ML4854IT Fairchild, ML4854IT Datasheet - Page 8

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

PRODUCT SPECIFICATION
Block Diagram
Functional Description
Boost regulator
ML4854 is an adjustable boost regulator that combines vari-
able ON and minimum OFF architecture with synchronous
rectification. Unique control circuitry provides high-effi-
ciency power conversion for both light and heavy loads by
transitioning between discontinuous and continuous conduc-
tion based on load conditions. There is no oscillator; a con-
stant-peak-current limit of 1.5A in the switch allows the
inductor current to vary between this peak limit and some
lesser value. The switching frequency depends upon the load
and the input voltage, and can range up to 650kHz.
The input voltage (V
external Inductor to the V
V
the feedback pin V
mines the output voltage. When V
V
to deliver a charge to the output by triggering the Variable
On-Time One Shot. This generates a pulse at the gate of the
Power NMOS transistor Q1. This transistor will charge the
Inductor L1 for the time interval (T
current given by:
8
LBI
I
OUT
REF
V
L PEAK
IN
(
3
1
=1.23V, the error amplifier A1 signals to the regulator
closes through the external resistive voltage divider to
0.39V
)
Minimum
Off-Time
Logic
=
T
-------------------------- -
ON
–
+
L
V
×
A3
FB
OUT
1
I
LIMIT
V
IN
. The transfer ratio of this divider deter-
IN
) comes to V
SHDN
L
Start-Up
pin of the device. The loop from
LBO
One Shot
On-Time
Variable
4
FB
ON
IN
voltage drops below
) resulting in a peak
pin and through the
Q
1
7
V
N
L
I
LIMIT
Synchronous
When the one shot times out, the Q1 transistor releases the
V
charge the output through the body diode of the transistor
Q2. But, as the voltage across the Q2 changes polarity, its
gate will be driven low by the Synchronous Rectifier Control
Circuit (SRC), causing Q2 to short out its body diode. The
inductor then delivers the charge to the load by discharging
into it through Q2.
Under lightly loaded conditions, the amount of energy
delivered in this single pulse satisfies the voltage-control
loop, and the converter does not command any more energy
pulses until the output again drops below the lower-voltage
threshold. Under medium and heavy loads, a single energy
pulse is not sufficient to force the output voltage above its
upper threshold before the minimum off time has expired
and a second charge cycle is commanded. Since the inductor
current has not reached zero in this case, the peak current is
greater than the previous value at the end of the second
cycle. The result is a ratcheting of inductor current until
either the output voltage is satisfied, or the converter reaches
its set current limit.
After a period of time T
mum Off–Time Logic and if V
Variable On-Time One Shot will be turned ON again and the
process repeats.
The output capacitor of the converter (see Test circuit) filters
the variable component, limiting the output voltage ripple to
a value determined by its capacitance and its ESR.
Rectifier
Control
L
1
pin, allowing the inductor to fly-back and momentary
Current
Control
Limit
A1
Q
A2
–
+
2
+
–
I
V
LIMIT
OFF
REF
> 0.5µS, determined by Mini-
OUT
Control
Logic
is low (V
GND
8
REV. 1.0.7 5/6/03
FB
SHDN
<V
REF
ML4854
2
6
5
), the
SHDN
V
V
OUT
FB