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SLOS390A – NOVEMBER 2001– REVISED MAY 2002
For worst case conditions, the on-resistance of the output
transistors has been ignored to give the maximum
theoretical ripple current. In reality, the voltage drop across
LC FILTER IN THE TIME DOMAIN
The ripple current of an inductor may be calculated using
equation (4):
the output transistors decreases the maximum V as the
O
output current increases. It can be shown using equation
(4) that this decreases the inductor ripple current, and
therefore the TEC ripple current.
(4)
ǒV –V ǓDT
s
O
TEC
L
DI
+
L
POWER SUPPLY DECOUPLING
D + duty cycle (0.5 worst case)
T + 1ńf + 1ń500 kHz
To reduce the effects of high-frequency transients or
spikes, a small ceramic capacitor, typically 0.1 µF to 1 µF,
should be placed as close to each set of PVDD pins of the
DRV592 as possible. For bulk decoupling, a 10 µF to
100 µF tantalum or aluminum electrolytic capacitor should
be placed relatively close to the DRV592.
s
s
For V = 5 V, V
= 2.5 V, and L = 10 µH, and a switching
O
TEC
frequency of 500 kHz; the inductor ripple current is
250 mA. To calculate how much of that ripple current flows
through the TEC element, however, the properties of the
filter capacitor must be considered.
SHUTDOWN OPERATION
For relatively small capacitors (less than 22 µF) with very
low equivalent series resistance (ESR, less than 10 mΩ),
such as ceramic capacitors, the following equation (5) may
be used to estimate the ripple voltage on the capacitor due
to the change in charge:
The DRV592 includes a shutdown mode that disables the
outputs and places the device in a low supply current state.
The SHUTDOWN pin may be controlled with a TTL logic
signal. When SHUTDOWN is held high, the device
operates normally. When SHUTDOWN is held low, the
device is placed in shutdown. The SHUTDOWN pin must
not be left floating. If the shutdown feature is unused, the
pin may be connected to VDD.
2
(5)
f
2
o
p
ǒ
Ǔ
1–D ǒ Ǔ VTEC
DV
+
C
2
f
s
D + duty cycle
FAULT REPORTING
f + DRV592 switching frequency
The DRV592 includes circuitry to sense three faults:
s
1
D
D
D
Overcurrent
f
+
o
Ǹ
2p LC
Undervoltage
Overtemperature
For L = 10 µH and C = 10 µF, the cutoff frequency, f , is
15.9 kHz. For worst case duty cycle of 0.5 and
o
These three fault conditions are decoded via the FAULT1
and FAULT0 terminals. Internally, these are open-drain
outputs, so an external pull-up resistor of 5 kΩ or greater
is required.
V
= 2.5 V, the ripple voltage on the capacitors is
TEC
6.2 mV. The ripple current may be calculated by dividing
the ripple voltage by the TEC resistance of 1.5 Ω, resulting
in a ripple current through the TEC element of 4.1 mA.
Note that this is similar to the value calculated using the
frequency domain approach.
Table 1. Fault Indicators
FAULT1
FAULT0
0
0
1
1
0
1
0
1
Overcurrent
For larger capacitors (greater than 22 µF) with relatively
high ESR (greater than 100 mΩ), such as electrolytic
capacitors, the ESR dominates over the charging-
discharging of the capacitor. The following simple equation
(6) may be used to estimate the ripple voltage:
Undervoltage
Overtemperature
Normal operation
The over-current fault is reported when the output current
exceeds four amps. As soon as the condition is sensed,
the over-current fault is set and the outputs go into a
high-impedance state for approximately 3 µs to 5 µs
(500 kHz operation). After 3 µs to 5 µs, the outputs are
re-enabled. If the over-current condition has ended, the
fault is cleared and the device resumes normal operation.
If the over-current condition still exists, the above
sequence repeats.
(6)
DV + DI R
L
C
ESR
DI + inductor ripple current
L
R
+ filter capacitor ESR
ESR
For a 100 µF electrolytic capacitor, an ESR of 0.1 Ω is
common. If the 10 µH inductor is used, delivering 250 mA
of ripple current to the capacitor (as calculated above),
then the ripple voltage is 25 mV. This is over ten times that
of the 10 µF ceramic capacitor, as ceramic capacitors
typically have negligible ESR.
The under-voltage fault is reported when the operating
voltage is reduced below 2.8 V. This fault is not latched, so
as soon as the power-supply recovers, the fault is cleared
10
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