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EP53A7LQ Datasheet(PDF) 12 Page - Enpirion, Inc.

Part No. EP53A7LQ
Description  1000mA Synchronous Buck Regulator with Integrated Inductor
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Maker  ENPIRION [Enpirion, Inc.]
Homepage  http://www.enpirion.com
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EP53A7LQ Datasheet(HTML) 12 Page - Enpirion, Inc.

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01543
11/11/2009
Rev:B
EP53A7LQI/EP53A7HQI
©Enpirion 2009 all rights reserved, E&OE
12
www.enpirion.com
VOUT
IOUT
LLM
Ripple
PWM
Ripple
VMAX
VNOM
VMIN
Load
Step
Figure 6: VOUT Behavior in LLM Operation
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
(shown in Figure 6).
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 EP53A7xQI 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.
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
LLM Threshold Current vs. VOUT
0
50
100
150
200
250
0.8
1.1
1.4
1.7
2.0
2.3
2.6
2.9
3.2
VOUT (V)
VIN=5V (top curve)
VIN=4.2V
VIN=3.7V
VIN=3.3V (bottom curve)
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
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
VIN
VOUT
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
complete; disable LLM before VIN ramp down.
For applications with ENABLE control, tie LLM
to ENABLE; enable device after VIN ramp up
complete and disable device before VIN ramp
Device exits LLM,
tests load current


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