ISL78213 Datasheet, PDF (13/15 Pages) Intersil Corporation – 3A Low Quiescent Current, High Efficiency Synchronous Buck Regulator

ISL78213
PWM
PFM
CLOCK
I_L
0
8 CYCLES
NOMINAL +1.5%
PFM CURRENT LIMIT
LOAD CURRENTT
V_OUT
NOMINAL
FIGURE 37. SKIP MODE OPERATION WAVEFORMS
PG
During power-up, the open-drain power good output
holds low for about 1ms after V_OUT reaches the
regulation voltage. The PG output also serves as a 1ms
delayed the Power Good signal when the pull-up resistor
R₁ is installed.
UVLO
When the input voltage is below the undervoltage
lock-out (UVLO) threshold, the regulator is disabled.
Soft Start-Up
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 pre-biased 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)
When a transition to shutdown mode occurs or the V_IN
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 ISL78213 features 100% duty cycle operation to
maximize the battery life. When the battery voltage
drops to a level that the ISL78213 can no longer
maintain the regulation at the output, the regulator
completely turns on the P-MOSFET. The maximum
dropout voltage under the 100% duty-cycle operation is
the product of the load current and the ON-resistance of
the P-MOSFET.
Thermal Shut-Down
The ISL78213 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 ISL78213 resumes operation by stepping
through the soft-start.
Applications Information
Output Inductor and Capacitor Selection
To consider steady state and transient operations,
ISL78213 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:
V_O
⋅
⎛
⎜
⎝
1
–
V^-V----_I^-O-_N^--_{⎠⎟⎞}
ΔI
=
-------------------------------------
L ⋅ f_S
(EQ. 1)
The inductor’s saturation current rating needs to be at
least larger than the peak current. The ISL78213
protects the typical peak current 4.8A. The saturation
current needs be over 5.5A for maximum output current
application.
ISL78213 uses an internal 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. In Table 1,
the minimum output capacitor value is given for the
different output voltage to make sure that the whole
converter system is stable. Additional output capacitance
should be added for better performances in applications
13
FN7550.1
March 16, 2010