6
UC1625
UC2625
UC3625
PDA, PDB, PDC: These outputs can drive the gates of
N-Channel power MOSFETs directly or they can drive
the bases of power Darlingtons if some form of current
limiting is used. They are meant to drive low-side power
devices in high-current output stages. Current available
from these pins can peak as high as 0.5A. These out
-
puts feature a true totem-pole output stage. Beware of
exceeding IC power dissipation limits when using these
outputs for high continuous currents. These outputs pull
high to turn a “low-side” device on (active high).
PUA, PUB, PUC: These outputs are open-collector,
high-voltage drivers that are meant to drive high-side
power devices in high-current output stages. These are
active low outputs, meaning that these outputs pull low
to command a high-side device on. These outputs can
drive low-voltage PNP Darlingtons and P-channel
MOSFETs directly, and can drive any high-voltage de
-
vice using external charge-pump techniques, trans
-
former signal coupling, cascode level-shift transistors, or
opto-isolated drive (high-speed opto devices are recom-
mended). (See applications).
PWR V
CC
: This supply pin carries the current sourced
by the PD outputs. When connecting PD outputs directly
to the bases of power Darlingtons, the PWR V
CC
pin can
be current limited with a resistor. Darlington outputs can
also be "Baker Clamped" with diodes from collectors
back to PWR V
CC
. (See Applications)
Quad Sel: The IC can chop power devices in either of
two modes, referred to as “two-quadrant” (Quad Sel low)
and “four-quadrant” (Quad Sel high). When
two-quadrant chopping, the pull-down power devices
are chopped by the output of the PWM latch while the
pull-up drivers remain on. The load will chop into one
commutation diode, and except for back-EMF, will ex
-
hibit slow discharge current and faster charge current.
Two-quadrant chopping can be more efficient than
four-quadrant.
When four-quadrant chopping, all power drivers are
chopped by the PWM latch, causing the load current to
flow into two diodes during chopping. This mode exhibits
better control of load current when current is low, and is
preferred in servo systems for equal control over accel
-
eration and deceleration. The Quad Sel input has no ef
-
fect on operation during braking.
RC-Brake: Each time the Tach-Out pulses, the capaci
-
tor tied to RC-Brake discharges from approximately
3.33V down to 1.67V through a resistor. The tachometer
pulse width is approximately T = 0.67 R
T
C
T
, where R
T
and C
T
are a resistor and capacitor from RC-Brake to
ground. Recommended values for R
T
are 10kΩ to
500kΩ, and recommended values for C
T
are 1nF to
100nF, allowing times between 5µs and 10ms. Best ac
-
curacy and stability are achieved with values in the cen
-
ters of those ranges.
RC-Brake also has another function. If RC-Brake pin is
pulled below the brake threshold, the IC will enter brake
mode. This mode consists of turning off all three
high-side devices, enabling all three low-side devices,
and disabling the tachometer. The only things that in
-
hibit low-side device operation in braking are
low-supply, exceeding peak current, OV-Coast com
-
mand, and the PWM comparator signal. The last of
these means that if current sense is implemented such
that the signal in the current sense amplifier is propor
-
tional to braking current, the low-side devices will brake
the motor with current control. (See applications) Sim
-
pler current sense connections will result in uncontrolled
braking and potential damage to the power devices.
RC-Osc: The UC3625 can regulate motor current using
fixed-frequency pulse width modulation (PWM). The
RC-Osc pin sets oscillator frequency by means of timing
resistor R
OSC from the RC-Osc pin to V
REF
and capaci-
tor C
OSC
from RC-Osc to Gnd. Resistors 10kΩ to
100kΩ and capacitors 1nF to 100nF will work best, but
frequency should always be below 500kHz. Oscillator
frequency is approximately:
()
F
RC
OSC OSC
=
•
2
Additional components can be added to this device to
cause it to operate as a fixed off-time PWM rather than
a fixed frequency PWM, using the RC-Osc pin to select
the monostable time constant.
The voltage on the RC-Osc pin is normally a ramp of
about 1.2V peak-to-peak, centered at approximately
1.6V. This ramp can be used for voltage-mode PWM
control, or can be used for slope compensation in cur
-
rent-mode control.
S
START
: Any time that V
CC
drops below threshold or the
sensed current exceeds the over-current threshold, the
soft-start latch is set. When set, it turns on a transistor
that pulls down on S
START
.
Normally, a capacitor is con
-
nected to this pin, and the transistor will completely dis
-
charge the capacitor. A comparator senses when the
NPN transistor has completely discharged the capacitor,
and allows the soft-start latch to clear when the fault is
removed. When the fault is removed, the soft-start ca
-
pacitor will charge from the on-chip current source.
PIN DESCRIPTIONS (cont.)
