lm2788mmx-1.8 National Semiconductor Corporation, lm2788mmx-1.8 Datasheet - Page 9

• Manufacturer Part Number: lm2788mmx-1.8
• Description: 120ma High Efficiency Step-down Switched Capacitor Voltage Converter
• Manufacturer: National Semiconductor Corporation

Operation Description
holds true for the other base-gain regions. At the base-gain
transitions (V
transition as the part switches base-gains. The 10mA load
curve gives a clear picture of how base-gain affects overall
converter efficiency. With a 10mA output current, the gain of
the LM2788-1.8 is equal to the base-gain over the entire
operating input voltage range. Additionally, with a 10mA load,
internal supply current has a minimal impact on efficiency
(Supply current does have a small affect: it is why the 10mA
load curve is slightly below the ideal efficiency gradients in
each of the base-gain regions).
The 120mA-load curve in Figure 1 illustrates the effect of
gain hopping on converter efficiency. Gain hopping is imple-
mented to overcome output voltage droop that results from
charge-pump non-idealities. In an ideal charge pump, the
output voltage is equal to the product of the gain and the
input voltage. Non-idealities such as finite switch resistance,
capacitor ESR, and other factors result in the output of
practical charge pumps being below the ideal value, how-
ever. This output droop is typically modeled as an output
resistance, R
creases linearly with load current.
The LM2788 compensates for output voltage droop under
high load conditions by gain hopping: when the base-gain is
not sufficient to keep the output voltage in regulation, the
part will temporarily switch up to the next highest gain setting
to provide an intermittent boost in output voltage. When the
output voltage is sufficiently boosted, the gain configuration
reverts back to the base-gain setting. If the load remains
high, the part will continue to hop back and forth between the
base-gain and the next highest gain setting, and the output
voltage will remain in regulation. In contrast to the base-gain
decision, which is made based on the input voltage, the
decision to gain hop is made by monitoring the voltage at the
output of the part.
The efficiency curve of the LM2788-1.8 with a 120mA output
current, also contained in Figure 1, shows the effect that gain
hopping has on efficiency. Comparing the 120mA load curve
to the 10mA load curve, it is plain to see that to the right of
the base-gain transitions, the efficiency of the 120mA curve
increases gradually whereas the 10mA curve makes a sharp
transition. The base-gain of both curves is the same for both
loads. The difference comes in gain hopping. With the
120mA load, the part will spend a percentage of time in the
base-gain setting and the rest of the time in the next-highest
gain setting. The percentage of time gain hopping decreases
as the input voltage rises, as less gain-hopping boost is
required with increased input voltage. When the input volt-
age in a given base-gain region is large enough so that no
extra boost from gain hopping is required, the 120mA-load
efficiency curve mirrors the 10mA efficiency curve.
Gain hopping contributes to the overall high efficiency of the
LM2788. Gain hopping only occurs when required for keep-
ing the output voltage in regulation. This allows the LM2788
Real Charge Pump: V
TABLE 2. LM2788-1.8 Gain Hopping Regions
Input Voltage
3.0V - 3.3V
3.8V - 4.4V
Ideal Charge Pump: V
IN
OUT
= 2.9V, 3.8V), the 10mA curve makes sharps
, because the magnitude of the droop in-
OUT
Base Gain
(G
= (G x V
2
1
⁄
⁄
B
3
2
)
OUT
IN
= G x V
(Continued)
) - (I
Gain Hop
Setting
OUT
2
IN
1
⁄
3
x R
OUT
)
9
to operate in the higher efficiency base-gain setting as much
as possible. Gain hopping also allows the base-gain transi-
tions to be placed at input voltages that are as low as
practically possible. This maximizes the peaks, and mini-
mizes the valleys, of the efficiency ’saw-tooth’ curves, again
maximizing total solution efficiency.
SHUTDOWN
The LM2788 is in shutdown mode when the voltage on the
active-low logic enable pin (EN) is low. In shutdown, the
LM2788 draws virtually no supply current. When in shut-
down, the output of the LM2788 is completely disconnected
from the input, and will be 0V unless driven by an outside
source.
In some applications, it may be desired to disable the
LM2788 and drive the output pin with another voltage
source. This can be done, but the voltage on the output pin
of the LM2788 must not be brought above the input voltage.
The output pin will draw a small amount when driven exter-
nally due the internal feedback resistor divider connected
between V
SOFT START
The LM2788 employs soft start circuitry to prevent excessive
input inrush currents during startup. The output voltage is
programmed to rise from 0V to the nominal output voltage in
approximately 400µs (typ.). With the input voltage estab-
lished, soft-start is engaged when a part is enabled by
pulling the voltage on the EN pin high. Soft-start also en-
gages when voltage is established simultaneously to the V
and EN pins
THERMAL SHUTDOWN
Protection from overheating-related damage is achieved
with a thermal shutdown feature. When the junction tem-
perature rises to 150
down mode. The LM2788 disengages thermal shutdown
when the junction temperature of the part is reduced to
130
not activate thermal shutdown (or exhibit related thermal
cycling) when the part is operated within specified input
voltage, output current, and ambient temperature operating
ratings.
SHORT-CIRCUIT PROTECTION
The LM2788 short-circuit protection circuitry that protects the
device in the event of excessive output current and/or output
shorts to ground. A graph of ’Short-Circuit Current vs. Input
Voltage’ is provided in the Performance Characteristics
section.
Application Information
OUTPUT VOLTAGE RIPPLE
The voltage ripple on the output of the LM2788 is highly
dependent on the application conditions. The output capaci-
tor, the input voltage, and the output current each play a
significant part in determining the output voltage ripple. Due
to the complexity of LM2788 operation, providing equations
or models to approximate the magnitude of the ripple cannot
be easily accomplished. The following general statements
can be made however
The output capacitor will have a significant effect on output
voltage ripple magnitude. Ripple magnitude will typically be
linearly proportional to the output capacitance present. A
o
C (typ.). Due to its high efficiency, the LM2788 should
OUT
and GND.
o
C (typ.), the part switches into shut-
www.national.com
IN