MAX8770GTL+T Maxim Integrated Products, MAX8770GTL+T Datasheet - Page 34

• Manufacturer Part Number: MAX8770GTL+T
• Description: IC CTLR PS 2/1PH QUICK PWM 40QFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX8770GTL+T

• Applications: Controller, Intel IMVP-6
• Voltage - Input: 4 ~ 26 V
• Number Of Outputs: 1
• Voltage - Output: 0.125 ~ 1.5 V
• Operating Temperature: -40°C ~ 105°C
• Mounting Type: *
• Package / Case: *
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

PWM Controller for IMVP-6+ CPU Core Power Supplies
CONFIDENTIAL INFORMATION – RESTRICTED TO INTEL
During soft-start, soft-shutdown, and normal opera-
tion—when the CPU is actively running (DPRSLPVR =
low, Table 5)—the MAX8770/MAX8771/MAX8772 oper-
ate with the low-noise, forced-PWM control scheme.
Forced-PWM operation disables the zero-crossing
comparators of all active phases, forcing the low-side
gate-drive waveforms to be constantly the complement
of the high-side gate-drive waveforms. This keeps the
switching frequency constant and allows the inductor
current to reverse under light loads, providing fast,
accurate negative-output-voltage transitions by quickly
discharging the output capacitors.
Forced-PWM operation comes at a cost; the no-load
+5V bias supply current remains between 10mA to
50mA per phase, depending on the external MOSFETs
and switching frequency. To maintain high efficiency
under light-load conditions, the processor may switch
the controller to a low-power, pulse-skipping control
scheme after entering suspend mode.
PSI determines how many phases are active when
operating in forced-PWM mode (DPRSLPVR = low).
When PSI is pulled low, the main phase remains active
but the secondary phase is disabled (DH2 and DL2
forced low).
When DPRSLPVR is pulled high, the MAX8770/
MAX8771/MAX8772 operate with a single-phase,
pulse-skipping mode. The pulse-skipping mode
enables the driver’s zero-crossing comparator, so the
controller pulls DL1 low when its current-sense inputs
detect “zero” inductor current. This keeps the inductor
from discharging the output capacitors and forces the
controller to skip pulses under light-load conditions to
avoid overcharging the output.
When pulse skipping, the controller blanks the upper
PWRGD and CLKEN thresholds, and also blanks
PHASEGD high impedance for the MAX8771. Upon
entering pulse-skipping operation, the controller tem-
porarily sets the OVP threshold to 1.80V, preventing
false OVP faults when the transition to pulse-skipping
operation coincides with a downward 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 MAX8770/MAX8771/
MAX8772 automatically use forced-PWM operation dur-
ing soft-start and soft-shutdown, regardless of the
DPRSLPVR and PSI configuration.
34
______________________________________________________________________________________
Forced-PWM Operation (Normal Mode)
Light-Load Pulse-Skipping Operation
MAX8770/MAX8771/MAX8772 Dual-Phase, Quick-
(Deeper Sleep)
In skip mode (DPRSLPVR = high), an inherent automat-
ic switchover to PFM takes place at light loads (Figure
8). This switchover is affected by a comparator that
truncates the low-side switch on-time at the inductor
current’s zero crossing. The zero-crossing comparator
senses the inductor current across the low-side
MOSFETs. Once V
comparator threshold (see the Electrical Characteristics
table), the comparator forces DL low (Figure 2). This
mechanism causes the threshold between pulse-skip-
ping PFM and nonskipping PWM operation to coincide
with the boundary between continuous and discontinu-
ous 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 bat-
tery-input 7V to 20V range, 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
where η
The switching waveforms may appear noisy and asyn-
chronous when light loading activates pulse-skipping
operation, but this is a normal operating condition that
results in high light-load efficiency. Trade-offs between
PFM noise and light-load efficiency are made by vary-
Figure 8. Pulse-Skipping/Discontinuous Crossover Point
LOAD(SKIP)
I
LOAD SKIP
0
TOTAL
(
∆i
∆t
) is approximately:
ON-TIME
is the number of active phases.
Automatic Pulse-Skipping Switchover
)
V
=
BATT
η
TOTAL
– V
LX
L
®
OUT
drops below the zero crossing
IMVP-6 LICENSEES
⎛
⎜
⎝
T
SW OUT
V
L
TIME
⎞
⎟
⎠
⎛
⎜
⎝
V
IN
−
V
I
LOAD
IN
V
OUT
I
PEAK
= I
PEAK
⎞
⎟
⎠
/2