Rainbow Electronics MAX1712 - Manual (Page 16)

MAX1710/MAX1711/MAX1712
High-Speed, Digitally Adjusted
Step-Down Controllers for Notebook CPUs
16 ______________________________________________________________________________________
pullups on each input in order to eliminate external resis-
tors.
When changing MAX1710 DAC codes while powered
up, the over/undervoltage protection features can be
activated if the code is changed more than 1LSB at a
time. For applications needing the capability of changing
DAC codes “on-the-fly,” use the MAX1711/MAX1712.
POR, UVLO, and Soft-Start
Power-on reset (POR) occurs when V CC rises above
approximately 2V, resetting the fault latch and soft-start
counter, and preparing the PWM for operation. V
CC
undervoltage lockout (UVLO) circuitry inhibits switching
and forces the DL gate driver high (in order to enforce
output overvoltage protection) until V
CC rises above
4.2V, whereupon an internal digital soft-start timer begins
to ramp up the maximum allowed current limit. The ramp
occurs in five steps: 20%, 40%, 60%, 80%, and 100%,
with 100% current available after 1.7ms ±50%.
A continuously adjustable, analog soft-start function can
be realized by adding a capacitor in parallel with R
LIM at
ILIM. This soft-start method requires a minimum interval
between power-down and power-up to allow R
LIM to dis-
charge the capacitor.
Power-Good Output (PGOOD)
The output (FB) is continuously monitored for undervolt-
age by the PGOOD comparator, except in shutdown or
standby mode. The -5% undervoltage trip threshold is
measured with respect to the nominal unloaded output
voltage, as set by the DAC. If the DAC code increases in
steps greater than 1LSB, it is likely that PGOOD will
momentarily go low. In shutdown and standby modes,
PGOOD is actively held low. The PGOOD output is a true
SHDN SKIP OVP DL MODE COMMENTS
1 X
0 X 0 High Shutdown1 Low-power shutdown state. DL is forced to V DD , enforcing OVP. I CC < 1µA typ.
X Low
0 X 1 Low Shutdown2
Low-power shutdown state. DL is forced to GND, disabling OVP. I CC < 1µA typ.
Exiting shutdown triggers a soft-start cycle.
Shutdown3
(MAX1711/
MAX1712)
DAC code = X1111 (MAX1711), DAC code = 11111 (MAX1712) (Table 2). DL is
forced to PGND, DH is forced to LX. The MAX1711/MAX1712 eventually goes
into UVP fault mode as the load current discharges the output.
1
Below
GND
X Switching No fault
Test mode with OVP, UVP, and thermal faults disabled and latches cleared.
Otherwise normal operation, with automatic PWM/PFM switchover for pulse
skipping at light loads (Figure 6).
1 X 1 Switching No OVP
OVP faults disabled and OVP latch cleared. Otherwise normal operation,
with SKIP controlling PWM/PFM switchover.
1 V CC X Switching
Run (PWM),
Low Noise
Low-noise operation with no automatic switchover. Fixed-frequency PWM action
is forced regardless of load. Inductor current reverses at light load levels.
I
CC draw = 750µA typ. I DD draw = 15mA typ.
1 GND X Switching
Run
(PFM/PWM)
Normal operation with automatic PWM/PFM switchover for pulse skipping at light
loads. I
CC = 600µA typ. I DD draw = load dependent.
1 X X High Fault
Fault latch has been set by OVP, output UVLO, or thermal shutdown. Device will
remain in FAULT mode until V
CC power is cycled, SKIP is forced below ground,
or SHDN is toggled.
Table 4. Operating Mode Truth Table
Good operating point for
compound buck designs
or desktop circuits.
+5V-input notebook
CPU core
550
400
3-cell Li+ notebook
CPU core
Useful in 4-cell systems
for lighter loads than the
CPU or where size is key.
Considered mainstream
by current standards. 4-cell Li+ notebook
CPU core
300
Table 5. Frequency Selection Guidelines
FREQUENCY
(kHz)
TYPICAL
APPLICATION
COMMENT
200
4-cell Li+ notebook
CPU core Use for absolute best
efficiency.