ML4854IT Fairchild, ML4854IT Datasheet - Page 9

• Manufacturer Part Number: ML4854IT
• Description: Adjustable / Low-Current / 2-Cell Boost Regulator with Shutdown and Low Battery Detect
• Manufacturer: Fairchild

ML4854
The synchronous rectifier significantly improves efficiency
without the addition of an external component, so that
conversion efficiency can be as high as 94% over a large load
range, as shown in the “Typical Operating Characteristics.”
Even at light loads, the efficiency stays high because the
switching losses of the converter are minimized by reducing
the switching frequency.
Error Detection Comparator (LBI – LBO)
An additional comparator A3 is provided to detect low V
or any other error conditions that is important to the user.
The non-inverting input of the comparator is internally
connected to a reference threshold voltage V
inverting input is connected to the LBI pin. The output of
the low battery comparator is a simple open-drain output
that goes active low if the battery voltage drops below the
programmed threshold voltage on LBI. The output requires a
pull-up resistor, with a recommended value of 100 kΩ, be
connected only to V
The low-battery detector circuit is typically used to supervise
the battery voltage and to generate an error flag or a RESET
command when the battery voltage drops below a user-set
threshold voltage. The function is active only when the
device is enabled. When the device is disabled, the LBO-pin
is high impedance.
Shutdown
The device enters shutdown when V
mately less than 0.5V
stops switching, all internal control circuitry including the
low-battery comparator is switched off and the load is
disconnected from the input. The output voltage may drop
below the input voltage during shutdown. The typical depen-
dence shutdown voltage versus input voltage and the timing
process of the exiting shutdown are shown in the “Typical
Operating Characteristics.” For normal operation V
should be driven up 0.8V
Application Information
Selecting the Output Voltage
The output voltage V
choosing resistors R4 and R5 of the divider in the feedback
circuit (see Test Circuit). The value of the R5 is recom-
mended to be less than 270k. R4 can be calculated using
the following equation:
R4= R5[(V
where V
A compensation capacitor C3=18pF parallel with R4 pro-
vides better pulse grouping.
REV. 1.0.7 5/6/03
REF
OUT
= 1.23V
/V
REF
) – 1]
OUT
OUT
IN
). During shutdown the regulator
.
IN
can be adjusted from 3V to 5V,
or connected to the V
SHDN
is low (approxi-
th
while the
IN
SHDN
.
IN
Setting the LBI Threshold of Low-Battery
Detector Circuit
The LBO-pin goes active low when the voltage on the
LBI-pin decreases below the set threshold typical voltage of
390 mV, which is set by the internal reference voltage.
The battery voltage, at which the detection circuit switches,
can be programmed with a resistive divider connected to
the LBI-pin. The resistive divider scales down the battery
voltage to a voltage level of tenths of volt, which is then
compared to the LBI threshold voltage. The LBI-pin has a
built-in hysteresis of 25 mV. The resistor values R1 and R2
can be calculated using the following equation:
V
The value of R2 should be 270k or less to minimize bias cur-
rent errors. R1 is then found by rearranging the equation:
R1=R2 x ( V
If the low-battery detection circuit is not used, the LBI-pin
should be connected to GND (or to V
can be left unconnected or tied to GND. Do not let the LBI-
pin float.
Component selection
Output capacitor selection
The major parameter necessary to define the output capacitor
is the maximum allowed output voltage ripple of the con-
verter. This ripple is determined by two parameters of the
capacitor, the capacitance and the ESR.
The contribution due to the capacitance can be determined
by looking at the change in capacitor voltage required to
store the energy delivered by the inductor in a single charge
–discharge cycle, as determined by the formula:
For example, if V
C=47µF, the calculation by this formula gives an expected
output ripple due to only the capacitor value of 6.5mV.
In continuous inductor mode operation, this additional com-
ponent of the ripple, due to capacitor ESR, can be calculated
using equation:
Where D is the duty cycle.
An additional ripple of 28 mV, at 100mA load current, is the
result of using a ceramic capacitor with an ESR of 70mΩ.
The total ripple is the sum of the ripple caused by the capaci-
tance and the ripple caused by the ESR of the capacitor. In
this example, the total ripple is 34.5mV. It is possible to
∆V
∆V
IN_MIN
OUT
ESR
=
= 0.39 x (R1+R2)/R2
=
(
---------------------------------------------------------- -
2 L
ESR
IN_MIN
×
T
)
×
×
ON
IN
/0.39 - 1)
C V


(
=3V, V
2
------------ -
1 D
I
OUT
×
–
OUT
V
IN
+
–
OUT
2
V
------------------------ -
V
IN
IN
=5V, L=10µH, T
2L
×
)
t
ON
PRODUCT SPECIFICATION


IN
) and the LBO-pin
ON
=1.2µs,
9