Power-up sequence (por, uvlo) – Rainbow Electronics MAX17036 User Manual
Page 30
MAX17030/MAX17036
1/2/3-Phase Quick-PWM
IMVP-6.5 VID Controllers
30
______________________________________________________________________________________
When pulse-skipping, the controller blanks the upper
PWRGD and CLKEN thresholds. Upon entering pulse-
skipping operation, the controller temporarily sets the
OVP threshold to 1.5V, preventing false OVP faults
when the transition to pulse-skipping operation coin-
cides with a VID code change. Once the error amplifier
detects that the output voltage is in regulation, the OVP
threshold tracks the selected VID DAC code. The
MAX17030/MAX17036 automatically use forced-PWM
operation during soft-start and soft shutdown, regard-
less of the DPRSLPVR and PSI configuration.
Automatic Pulse-Skipping Switchover
In skip mode (DPRSLPVR = high), an inherent automatic
switchover to PFM takes place at light loads (Figure 8).
This switchover is affected by a comparator that trun-
cates the low-side switch on-time at the inductor cur-
rent’s zero crossing. The zero-crossing comparator
senses the inductor current across the low-side
MOSFETs. Once V
LX
drops below the zero-crossing
comparator threshold (see the
Electrical Characteristics
),
the comparator forces DL low. This mechanism causes
the threshold between pulse-skipping PFM and non-
skipping PWM operation to coincide with the boundary
between continuous and discontinuous inductor-current
operation. The PFM/PWM crossover occurs when the
load current of each phase is equal to 1/2 the peak-to-
peak ripple current, which is a function of the inductor
value (Figure 8). For a battery input range of 7V to 20V,
this threshold is relatively constant, with only a minor
dependence on the input voltage due to the typically
low duty cycles. The total load-current at the PFM/PWM
crossover threshold (I
LOAD(SKIP)
) is approximately:
Power-Up Sequence (POR, UVLO)
The MAX17030/MAX17036 are enabled when SHDN is
driven high (Figure 9). The reference powers up first.
Once the reference exceeds its undervoltage-lockout
(UVLO) threshold, the internal analog blocks are turned
on and masked by a 150µs one-shot delay. The PWM
controller then begins switching.
I
T
V
L
V
V
V
LOAD SKIP
SW OUT
IN
OUT
IN
(
)
= ⎛
⎝⎜
⎞
⎠⎟
×
⎛
⎝⎜
⎞
-
2
⎠⎠⎟
I
PEAK
I
LOAD
= I
PEAK
/2
INDUCTOR CURRENT
TIME
0
L
V
IN
– V
OUT
∆t
∆I
=
ON-TIME
Figure 8. Pulse-Skipping/Discontinuous Crossover Point
FORCED-PWM
t
BLANK
20
µs TYP
t
BLANK
60
µs TYP
t
BLANK
5ms TYP
t
BLANK
20
µs TYP
VID (D0–D6)
V
CORE
IMVPOK
INTERNAL
PWM CONTROL
V
CC
SOFT-START
1/4 SLEW RATE SET
BY R
TIME
SHDN
IGNORE
VID
IGNORE
VID
PULSE-SKIPPING
CLKEN
SOFT-SHUTDOWN
1/4 SLEW RATE SET
BY R
TIME
Figure 9. Power-Up and Shutdown Sequence Timing Diagram