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EP5357LUI Datasheet, PDF (11/17 Pages) Enpirion, Inc. – 600mA Synchronous Buck Regulator with Integrated Inductor
exit LLM operation and begin normal PWM
operation. Figure 6 demonstrates VOUT
behavior during transition into and out of LLM
operation.
VMAX
VNOM
VMIN
LLM
Ripple
PWM
Ripple
VOUT
Load
Step
IOUT
Figure 6: VOUT Behavior in LLM Operation
Device exits LLM,
tests load current
Figure 7: VOUT Droop during Periodic LLM Exit
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 EP5357xUI
periodically exits LLM mode into PWM mode
and measures the load current. If the load
current is above the LLM threshold current, the
device will remain in PWM mode. If the load
current is below the LLM threshold, the device
will re-enter LLM operation. There will be a
small droop in VOUT at the point where the
device exits and re-enters LLM, as shown in
Figure 7.
EP5357LUI/EP5357HUI
LLM Threshold Current vs. VOUT
250
200
150
VIN=5V (top curve)
100
VIN=4.2V
VIN=3.7V
50
VIN=3.3V (bottom curve)
0
0.8
1.1
1.4
1.7
2.0
2.3
2.6
2.9
3.2
VOUT (V)
Figure 8: Typical load current for LLM engage and
disengage versus VOUT for selected input voltages
Table 1: Load current below which the device can be
certain to be in LLM operation. These values are
guaranteed by design
VIN
VOUT
3.3
3.7
4.3
5.0
3.30
105
147
3.00
62
122
156
2.90
89
126
158
2.60
56
106
136
162
2.50
69
111
138
162
2.20
101
120
141
160
2.10
105
122
141
158
1.80
111
124
138
150
1.50
111
120
130
138
1.45
111
119
128
136
1.20
105
111
117
122
1.15
103
108
114
119
1.10
101
106
111
116
1.05
99
104
108
113
0.80
87
89
92
94
The load current at which the device will enter
LLM mode is a function of input and output
voltage. Figure 8 shows the typical value at
which the device will enter LLM operation. The
actual load current at which the device will
enter LLM operation can vary by +/-30%. Table
1 shows the minimum load current below which
the device is guaranteed to be in LLM
operating mode.
To ensure normal LLM operation, LLM mode
should be enabled/disabled with specific
sequencing. For applications with explicit LLM
pin control, enable LLM after VIN ramp up is
complete; disable LLM before VIN ramp down.
©Enpirion 2012 all rights reserved, E&OE
03409
11
September 12, 2012
www.enpirion.com
Rev: D