MAX1845 Maxim, MAX1845 Datasheet - Page 11

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

time is determined solely by a one-shot whose pulse
width is inversely proportional to input voltage and
directly proportional to output voltage. Another one-shot
sets a minimum off-time (400ns typ). The on-time one-
shot is triggered if the error comparator is low, the low-
side switch current is below the current-limit threshold,
and the minimum off-time one-shot has timed out
(Table 3).
The heart of the PWM core is the one-shot that sets the
high-side switch on-time for both controllers. This fast,
low-jitter, adjustable one-shot includes circuitry that
varies the on-time in response to battery and output
voltage. The high-side switch on-time is inversely pro-
portional to the battery voltage as measured by the V+
input, and proportional to the output voltage. This algo-
rithm results in a nearly constant switching frequency
despite the lack of a fixed-frequency clock generator.
The benefits of a constant switching frequency are
twofold: First, the frequency can be selected to avoid
noise-sensitive regions such as the 455kHz IF band;
second, the inductor ripple-current operating point
Figure 1. Standard Application Circuit
V
BIAS SUPPLY
4.7µF
DD
C9
= 5V
1µF
______________________________________________________________________________________
C8
3
✕
C11
1µF
Controller with Accurate Current Limit
470µF
On-Time One-Shot (TON)
C3
OUTPUT1
1.8V, 8A
R1
20Ω
5mΩ
3
R1
✕
2.2µH
10µF
L1
C1
Dual, High-Efficiency, Step-Down
D1
0.22µF
Q1
Q2
C7
0.1µF
C5
21
22
25
26
27
24
28
10
23
13
9
3
5
1
2
V
UVP
V
ILIM1
ILIM2
BST1
DH1
LX1
DL1
TON
CS1
OUT1
REF
FB1
GND
DD
CC
remains relatively constant, resulting in easy design
methodology and predictable output voltage ripple.
The on-times for side 1 are set 35% higher than the on-
times for side 2. This is done to prevent audio-frequen-
cy “beating” between the two sides, which switch
asynchronously for each side. The on-time is given by:
where K is set by the TON pin-strap connection (Table
4), and 0.075V is an approximation to accommodate
for the expected drop across the low-side MOSFET
switch. One-shot timing error increases for the shorter
on-time settings due to fixed propagation delays; it is
approximately ±12.5% at higher frequencies and ±10%
at lower frequencies. This translates to reduced switch-
ing-frequency accuracy at higher frequencies (Table
4). Switching frequency increases as a function of load
current due to the increasing drop across the low-side
MOSFET, which causes a faster inductor-current dis-
charge ramp. The on-times guaranteed in the Electrical
Characteristics tables are influenced by switching
delays in the external high-side power MOSFET.
MAX1845EEI
V+
4
D3
CMPSH-3A
PGOOD
OUT2
BST2
SKIP
ON1
ON2
OVP
DH2
CS2
DL2
LX2
FB2
On-Time = K (V
11
12
19
18
17
20
16
15
14
8
6
7
C6
0.1µF
5V
100kΩ
Q3
Q4
OUT
D2
C2
2
V
7V TO 24V
✕
IN
4.7µH
+ 0.075V) / V
10µF
L2
R2
10mΩ
POWER-GOOD
INDICATOR
ON/OFF
CONTROLS
OUTPUT2
2.5V, 4A
C4
470µF
IN
11