Datasheet

LED7707 Application information
25/47
6 Application information
6.1 System stability
The boost section of the LED7707 is a fixed frequency, current-mode converter. During
normal operation, a minimum voltage selection circuit compares all the voltage drops across
the active current generators and provides the minimum one to the error amplifier. The
output voltage of the error amplifier determines the inductor peak current in order to keep its
inverting input equal to the reference voltage (700 mV typ). The compensation network
consists of a simple RC series (R
COMP
- C
COMP
) between the COMP pin and ground.
The calculation of R
COMP
and C
COMP
is fundamental to achieve optimal loop stability and
dynamic performance of the boost converter and is strictly related to the operating
conditions.
6.1.1 Loop compensation
The compensation network can be quickly calculated using equations 11 to 16. Once both
R
COMP
and C
COMP
have been determined, a fine-tuning phase may be required in order to
get the optimal dynamic performance from the application.
The first parameter to be fixed is the switching frequency. Normally, a high switching
frequency allows reducing the size of the inductor and positively affects the dynamic
response of the converter (wider bandwidth) but increases the switching losses. For most of
applications, the fixed value (660 kHz) represents a good trade-off between power
dissipation and dynamic response, allowing to save an external resistor at the same time. In
low-profile applications, the inductor value is often kept low to reduce the number of turns;
an inductor value in the 4.7 µH-15 µH range is a good starting choice.
In order to avoid instability due to interaction between the DC-DC converter's loop and the
current generators' loop, the bandwidth of the boost should not exceed the bandwidth of the
current generators. A unity-gain frequency (f
U
) in the order of 30-40 kHz is acceptable. Also,
take care not to exceed the CCM-mode right half-plane zero (RHPZ).
Equation 11
Equation 12
Equation 13 a
SWU
F2.0f
⋅
≤
L2
I
V
V
V
2.0
L2
RM
2.0f
OUT
OUT
2
OUT
min,IN
2
U
⋅π
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
⋅=
⋅π
⋅≤
OUT
min,IN
V
V
M =