Analog Integrated Circuit Device Data
8 Freescale Semiconductor
33285
FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
INTRODUCTION
The 33285 contains only one charge pump for two
outputs.The outputs, OUT1 and OUT2, are switched ON and
OFF by the input (IN) .There are three ways to control the
outputs:
OUT1 can be switched alone
OUT1 and OUT2 can be switched together
OUT2 can be switched when OUT1 is already on
In the last case, the voltage drop on OUT1 when charging
OUT2 is limited.
The external capacitor (C
CP
) connected to the CHARGE
PUMP (CP) pin is used to store the charge continuously
delivered by the charge pump. The voltage on this pin is
limited to a maximum value V
CPMAX
. Both outputs are
sourced with a constant current from C
CP
to switch them ON.
Additionally, the gates of the power FETs are precharged
from voltage power supply (V
CC
)
to prevent C
CP
from being
discharged by a voltage on OUT1 or OUT2, is still lower than
V
VCC
. The values of the output voltages are limited to
V
OUT1MAX
and V
OUT2MAX
.
The power FET on OUT1 is protected against an
exceeded gate-source voltage by an internal zener diode.
Channel One protects the N-Channel power FET on OUT1
undercurrent and short-circuit conditions. The drain-source
voltage of the FET on OUT1 is checked if Channel One is
switched ON. The internal error voltage threshold determines
the maximum drain-source voltage allowing the power FET to
stay in the ON state. If the measured drain-source voltage
exceeds the internal error voltage threshold, the output of the
short-circuit protection comparator (SCPC) is enabled. If the
output of the SCPC is active longer than t
OCDET
, output
OUT1 is switched OFF.
After switching OFF the power FET on OUT1 by an short-
circuit condition, the power FET can only be turned ON again
by the input IN.
When switching OFF the power FETs their gate capacities
are discharged by a constant current, I
OUTOFF
.
If the input IN is disconnected, the 33285 outputs, OUT1
and OUT2, are in the OFF state.
If overvoltage occurs on the DRAIN (DRN) pin for a time
period longer than t
LDDET
, OUT2 is switched ON for the time
t
OUT2ACT
. In an overvoltage condition OUT1 is OFF if IN is
below V
IH
.
INTERNAL ZENER DIODE
An on-chip zener diode is placed between OUT1 and The
SOURCE (SRC). Design guarantees V
Z
> V
TH1
. Zener
clamping voltage between OUT1 and SRC is V
TH1
< V
Z
<
20 V
PWM CAPABILITY
The C
PIC2
is PWM capable on OUT2. The loss of charge
ON C
CP
when switching ON OUT2 is refreshed until the Start
on the next PWM cycle to a value sufficient to guarantee the
specified turn ON behavior.
The PWM capability is measured with a test circuit and
load conditions:
• PWM cycle is period T = 20 ms ; OUT2 is switched ON
from 10 to 90 percent of T
• Test condition V
IN
ramps 2.5 V ≤ 5.0 V according to PWM
cycle defined above
CROSS TALK BETWEEN OUT1 AND OUT2
If output OUT2 is switched ON while OUT1 is already ON,
the voltage drop occurring on OUT1 is limited.
Voltage drop on OUT1:
•10 V < V
VCC
< 20 V : OUT1 not below V
VCC
+ 7.0 V
• 7.0 V < V
VCC
< 20 V : OUT1 not below V
VCC
+ 7.0 V
Each time OUT1 is switched ON, a current I
LCDET
is
sourced out of the SRC pin for the time t
LCDET
to check if
there is an external leakage current on that node in the
application. The high-side switch on OUT1 is turned ON only
if the test is successful.
