EP5357LUI datasheet(11/17 Pages) ENPIRION

11 / 17 page

03409

September 12, 2012

Rev: D

EP5357LUI/EP5357HUI

www.enpirion.com

exit LLM operation and begin normal PWM

operation.

Figure

demonstrates

VOUT

behavior during transition into and out of LLM

operation.

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.

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.

LLM Threshold Current vs. VOUT

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

122

156

2.90

126

158

2.60

106

136

162

2.50

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

104

108

113

0.80

VIN

VOUT

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.

Device exits LLM,

tests load current

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