Rainbow Electronics MAX1712 - Manual (Page 12)

MAX1710/MAX1711/MAX1712
High-Speed, Digitally Adjusted
Step-Down Controllers for Notebook CPUs
12 ______________________________________________________________________________________
low-side switch current is below the current-limit thresh-
old, and the minimum off-time one-shot has timed out.
On-Time One-Shot (TON)
The heart of the PWM core is the one-shot that sets the
high-side switch on-time. 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 proportional to the battery
voltage as measured by the V+ input, and directly pro-
portional to the output voltage as set by the DAC code.
This algorithm results in a nearly constant switching fre-
quency despite the lack of a fixed-frequency clock gen-
erator. 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:
On-Time = K (V OUT + 0.075V) / V IN
where K is set by the TON pin-strap connection 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 and is approxi-
mately ±12.5% at 550kHz and 400kHz, and ±10% at the
two slower settings. This translates to reduced switch-
ing-frequency accuracy at higher frequencies (Table 5).
Switching frequency increases as a function of load cur-
rent due to the increasing drop across the low-side
Table 1. MAX1710 FB Output Voltage
DAC Codes
D3 D2 D1 D0
OUTPUT
VOLTAGE (V)
1 0 0 0 1.60
0 0 0 0 2.00
0 0 0 1 1.95
0 0 1 0 1.90
0 0 1 1 1.85
0 1 0 0 1.80
0 1 0 1 1.75
0 1 1 0 1.70
0 1 1 1 1.65
1 0 0 1 1.55
1 0 1 0 1.50
1 0 1 1 1.45
1 1 0 0 1.40
1 1 0 1 1.35
1 1 1 0 1.30
1 1 1 1 1.25
Table 2. MAX1711 FB Output Voltage
DAC Codes
D4 D3 D2 D1
OUTPUT
VOLTAGE (V)
0 1 0 0 1.60
0 0 0 0 2.00
0 0 0 0 1.95
0 0 0 1 1.90
0 0 0 1 1.85
0 0 1 0 1.80
0 0 1 0 1.75
0 0 1 1 1.70
0 0 1 1 1.65
0 1 0 0 1.55
0 1 0 1 1.50
0 1 0 1 1.45
0 1 1 0 1.40
0 1 1 0 1.35
0 1 1 1 1.30
0 1 1 1 Shutdown3*
1 1 0 0 1.075
1 0 0 0 1.275
1 0 0 0 1.250
1 0 0 1 1.225
1 0 0 1 1.200
1 0 1 0 1.175
1 0 1 0 1.150
1 0 1 1 1.125
1 0 1 1 1.100
1 1 0 0 1.050
1 1 0 1 1.025
1 1 0 1 1.000
1 1 1 0 0.975
1 1 1 0 0.950
1 1 1 1 0.925
1 1 1 1 Shutdown3*
D0
0
0
1
0
1
0
1
0
1
1
0
1
0
1
0
1
0
0
1
0
1
0
1
0
1
1
0
1
0
1
0
1
*See Table 4.