Rainbow Electronics MAX1845 - Manual (Page 11)

MAX1845
Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limit
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V DD = 5V
BIAS SUPPLY
POWER-GOOD
INDICATOR
MAX1845
V CC
OUTPUT1
1.8V, 8A
V IN
7V TO 24V
D3
CMPSH-3A
ILIM1
DL1
TON
CS1
OUT1
GND
C3
3 ✕ 470 µF
C4
470
µF D1
Q4
Q3 Q1
Q2
LX1
DH1
C5
0.1 µF
C6
0.1
µF
C7
0.22 µF
FB1
V
DD
UVP
C8
1 µF
C1
3 ✕ 10 µF
C2
2
✕ 10 µF
11
12
8
19
18
17
20
16
15
6
14
7
22
25
26
27
24
5
10
2
23
21
9
C11
1
µF
L1
2.2 µH
L2
4.7 µH
13
3
28
1
BST1
ILIM2
REF
ON1
ON2
OVP
DL2
CS2
5V
100k Ω
OUT2
LX2
DH2
FB2
PGOOD
V+
4
BST2
SKIP
C9
4.7 µF
R1
20
Ω
R1
5m Ω
OUTPUT2
2.5V, 4A
R2
10m
Ω
D2
ON/OFF
CONTROLS
Figure 1. Standard Application Circuit
On-Time One-Shot (TON)
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
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 asyn-
chronously for each side. The on-time is given by:
On-Time = K (V OUT + 0.075V) / V IN
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.