Diodes AP3585A/B/C - Manual (Page 10)

AP3585A/B/C
Document number: DS36819 Rev. 1 - 2
10 of 18
www.diodes.com
January 2014
© Diodes Incorporated
AP3585A/B/C
A Product Line of
Diodes Incorporated
NEW PROD
UC
T
Functional Descriptions
(Cont.)
Power Input Detection
The AP3585A/B/C detects PHASE voltage for the present of power input V
IN
when UGATE turns on the first time. If the PHASE voltage does not
exceed 2.0V when UGATE turns on, AP3585A/B/C asserts that V
IN
is not ready and stops the soft start cycle. However, the internal SS continues
ramping up to V
DD
. Another soft start is initiated after SS ramps up to V
DD
. The hiccup period is about 1ms. Figure 3 shows the start-up waveform
where V
IN
does not present initially.
V
IN
10V/Div
V
OUT
0.5V/Div
I
L
10A/Div
LGATE 10V/Div
1ms/Div
Figure 3. Soft Start Where V
IN
Does Not Present Initially
Over Current Protection (OCP)
A resistor R
OCSET
connected from LGATE pin sets the threshold. An internal current source I
OCSET
(10µA typically), flowing through R
OCSET
determines the OCP trigger point, which can be calculated using the following equation:
If R
OCSET
is absent or larger than 40kΩ, AP3585A/B/C will work in build-in Maximum OCP Mode. Then the current limit will be:
Because the R
DS(ON)
of MOSFET increases with temperature, it is necessary to take this thermal effect into consideration in calculating OCP point.
When OCP is triggered, both UGATE and LGATE will go low to stop the energy transfer to the load. Controller will try to restart in a hiccupped
way. Figure 4 shows the hiccupped over current protection.
Figure 4. Hiccupped Over Current Protection
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.
The AP3585A/B/C provides full-time over voltage protection whenever soft start completes or not. The typical OVP threshold is 137.5% of the
internal reference voltage V
REF
. AP3585A/B/C provides non-latched OVP. The controller will return to normal operation if over voltage condition is
removed.
Under Voltage Protection (UVP)
The feedback voltage is also monitored for under voltage protection. The under voltage protection has 15µs triggered delay. When UVP is
triggered, both UGATE and LGATE will go low. Unlike OCP, UVP is not a latched protection; controller will always try to restart in a hiccupped way.
MOSFET side low the of R
R I
I
DS(ON)
OCSET OCSET
LIMIT


) ( LIMIT
/ 375 I
ON DS
R mV 