EP53A7LQ Datasheet, PDF (12/17 Pages) Enpirion, Inc. – 1000mA Synchronous Buck Regulator with Integrated Inductor

V_MAX
V_NOM
V_MIN
Load
Step
LLM
Ripple
PWM
Ripple
V_OUT
I_OUT
Figure 6: V_OUT Behavior in LLM Operation
Device exits LLM,
tests load current
Figure 7: V_OUT 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 V_OUT below the V_MIN 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 V_OUT
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
©Enpirion 2009 all rights reserved, E&OE
01543
EP53A7LQI/EP53A7HQI
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 V_OUT 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
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
12
11/11/2009
www.enpirion.com
Rev:B