10
voltage, the integrator capacitor voltage is raised or lowered
to compensate for the systematic offset at the error amplifier.
Compensation is limited to ±5% to minimize transient
overshoot when the device goes out of dropout, current limit,
or thermal shutdown.
Shutdown
Driving the EN_LDO pin low will put LDO1 and LDO2 into
the shutdown mode. Driving the EN_PWM pin low will put
the PWM into shutdown mode. Pulling the EN_PWM and
EN_LDO both pins low simultaneously, puts the complete
chip into shutdown mode, and supply current drops to 15µA
typical.
Protection Features for the LDOs
Current Limit
The ISL6413 monitors and controls the pass transistor’s
gate voltage to limit the output current. The current limit for
LDO1 is 330mA and LDO2 is 250mA. The output can be
shorted to ground without damaging the part due to the
current limit and thermal protection features.
Thermal Overload Protection
Thermal overload protection limits total power dissipation in
the ISL6413. When the junction temperature (T
J
) exceeds
+150°C, the thermal sensor sends a signal to the shutdown
logic, turning off the pass transistor and allowing the IC to
cool. The pass transistor turns on again after the IC’s
junction temperature typically cools by 20°C, resulting in a
pulsed output during continuous thermal overload
conditions. Thermal overload protection protects the
ISL6413 against fault conditions. For continuous operation,
do not exceed the absolute maximum junction temperature
rating of +150°C.
Operating Region and Power Dissipation
The maximum power dissipation of ISL6413 depends on the
thermal resistance of the IC package and circuit board, the
temperature difference between the die junction and ambient
air, and the rate of air flow. The power dissipated in the
device is:
PT = P1 + P2 + P3, where
P1 = I
OUT1
x V
OUT1
/I
IN
x V
IN
P2 = I
OUT2
(V
IN
– V
OUT2
)
P3 = I
OUT3
(V
IN
- V
OUT3
)
The maximum power dissipation is:
Pmax = (Tjmax – T
A
)/θJA
Where Tjmax = 150
o
C, T
A
= ambient temperature, and θJA
is the thermal resistance from the junction to the surrounding
environment.
The ISL6413 package features an exposed thermal pad on
its underside. This pad lowers the thermal resistance of the
package by providing a direct heat conduction path from the
die to the PC board. Additionally, the ISL6413’s ground
(GND_LDO and PGND) performs the dual function of
providing an electrical connection to system ground and
channeling heat away. Connect the exposed backside pad
direct to the GND_LDO ground plane.
Applications Information
LDO Regulator Capacitor Selection and Regulator
Stability
Capacitors are required at the ISL6413 LDO Regulators’
input and output for stable operation over the entire load
range and the full temperature range. Use >1µF capacitor at
the input of LDO Regulators, V
IN
_LDO pins. The input
capacitor lowers the source impedance of the input supply.
Larger capacitor values and lower ESR provides better
PSRR and line transient response. The input capacitor must
be located at a distance of not more then 0.5 inches from the
V
IN
pins of the IC and returned to a clean analog ground.
Any good quality ceramic capacitor can be used as an input
capacitor.
The output capacitor must meet the requirements of
minimum amount of capacitance and ESR for both LDO’s.
The ISL6413 is specifically designed to work with small
ceramic output capacitors. The output capacitor’s ESR
affects stability and output noise. Use an output capacitor
with an ESR of 50mΩ or less to insure stability and optimum
transient response. For stable operation, a ceramic
capacitor, with a minimum value of 3.3µF, is recommended
for V
OUT1
for 300mA output current, and 3.3µF is
recommended for V
OUT2
at 200mA load current. There is no
upper limit to the output capacitor value. Larger capacitor
can reduce noise and improve load transient response,
stability and PSRR. Higher value of output capacitor (10µF)
is recommended for LDO2 when used to power VCO
circuitry in wireless chipsets. The output capacitor should be
located very close to V
OUT
pins to minimize impact of PC
board inductances and the other end of the capacitor should
be returned to a clean analog ground.
PWM Regulator Component Selection
INDUCTOR SELECTION
A 10µH minimum output inductor is used with the ISL6413
PWM section. Values larger then 15µH or less then 10µH
may cause stability problems because of the internal
compensation of the regulator. The important parameters of
the inductor that need to be considered are the current rating
of the inductor and the DC resistance of the inductor. The dc
resistance of the inductor will influence directly the efficiency
of the converter. Therefore, an inductor with lowest dc
resistance should be selected for highest efficiency.
ISL6413
