MAX1845 Maxim, MAX1845 Datasheet - Page 13

• Manufacturer Part Number: MAX1845
• Description: Dual / High-Efficiency / Step-Down Controller with Accurate Current Limit
• Manufacturer: Maxim

Figure 2. Functional Diagram
current level at which PFM/PWM crossover occurs,
I
rent, which is a function of the inductor value (Figure
4). For example, in the standard application circuit with
V
switchover to pulse-skipping operation occurs at I
= 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.
where K is the on-time scale factor (Table 4). The load-
LOAD(SKIP)
OUT1
= 2.5V, V
5V INPUT
, is equal to 1/2 the peak-to-peak ripple cur-
V+
V
DD
IN
______________________________________________________________________________________
V
DD
= 15V, and K = 2.96µs (Table 4),
Controller with Accurate Current Limit
LX1
CS1
V
PGND*
BST1
DH1
DL1
OUT1
FB1
UVP
OVP
PGOOD
ON2
TON
SKIP
ON1
DD
V
DD
Dual, High-Efficiency, Step-Down
0.5V
* IN THE MAX1845EEI, AGND AND PGND ARE INTERNALLY CONNECTED AND CALLED GND.
V
CC
CONTROLLER
- 1V
(FIGURE 3)
I
LIM1
PWM
LOAD
MAX1845
V+
V+
CONTROLLER
(FIGURE 3)
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
2V TO 28V
PWM
V
CC
- 1V
I
LIM2
0.5V
FAULT1
FAULT2
REF
2V
V
DD
AGND*
OUT 2
BST2
V
DH2
CS2
FB2
REF
LX2
DL2
CC
V
DD
V+
20Ω
V
DD
13