
NCV7510
http://onsemi.com
20
DETAILED OPERATING DESCRIPTION (continued)
MOSFET Predrivers
The NCV7510 employs crossconduction suppression
for the external high side and clamp MOSFETs. The
CLAMP antisaturation circuitry is used to detect the
turnoff of the high side MOSFET and the voltage at the
CLAMP pin is monitored to detect turnoff of the CLAMP
MOSFET. Figure 17 shows the simplified predriver circuits.
The high side predriver is designed to allow external
system level diagnostics to be implemented at the SRC pin.
The driver is constructed to provide a typical 20 discharge
path for 10 s at turnoff and a 60 k gatesource bleed
resistance to help prevent MOSFET turnon from leakage
or noise. The R
G
resistor must be carefully chosen to ensure
full depletion of the high side MOSFET’s gate charge in less
than 10 s.
Current for the GATE predrive output is supplied from the
V
B
voltage developed by an external bootstrap circuit or
boost power supply. Current for the CLAMP predrive output
is supplied from the V
DD
power supply.
While the IC contains no slew rate control circuitry, slew
rate control of the high side MOSFET can be achieved by the
use of a series gate resistor (R
G
.) Since the body diode of the
CLAMP MOSFET conducts the load current immediately
after high side turn off, slew rate control of the CLAMP
MOSFET gives no benefit and the use of a series gate
resistor will interfere with crossconduction suppression.
Bootstrap Circuit
A bootstrap circuit can be constructed using a diode,
resistor, and capacitor to generate the V
B
voltage necessary
to put the external high side MOSFET in full conduction
(refer to Figure 17). The circuit charges C
BOOT
through D2
and R
LIM
when the SRC pin is low. The charge is then
transferred to the high side MOSFET when M1 in the GATE
predriver is turned on, and the capacitor rides up with the
voltage at the SRC pin. The charge is continually refreshed
as a result of alternate switching of the high side and clamp
MOSFETs. A clamp diode at the V
B
input (see Application
Diagram – Diode D3) may be needed to keep the NCV7510
within its maximum ratings during overvoltage transients.
D4 ensures that the high side MOSFET’s maximum V
GS
is
not exceeded (refer to Figure 17).
While simple and straightforward in operation, a
bootstrap circuit depends on periodic refresh and thus
cannot run at 100% duty cycle. During engine cranking, the
PKHI program value may not be reached and a state change
in the control loop may not occur, possibly fully depleting
(and preventing recharge) of the bootstrap capacitor.
With the use of logiclevel MOSFETs and careful design,
sufficient V
GS
should be available during start up. Attention
to leakage paths (such as external gatesource bleed
resistors) and the V
B
input bias current will help ensure that
gate charge is available when recharge does not occur.
M1
20
M2
20
GATE
M3
60 k
I
VB
IN
SRC
R
G
C
BOOT
R
LIM
D2
M4
20
M5
20
200 k
CLAMP
Q
VBAT
PGND
R
SOL
L
SOL
R
SNS
M
HS
M
CL
Figure 17. Simplified GATE and CLAMP Predrivers
10 s
V
DD
V
B
D4