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ISL9105 Datasheet(PDF) 9 Page - Intersil Corporation

Part No. ISL9105
Description  600mA Low Quiescent Current 1.6MHz High Efficiency Synchronous Buck Regulator
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Maker  INTERSIL [Intersil Corporation]
Homepage  http://www.intersil.com/cda/home
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ISL9105 Datasheet(HTML) 9 Page - Intersil Corporation

 
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9
FN6415.1
February 13, 2007
RSI Signal
The RSI signal is an input signal, which can reset the POR
signal. As shown in Figure 15, the power-good signal is
gated by the RSI signal. When the RSI is high, the POR
signal will remain low, regardless of the power-good signal.
Overcurrent Protection
The overcurrent protection can protect ISL9105 itself as well
as other external components when over load condition
happens. It is realized by monitoring the CSA output with the
OCP comparator, as shown in Figure 15. The current
sensing circuit has a gain of 0.4V/A. When the CSA output
reaches 0.4V, (which means the current at P-Channel
MOSFET reaches 1A) the OCP comparator is triggered to
turn off the P-Channel MOSFET immediately.
Short-Circuit Protection
ISL9105 has a Short-Circuit Protection (SCP) comparator
monitors the FB pin voltage for output short-circuit
protection. When the FB is lower than 0.2V, the SCP
comparator forces the PWM oscillator frequency to drop to
1/3 of the normal operation value. This comparator is
effective during start-up or an output short-circuit event.
UVLO
When the input voltage is below the Under-Voltage Lock Out
(UVLO) threshold, the regulator is disabled.
Soft-Start-Up
The soft-start-up eliminates the in-rush current during the
start-up. The soft-start block outputs a ramp reference to
both the voltage loop and the current loop. The two ramps
limit the inductor current rising speed as well as the output
voltage speed so that the output voltage rises in a controlled
fashion. At the very beginning of the start-up, the output
voltage is less than 0.2V; hence the PWM operating
frequency is 1/3 of the normal frequency.
Power MOSFETs
The two power MOSFETs are optimized to achieve better
efficiency. The on resistance for the P-Channel MOSFET is
typically 160m
Ω and the on resistance for the N-Channel
MOSFET is typically 140m
Ω.
100% Duty Cycle Operation
The ISL9105 features 100% duty cycle operation to
maximize the battery life. When the input voltage drops to a
level that the ISL9105 can no longer maintain the switching
regulation at the output, the P-Channel MOSFET is
completely turned on. The maximum drop out voltage under
the 100% duty-cycle operation is the product of the load
current and the on resistance of the P-Channel MOSFET.
Minimum input voltage VIN under this condition is the sum of
output voltage and the voltage drop cross the output inductor
and P-Channel MOSFET.
Thermal Shut Down
The ISL9105 provides built-in thermal protection. When the
internal temperature reaches +150°C, the regulator is
completely shutdown. As the temperature drops to +125°C,
the ISL9105 resumes operation by stepping through a
soft-start-up.
Applications Information
Output Inductor and Capacitor Selection
To achieve better steady state and transient operation,
ISL9105 typically uses a 4.7µH output inductor. Higher or
lower inductor value can be used to optimize the total
converter system performance. For example, for higher
output voltage 3.3V application, in order to decrease the
inductor current ripple and output voltage ripple, the output
inductor value can be increased. The peak-to-peak inductor
current ripple can be expressed in Equation 1:
In Equation 1, the inductance should consider the value with
worst case tolerances; and for switching frequency fS, the
minimum fS from the Electrical Specifications Table on
page 2 can be used.
To select the inductor, its saturation current rating should be
at least higher than the sum of the maximum output current
and (
ΔI)/2 from Equation 1.
ISL9105 uses internal compensation network and the output
capacitor value is dependant on the output voltage. The
ceramic capacitor is recommended to be X5R or X7R.
Input Capacitor Selection
The main functions for the input capacitor is to provide
decoupling of the parasitic inductance and to provide filtering
function to prevent the switching current flowing back to the
battery rail. A 10
μF/6.3V ceramic capacitor (X5R or X7R) is
a good starting point for the input capacitor selection.
Output Voltage Setting Resistor Selection
The voltage divider resistors, R2 and R3, shown in Figure 14
set the output voltage. The output voltage can be calculated
by Equation 2:
where the 0.8V is the reference voltage. The voltage divider,
which consists of R2 and R3, increases the quiescent
current by VO/(R2+R3), so larger resistance is desirable. On
the other hand, the FB pin has leakage current that will
cause error in the output voltage setting. The leakage
current is typically 0.1
μA. To minimize the accuracy impact
on the output voltage, select the R3 no larger than 200k
Ω.
ΔI
V
OUT
1
V
OUT
V
IN
----------------
⎝⎠
⎜⎟
⎛⎞
Lf
S
-----------------------------------------------------
=
(EQ. 1)
V
O
0.8
1
R
2
R
3
-------
+
⎝⎠
⎜⎟
⎛⎞
=
(EQ. 2)
ISL9105


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