ISL78214_15 Datasheet, PDF (13/15 Pages) Intersil Corporation – 4A Low Quiescent Current High Efficiency Synchronous Buck Regulator

ISL78214
UVLO
Thermal Shutdown
When the input voltage is below the Undervoltage Lockout
(UVLO) threshold, the regulator is disabled.
Soft Start-up
The ISL78214 has built-in thermal protection. When the internal
temperature reaches +140°C, the regulator is completely shut
down. As the temperature drops to +115°C, the ISL78214
resumes operation by stepping through the soft-start.
The soft start-up reduces the inrush current during the start-up.
The soft-start block outputs a ramp reference to the input of the
error amplifier. This voltage ramp limits the inductor current as
well as the output voltage speed so that the output voltage rises
in a controlled fashion. When VFB is less than 0.2V at the
beginning of the soft-start, the switching frequency is reduced to
1/3 of the nominal value so that the output can start up
smoothly at light load condition. During soft-start, the IC operates
in the SKIP mode to support prebiased output condition.
Enable
The enable (EN) input allows the user to control turning on or off
the regulator for purposes such as power-up sequencing. When
the regulator is enabled, there is typically a 600µs delay for
waking up the bandgap reference and then the soft start-up
begins.
Discharge Mode (Soft-stop)
Applications Information
Output Inductor and Capacitor Selection
To consider steady state and transient operations, ISL78214
typically uses a 1.5µH output inductor. The higher or lower
inductor value can be used to optimize the total converter system
performance. For example, for higher output voltage 3.3V
applications, in order to decrease the inductor current ripple and
output voltage ripple, the output inductor value can be increased.
It is recommended to set the ripple inductor current
approximately 30% of the maximum output current for optimized
performance. The inductor ripple current can be expressed as
shown in Equation 1:
I = ^V-----^O-----^---_L^{}---¹--_---_f^–--_S---^V-V--------^I--^O--^N-----^{}--
(EQ. 1)
When a transition to shutdown mode occurs or the VIN UVLO is
set, the outputs discharge to GND through an internal 100Ω
switch.
Power MOSFETs
The power MOSFETs are optimized for best efficiency. The
ON-resistance for the P-MOSFET is typically 50mΩ and the
ON-resistance for the N-MOSFET is typically 50mΩ.
100% Duty Cycle
The inductor’s saturation current rating needs to be at least
larger than the peak current. The ISL78214 protects the typical
peak current 6A. The saturation current needs be over 7A for
maximum output current application.
ISL78214 uses internal an compensation network and the
output capacitor value is dependent on the output voltage. The
ceramic capacitor is recommended to be X5R or X7R. The
recommended X5R or X7R minimum output capacitor values are
shown in Table 1.
The ISL78214 features 100% duty cycle operation to maximize
In Table 1, the minimum output capacitor value is given for the
the battery life. When the battery voltage drops to a level that the different output voltage to make sure that the whole converter
ISL78214 can no longer maintain the regulation at the output,
system is stable. Additional output capacitance should be added
the regulator completely turns on the P-MOSFET. The maximum for better performances in applications where high load transient
dropout voltage under the 100% duty-cycle operation is the
or low output ripple is required. It is recommended to check the
product of the load current and the ON-resistance of the
system level performance along with the simulation model.
P-MOSFET.
TABLE 1. OUTPUT CAPACITOR VALUE vs V_OUT
V_OUT
(V)
C_OUT
L
(µF)
(µH)
0.8
2 x 22
1.0~2.2
1.2
2 x 22
1.0~2.2
1.5
2 x 22
1.5~3.3
1.8
2 x 22
1.5~3.3
2.5
2 x 22
1.5~3.3
3.3
2 x 22
2.2~4.7
3.6
2 x 22
2.2~4.7
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FN7551.3
May 12, 2015