ML4866
ELECTRICAL CHARACTERISTICS
SYMBOL
SUPPLY
I
IN
V
IN
Current
I
OUT
= 0mA,
BURST
= 5V
I
OUT
= 0mA,
BURST
= 0V
SHDN
= 0V
Note 1: Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions.
(Continued)
CONDITIONS
MIN
TYP
MAX
UNITS
PARAMETER
400
120
20
500
220
35
µA
µA
µA
FUNCTIONAL DESCRIPTION
The ML4866 is a current-mode, step-down (buck)
converter designed to keep the buck inductor current in
the continuous conduction mode (CCM). Current-mode
operation provides faster output response to input voltage
and output current changes along with cycle-by-cycle
current limiting. CCM inductor current is preferred when
the highest conversion efficiencies are required.
For high efficiencies at low output current, the ML4866
contains an autoburst function which automatically
switches from pulse width modulation (PWM) to pulsed
frequency modulation (PFM) operation when the output
current drops below 100mA. Selection of either mode is
possible by applying the correct logic level signal to the
BURST
pin. When operating in PWM mode, loop
compensation of the ML4866 is simplified due to its
transconductance type error amplifier.
An under voltage lockout (UVLO) circuit within the
ML4866 enables the converter when the input voltage
is greater than 3.25V and disables it when the input
voltage is below 3.10V. The IC can also be disabled
externally by applying a logic low signal to the
SHDN
pin. When disabled, the ML4866 draws less than 20µA of
current.
The internal 1.25V bandgap reference is made available
via the V
REF
pin, and may be used for general
applications requiring less than 10µA of current. For
proper operation, this pin must always be bypassed to
GND with a 100nF capacitor.
ramping the inductor current down to 0mA. This action is
repeated until the output voltage returns to its nominal
setting and begins again when the output drops below its
nominal setting. The rate or frequency at which this
“bursting” occurs is directly proportional to the output
current. When the average output current rises above
130mA, the ML4866 returns to PWM operation.
For applications having a load current range of less than
100mA and greater than 130mA, the
BURST
pin should be
left open and bypassed to ground with a 15nF or larger
capacitor. It is possible to tailor an application for the
highest possible efficiency by externally forcing the
ML4866 into either control mode. Applying a logic low
level to
BURST
forces the IC into PFM mode. Conversely,
a logic high places it in PWM mode. Care should be
taken to avoid reducing the efficiency by placing the
controller in the least efficient mode for a given output
current.
BURST MODE
Burst (PFM) mode is a method of regulating the output
voltage by applying a variable frequency modulation
technique to the buck inductor. This method maintains
higher efficiencies at light loads than if PWM were used.
If
BURST
is left open, the ML4866 switches from PWM
mode to PFM mode when the output current falls below
100mA. When the output voltage falls out of regulation
while in PFM mode, the internal buck switch turns on and
ramps the inductor current up to 300mA. The buck switch
then turns off and the synchronous switch turns on,
V
IN
– V
OUT
L
–V
OUT
L
∆I
Figure 1. Inductor Current
4
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
