Rainbow Electronics MAX15011 - Manual (Page 16)

MAX15009/MAX15011
Leave CT open to select a typical reset timeout of 19µs.
To maintain reset accuracy, use a low-leakage type of
capacitor.
Setting the Switch Current Limit
The switch block features accurate current-limit sens-
ing circuitry. A resistor connected from ILIM to SGND
can be used to select the current-limit threshold using
the following relationship:
I
SW_LIM
(mA) = R
ILIM
(kΩ) x 1mA / kΩ
where 20kΩ ≤ R ILIM
≤ 200kΩ.
Connect ILIM to OUT_LDO to select the default current
limit of 200mA (typ).
Programming the Switch
Overcurrent Blanking Time
The switch provides an adjustable overcurrent blanking
time to allow the safe charge of large capacitive loads.
When an overcurrent event is detected, a delay period
elapses before the condition is latched and the internal
MOSFET is turned off. This period is the overcurrent
delay, t
OC_DELAY . Set the overcurrent delay using the
following equation:
t OC_DELAY = C OC_DELAY x V OC_DELAY / I OC_DELAY_UP
where t
OC_DELAY
is in seconds and C
OC_DELAY
is in
µF. V
OC_DELAY
is the overcurrent delay timeout thresh-
old voltage in volts and I OC_DELAY_UP is the overcur-
rent delay timeout pullup current in µA as seen in the
Electrical Characteristics table.
Ensure that the switch is not disabled due to a large
startup inrush current by selecting a large enough
value for overcurrent blanking time. Assume that the
current available for charging the total switch output
capacitance, C
OUT_SW , is the difference between the
current-limit threshold value, I
SW_LIM
, and the nominal
DC load current at OUT_SW, I OUT_SW_NOM and select
the C OC_DELAY using the following relationship:
C OC_DELAY also affects the length of time before the
MAX15009/MAX15011 attempt to turn the switch back
on. Set the autoretry delay using the following equation:
t OC_RETRY = C OC_DELAY x
V
OC_DELAY
/I
OC_DELAY_DOWN
where t OC_RETRY is in seconds, C OC_DELAY is in µF,
V OC_DELAY is in volts, and I OC_DELAY_DOWN is in µA.
C OC_DELAY should be a low-leakage type of capacitor
with a minimum value of 100pF.
Setting the Overvoltage Threshold
(MAX15009 Only)
The MAX15009 provides an accurate means to set the
overvoltage threshold for the OVP controller using
FB_PROT. Use a resistive voltage-divider to set the
desired overvoltage threshold (Figure 4). FB_PROT has
a rising 1.235V threshold with a 4% falling hysteresis.
Begin by selecting the total end-to-end resistance,
R
TOTAL = R 3 + R 4 . Choose R TOTAL to yield a total current
equivalent to a minimum of 100 x I FB_PROT (FB_PROT’s
input maximum bias current) at the desired overvoltage
threshold. See the
Electrical Characteristics table.
For example:
With an overvoltage threshold (V
OV
) set to 20V, R
TOTAL
< 20V / (100 x I FB_PROT ), where I FB_PROT is FB_PROT’s
maximum 100nA bias current:
R TOTAL < 2MΩ
C
I V C
V (I I )
OC_DELAY
OC_DELAY_UP OUT_LDO OUT_SW
OC_DELAY SW_LIM OUT_SW_NOM
≥
Ч Ч
Ч −
Automotive 300mA LDO Regulators with
Switched Output and Overvoltage Protector
16 ______________________________________________________________________________________
R4
R3
R5
R6
IN V
IN
FB_PROT
SGND
GATE
SOURCE
PROTECTOR
OUTPUT
MAX15009
IN V IN
FB_PROT
SGND
GATE
SOURCE
PROTECTOR
OUTPUT
MAX15009
Figure 4. Setting the Overvoltage Threshold (MAX15009)