BL3406B
1.5 MHz, 800mA Synchronous Buck Converter
Operation
The BL3406B uses a constant frequency,
current mode step-down architecture. Both
the main switch (P-channel MOSFET) and
the
synchronous
rectifier
(N-channel
MOSFET) are integrated internally. This
Step-Down DC-DC Converter can supply
800mA output current over a wide input
voltage range from 2.5V to 5.5V. The over
voltage comparator OVDET guards against
transient overshoots >7.8% by turning the
main switch off and keeping it off until the
fault is removed.
value of the output voltage, the BL3406B will
keep the main switch on for more than one
switching cycle and increases the duty cycle
(Note 6) until it reaches 100%. The output
voltage then is the input voltage minus the
voltage drop across the main switch and the
inductor. At low input supply voltage, the
R
DS(ON)
of the P-Channel MOSFET increases,
and the efficiency of the converter decreases.
Caution must be exercised to ensure the heat
dissipated not to exceed the maximum
junction temperature of the IC.
Note
6
:
The duty cycle D of a step-down converter is
defined as:
Current Mode PWM Control
Slope compensated current mode PWM
control provides stable switching and cycle-
by-cycle current limit for excellent load and
line responses. During normal operation, the
internal main switch is turned on for a certain
time to ramp the inductor current at each
rising edge of the internal oscillator, and
turned off when the peak inductor current
reaches the controlled value. When the main
switch is off, the synchronous rectifier will be
turned on immediately and stay on until either
the inductor current starts to reverse, as
indicated by the current reversal comparator,
I
RCMP
, or the beginning of the next clock cycle.
D
=
T
ON
×
f
OSC
×
100%
≈
V
OUT
×
100%
V
IN
where T
ON
is the main switch on time, and f
OSC
is the oscillator frequency (1.5MHz).
Short Circuit Protection
The BL3406B has short circuit protection.
When output is shorted to ground, the
oscillator frequency is reduced to prevent the
inductor current from increasing beyond the
PFET current limit. The PFET current limit is
also reduced to lower the short circuit current.
The frequency and current limit will return to
the normal values once the short circuit
condition is removed and the feedback
voltage reaches 0.6V.
Pulse Skipping Mode Operation
Maximum Load Current
At very light loads, the BL3406B will
automatically enter Pulse Skipping Mode to
increase efficiency, further extending battery
life. In this mode, the control loop skips PWM
pulses while maintaining output in regulation,
and the switching frequency depends on the
load condition. This is a kind of PFM mode
operation.
The BL3406B will operate with input supply
voltage as low as 2.5V, however the
maximum load current decreases at lower
input voltage due to large IR drop on the
main switch and synchronous rectifier.
Dropout Operation
When the input voltage decreases toward the
PPMIC BU
BL3406B Rev 1.5
10/2009
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