Ap3591, Application information – Diodes AP3591 User Manual
Page 13
AP3591
Document number: DS36906 Rev.
1 - 2
13 of 18
March 2014
© Diodes Incorporated
AP3591
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A Product Line of
Diodes Incorporated
Application Information
(Cont.)
7. Over Current Protection (OCP)
Figure 3 shows the over current protection (OCP) scheme of AP3591. In each switching cycle, the inductor current is sensed by monitoring the
low-side MOSFET in the OFF period. When the voltage between PGND pin and PHASE pin is larger than the over current trip level, the OCP is
triggered and the controller keeps the OFF state. Because the R
DS(ON)
of MOSFET increases with the temperature, I
OC
has 4500ppm/ºC
temperature coefficient to compensate this temperature dependency of R
DS(ON)
.
Figure 3. Over Current Scheme
A resistor R
OC-SET
should be connected from CS pin to GND. An internal current source I
OC
(10µA typically), flowing through R
OC-SET
determines
the OCP trip point I
OCSET
, which can be calculated using the following equation:
The load current at over-current threshold (I
O_OCSET
), can be calculated using the following equation:
V
OUT
is the output voltage, ΔI
L(PP)
is the inductor current ripple peak to peak value and f is the switching frequency.
8. Negative Over Current Protection (NOCP)
The AP3591 supports cycle by cycle negative over current limiting in CCM mode. The over current limit value is the same absolute value as the
positive over current limit. If the inductor reverse current is larger than NOCP current at OFF time, the LMOSFET is turned off. The reverse current
will flows to VIN through the body diode of HMOSFET. After 400ns delay, LMOSFET is turned on again. If the NOCP is released, the HMOSFET
is turned on and the device resumes normal operation.
9. Under Voltage Protection (UVP)
The output voltage is also monitored for under voltage protection. When the output voltage is less than 70% of the setting voltage threshold, under
voltage protection is triggered after 28µs delay to prevent false transition. When UVP is triggered, UGATE and LGATE will get low, and the output
is discharged with the internal 20
Ω transistor. UVP is a latched protection; it can only be released by VDD or EN/DEM power-on reset. The UVP
blanking time is 2ms during soft-start.
10. Under Voltage Lockout
The AP3591 provides an under voltage lockout circuit to prevent from undefined status at startup. The UVLO circuit shuts down the device when
V
DD
drops below 3.6V. The UVLO circuit has 300mV hysteresis, which means the device will start up again when V
DD
rises to 3.9V.
11. Over Voltage Protection (OVP)
The feedback voltage is continuously monitored for over voltage protection. When OVP is triggered, LGATE will go high and UGATE will go low to
discharge the output capacitor.
OC
COMPARATOR
CURRENT SENSE
Ioc
OPS
R
OC- SET
Q1
Q2
REVERSE BUFFER
)
(
R
A
10
)
(
V
SET
-
OC
CS
K
mV
m
IN
OUT
OUT
IN
PP
L
V
V
V
V
f
L
V
I
V
)
(
2
1
R
2
R
I
DS(ON)
CS
)
(
DS(ON)
CS
O_OCSET