MAX1870AETJ+ Maxim Integrated Products, MAX1870AETJ+ Datasheet - Page 20

• Manufacturer Part Number: MAX1870AETJ+
• Description: Battery Management Li+ Step Up/Step Down Battery Charger
• Manufacturer: Maxim Integrated Products

Specifications of MAX1870AETJ+

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

Step-Up/Step-Down
Li+ Battery Charger
up operation. During this mode, the MAX1870A regu-
lates the step-up on-time. Initially DBST switches M2 on
(state C) and the inductor current ramps up with a dI/dt
of V
current (set by the error integrators), DBST switches M2
off (state B) and the inductor current ramps down with
a dI/dt of (V
up off-time timer expires. This off-time is calculated
based on the input and output voltage to maintain
400kHz pseudo-fixed-frequency operation. The step-up
on-time is regulated by the error signal, set according
to the charge current or source current if either is at its
limit, or the battery voltage if both charge current and
source current are below their respective current limits.
The MAX1870A features a step-up/step-down mode
that eliminates dropout. Figure 8 shows the inductor
current in step-up/step-down operation. When V
within 10% of V
Figure 5. MAX1870A State Machine
20
IN
______________________________________________________________________________________
/ L. After the inductor current crosses the target
M1
BATT
(0.9 x V
D4
V
IN
BATT
Step-Up/Step-Down Operation
- V
STEP-DOWN OFF
, the MAX1870A alternates through
IN
STATE A
) / L. M2 remains off until a step-
BATT
M1
D2
V
IN
V
STEP-DOWN PFM
< V
OUT
D3
M2
IDLE STATE D
IN
STEP-DOWN
< 1.4 x V
PWM
V
OUT
M2
D3
M1
BATT
D4
V
IN
IN
STEP-DOWN ON
is
STEP-UP OFF
)
STATE B
states A, B, and C, following the order A, B, C, B, A, B,
C, etc., with the majority of the time spent in state B.
Since more time is spent in state B, the inductor ripple
current is reduced, improving efficiency.
The time in state C is peak-current regulated, and the
remaining time is spent in state B (Figure 8A). During
this operating mode, the average inductor current is
approximately 20% higher than the load current.
The time in state A is valley current and the remaining
time is spent in state B (Figure 8B). During this mode,
the average inductor current is approximately 10%
higher than the load current.
Alternative algorithms require inductor currents twice
as high, resulting in four times larger I
inductors typically four times larger in volume.
The MAX1870A state machine utilizes five comparators
to decide which state to be in and when to switch
states (Figure 3). The MAX1870A generates an error
V
M2
OUT
D3
STEP-UP
PWM
IMIN, IMAX, CCMP, and ZCMP
M1
D4
V
IN
STEP-UP ON
+
STATE C
-
V
OUT
M2
D3
2
R losses and