MAX15009 Datasheet, PDF (14/23 Pages) Maxim Integrated Products – Automotive 300mA LDO Regulators with Switched Output and Overvoltage Protector

Automotive 300mA LDO Regulators with
Switched Output and Overvoltage Protector
off the external MOSFET, disconnecting the power
source from the load. In this configuration, the voltage
at the source of the MOSFET is not monitored. When
the voltage at FB_PROT decreases below the overvolt-
age threshold, the MAX15009 raises the voltage at
GATE, reconnecting the load to the power source.
Overvoltage-Limiter Mode (MAX15009 Only)
When operating in overvoltage-limiter mode, the feed-
back path consists of SOURCE, FB_PROT’s internal
comparator, the internal gate charge pump/gate pull-
down, and the external n-channel MOSFET (Figure 2).
This configuration results in the external MOSFET oper-
ating as a hysteretic voltage regulator.
During normal operation, GATE is enhanced 8.1V above
V_IN. The external MOSFET source voltage is monitored
through a resistive voltage-divider between SOURCE
and FB_PROT. When V_SOURCE exceeds the adjustable
overvoltage threshold, an internal pulldown switch
discharges the gate voltage and quickly turns the
MOSFET off. Consequently, the source voltage begins
to fall. The V_SOURCE fall time is dependent on the MOS-
FET’s gate charge, the internal charge-pump current,
the output load, and any load capacitance at SOURCE.
When the voltage at FB_PROT is below the overvoltage
threshold by an amount equal to the hysteresis, the
charge pump restarts and turns the MOSFET back on.
In this way, the OVP controller attempts to regulate
V_SOURCE around the overvoltage threshold. SOURCE
remains high during overvoltage transients and the
MOSFET continues to conduct during an overvoltage
event. The hysteresis of the FB_PROT comparator and
the gate turn-on delay force the external MOSFET to
operate in a switched on/off sequence during an over-
voltage event.
Exercise caution when operating the MAX15009 in volt-
age-limiting mode for long durations. Care must be
taken against prolonged or repeated exposure to over-
voltage events while delivering large amounts of load
current as the power dissipation in the external MOS-
FET may be high under these conditions. To prevent
damage to the MOSFET, implement proper heatsinking.
The capacitor tied between SOURCE and ground may
also be damaged if the ripple current rating for the
capacitor is exceeded.
V_IN
IN
GATE
MAX15009
SOURCE
FB_PROT
SGND
PROTECTOR
OUTPUT
Figure 2. Overvoltage Limiter (MAX15009)
As the transient voltage decreases, the voltage at
SOURCE falls. For fast-rising transients and very large
MOSFETs, connect an additional capacitor from GATE
to PGND. This capacitor acts as a voltage-divider work-
ing against the MOSFET’s drain-to-gate capacitance. If
using a very low gate charge MOSFET, additional
capacitance from GATE to ground might be required to
reduce the switching frequency.
Control Logic
The MAX15009/MAX15011 LDO features two logic
inputs, EN_LDO and HOLD, making these devices suit-
able for automotive applications. For example, when
the ignition key signal drives EN_LDO high, the regula-
tor turns on and remains on even if EN_LDO goes low,
as long as HOLD is forced low and stays low after initial
regulator power-up. In this state, releasing HOLD turns
the regulator output (OUT_LDO) off. This feature makes
it possible to implement a self-holding circuit without
external components. Forcing EN_LDO low and HOLD
high (or unconnected) places the regulator into shut-
down mode, reducing the supply current to less than
16µA. Table 1 shows the state of OUT_LDO with
respect to EN_LDO and HOLD. Leave HOLD uncon-
nected or connect directly to OUT_LDO to allow the
EN_LDO input to act as a standard on/off logic input for
the regulator.
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