
Timer
Power-on-Reset
Initial Inrush Timeout
2
Shorted Inrush PWM
Shorted Circuit Timer
Second Inrush (Diode
‘OR’ing):
❒
Return to Limit
❒
Timeout
Auto-Retry
1
Duration
1
4.5ms
75ms
10µs
75ms
10µs
(100µs)
3
75ms
9sec
Now
∆T
/ Power = R
θ
JA
= 65
°
/3.84W = 20
°
C/W.
To achieve a R
θ
JA
of 20
°
C/W or better the table above shows
that it will be necessary to use a DPAK external heatsink or
IMS substrate.
1
This is the time from satisfying the undervoltage comparator. Each “bounce” will reset this
timer & therefore observed delay may be higher than this “ideal” delay.
2
Shorted-circuit timer starts after POR timer. If V
DS
drops more than ¼(V
PP
-V
NN
) after t
SC
then a shorted-circuit condition exists.
3
Limit within 10µs, but may take up to 100µs to settle.
This is the minimum value of the low to high thermal shutdown according to the electrical
specifications on pg. 2.
2
This is the maximum MOSFET on resistance at high temperature.
Current Sensing – No R
SENSE
Required
The HV111 uses an internal 6000:1 current mirror to
eliminate the need for a sense resistor. This saves energy
and eliminates the need for a power component. The current
mirror used by Supertex is unique in that it utilizes special
circuitry to normalize for the variations in V
DS
between the
primary pass element and the internal element which would
otherwise cause current mismatch – and forces competitors
to use internal sense resistors in similar applications. This
mechanism also provides the shorted circuit PWM
functionality which can help protect systems in the case of
severe short circuits.
5
SUPERTEX [ Supertex, Inc ]
