5v, v – Rainbow Electronics MAX1845 User Manual
Page 13
MAX1845
Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limit
______________________________________________________________________________________
13
where K is the on-time scale factor (Table 4). The load-
current level at which PFM/PWM crossover occurs,
I
LOAD(SKIP)
, is equal to 1/2 the peak-to-peak ripple cur-
rent, which is a function of the inductor value (Figure
4). For example, in the standard application circuit with
V
OUT1
= 2.5V, V
IN
= 15V, and K = 2.96µs (Table 4),
switchover to pulse-skipping operation occurs at I
LOAD
= 0.7A or about 1/6 full load. The crossover point
occurs at an even lower value if a swinging (soft-satu-
ration) inductor is used.
The switching waveforms may appear noisy and asyn-
chronous when light loading causes pulse-skipping
operation, but this is a normal operating condition that
results in high light-load efficiency. Trade-offs in PFM
noise vs. light-load efficiency are made by varying the
inductor value. Generally, low inductor values produce
a broader efficiency vs. load curve, while higher values
result in higher full-load efficiency (assuming that the
coil resistance remains fixed) and less output voltage
ripple. Penalties for using higher inductor values
FB2
OUT 2
PWM
CONTROLLER
(FIGURE 3)
V+
V+
2V
REF
GND
FAULT1
FAULT2
REF
20
Ω
V
DD
V
CC
OUT1
UVP
OVP
FB1
SKIP
TON
ON1
ON2
5V INPUT
DL1
V
DD
LX1
CS1
DH1
BST1
V
DD
V
DD
V
DD
V+
PWM
CONTROLLER
(FIGURE 3)
MAX1845
PGOOD
V
CC
- 1V
0.5V
I
LIM1
I
LIM2
DL2
V
DD
LX2
CS2
DH2
BST2
V
DD
V+
V
CC
- 1V
0.5V
2V TO 28V
Figure 2. Functional Diagram