EN6337QI
device will remain in PWM mode. If the load
current is below the LLM threshold, the device
will re-enter LLM operation. There may be a
small overshoot or undershoot in VOUT when
the device exits and re-enters LLM.
Light Load Mode (LLM) Operation
The EN6337QI uses a proprietary light load
mode to provide high efficiency at low output
currents. When the LLM/SYNC pin is high, the
device is in automatic LLM “Detection” mode.
When the LLM/SYNC pin is low, the device is
forced into PWM mode. In automatic LLM
“Detection” mode, when a light load condition
is detected, the device will:
The load current at which the device will enter
LLM mode is a function of input and output
voltage, and the RLLM pin resistor. Contact
Enpirion Applications support for details
regarding the optimization of this resistor for
specific operating conditions. For PWM only
operation, the RLLM pin can be left open.
(1) Step VOUT up by approximately 1.0% above
the nominal operating output voltage
setting, VNOM and as low as -0.5% below
VNOM, and then
To ensure normal LLM operation, LLM mode
should be enabled and disabled with specific
sequencing. For applications with explicit LLM
pin control, enable LLM after VIN ramp up is
complete. For applications with only ENABLE
control, tie LLM to ENABLE; enable the device
after VIN ramp up is complete, and disable the
device before VIN ramp down begins. For
designs with ENABLE and LLM tied to VIN,
make sure the device soft-start time is longer
than the VIN ramp-up time. LLM will start
operating after the soft-start time is completed.
(2) Shut down unnecessary circuitry, and then
(3) Monitor VOUT
.
When VOUT falls below VNOM, the device will
repeat (1), (2), and (3). The voltage step up, or
pre-positioning, improves transient droop when
a load transient causes a transition from LLM
mode to PWM mode. If a load transient
occurs, causing VOUT to fall below the threshold
VMIN, the device will exit LLM operation and
begin normal PWM operation. Figure 5
demonstrates VOUT behavior during transition
into and out of LLM operation.
NOTE: For proper LLM operation the
EN6337QI requires a minimum difference
between VIN and VOUT, and a minimum LLM
load requirement as specified in the Electrical
Characteristics Table. For LLM designs
requiring lower voltage headroom or a lower
minimum load, contact Enpirion Applications
support.
LLM
Ripple
VMAX
PWM
Ripple
VNOM
VOUT
VMIN
Load
Step
Over-Current Protection
The current limit function is achieved by
sensing the current flowing through the Power
PFET. When the sensed current exceeds the
over current trip point, both power FETs are
turned off for the remainder of the switching
cycle. If the over-current condition is removed,
the over-current protection circuit will enable
normal PWM operation. If the over-current
condition persists, the soft start capacitor will
gradually discharge causing the output voltage
to fall. When the OCP fault is removed, the
output voltage will ramp back up to the desired
voltage. This circuit is designed to provide high
noise immunity.
IOUT
Figure 5: VOUT behavior in LLM operation.
Many multi-mode DCDC converters suffer from
a condition that occurs when the load current
increases only slowly so that there is no load
transient driving VOUT below the VMIN threshold.
In this condition, the device would never exit
LLM operation. This could adversely affect
efficiency and cause unwanted ripple. To
prevent this from occurring, the EN6337QI
periodically exits LLM mode into PWM mode
and measures the load current. If the load
current is above the LLM threshold current, the
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11
www.enpirion.com
05800
6/17/2011
Rev: B
ENPIRION [ ENPIRION, INC. ]
