6/13
Rev. B, Sep 2005
Operation (2)
Low Noise Fixed Frequency Operation
Buck Region (VIN > VOUT)
Switch D is always on and switch C is always off during
this mode. When the internal control voltage, VCI is above
voltage V1, output A begins to switch. During the off time
of switch A, synchronous switch B turns on for the
remainder of the time. Switches A and B will alternate
similar to a typical synchronous buck regulator. As the
control voltage increases, the duty cycle of switch A
increases until the maximum duty cycle of the converter in
Buck mode reaches DMAX_BUCK, given by:
DMAX_BUCK =100 - D4SW %
where D4SW = duty cycle% of the four switch range
D4SW = (150ns f) 100%
where f = operating frequency, Hz.
Beyond this point the “four switch,” or Buck/Boost region is
reached.
Buck/Boost or four Switch (VIN - VOUT)
When the internal control voltage, VCI, is above voltage
V2, switch pair AD remain on for duty cycle DMAX-BUCK, and
the switch pair AC begins to phase in. As switch pair AC
phases in, switch pair BD phases out accordingly. When
the VCI voltage reaches the edge of the Buck/Boost range,
at voltage V3, the AC switch pair completely phase out the
BD pair, and the boost phase begins at duty cycle D4SW.
The input voltage, VIN, where there four switch region
begins is given by:
VOUT
VIN =
1- (150ns f )
V
The point at which the four switch region ends is given by:
VIN = VOUT (1-D) = VOUT (1-150ns f )V
Boost Region (VIN < VOUT)
Switch A is always on and switch B is always off during
this mode. When the internal control voltage, VCI, is above
voltage V3, switch pair CD will alternately switch to
provide a boosted output voltage. This operation is typical
to a synchronous boost regulator. The maximum duty
cycle of the converter is limited to 75% typical and is
reached when VCI is above V4.
Burst Mode Operation
Burst Mode Operation is when the IC delivers energy to the
output until is regulated and then goes into a sleep mode
where the outputs are off and the IC is consuming only
25uA. In this mode the output ripple has a variable
frequency component that depends upon load current.
During the period where the device is delivering energy to
the output, the peak current will be equal to 400mA typical
and the inductor current will terminate at zero current for
each cycle. In this mode the maximum average output
current is given by:
0.1 VIN
Iout (MAX)BURST ≈
VOUT + VIN
A
Burst Mode operation is user controlled, by driving the
MODE/SYNC pin high to enable and low to disable. The
peak efficiency during Burst Mode operation is less than
the peak efficiency during fixed frequency because the part
enters full-time 4-switch mode (when servicing the output)
with discontinuous inductor current as illustrated in Figures
3 and 4. During Burst Mode operation, the control loop is
nonlinear and cannot utilize the control voltage from the
error amp to determine the control mode, therefore full-time
4-switch mode is required to maintain the Buck/Boost
function. The efficiency below 1mA becomes dominated
primarily by the quiescent current and not the peak
efficiency. The equation is given by:
(ηbm) ILOAD
Efficiency Burst ≈
25uA + ILoad
Where (ηbm) is typically 79% during burst Mode operation
for an ESR of the inductor of 50mΩ. For 200mΩ of inductor
ESR, the peak efficiency (ηbm) drops to 75%.
Figure 3
Inductor Charge Cycle During Burst Mode Operation